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C ++值类别的快速参考：第2部分



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将参数传递给函数


将参数传递给函数时，传递的表达式的类别将隐式转换为函数参数的类别： void f(TO_TYPE p); FROM_TYPE x; f(x); void f(TO_TYPE p); FROM_TYPE x; f(x); 除了函数定义不能包含“自动”类型外，这种隐式转换的方式与分配期间的方式相同（请参见上面的“分配”部分）。



当函数具有相同数量和类型的参数但参数类别不同的​​多个重载时，重载机制将根据类别选择最适用的函数重载。 在下表中，为函数重载分配了一个顺序，在传递特定类别的表达式时，将选择每个重载（例如1表示，如果有可用，将始终首先选择此重载）：



脚注：


1-创建的用于保存引用初始化程序的临时目录会一直保留到函数作用域结束。


注意事项：


  • 如果函数按值接受参数，这并不意味着它总是创建对象的副本并且不能更改源对象。 例如，如果移动此函数的值，则将调用move构造函数，并更改源对象。
  • 您不能同时具有T f()和const T f()函数重载
  • 可以根据参数是否为引用来重载函数。 但是，如果两个函数都可以接受参数，则需要手动解决歧义（通过强制转换为相关的函数指针类型）。
  • 转发参考是最“贪婪的”-只要没有完全匹配的重载，它就会赢。 这就是通常不建议重载转发引用的原因（相反，避免FR或避免重载或使用标签分发或SFINAE）。 不应使用带有一个转发引用参数的构造函数，因为它可能与复制和移动构造函数冲突。
  • 转发引用会使编译器错误更难以理解和解决，特别是如果顺序使用它们多次。
  • 转发参考在以下情况下不起作用：
    • 您不能将{1, 2, 3}转发到带有转发参考的vector<int> 。
    • 您不能将0或NULL转发给具有转发引用的指针（改用nullptr ）。
    • 如果没有定义，则不能使用转发参考转发整数静态const和constexpr成员变量。
    • 您不能使用转发参考转发重载的函数或模板，因为这会造成歧义（可以通过指定类型来消除歧义）。

打印每个被调用的构造函数和运算符的选项的示例和测试：


打印每个被调用的构造函数和运算符的示例和测试
 #include <iostream> #include <iomanip> #include <map> #include <vector> #include <string> using namespace std; template<class C, class T> auto contains(const C& v, const T& x) -> decltype(end(v), true) { return end(v) != std::find(begin(v), end(v), x); } template <class... Types> constexpr inline __attribute__((__always_inline__)) int UNUSED(Types&&...) { return 0; }; map<string, map<string, string>> res; vector<string> froms; vector<string> tos; string from; string to; int ready = 0; void report(string st) { if (!from.empty() && !to.empty()) { res[from][to] += st; } if (ready) cout << st << " "; } struct T { T() { report("Dc"); } T(int va) : a(va) { report("Pc"); } T(const T& other) : a(other.a) { report("Cc"); } T(T&& other) : a(std::exchange(other.a, 0)) { report("Mc"); } T& operator=(int va) { report("Va"); a = va; return *this; } T& operator=(const T& rhs) { report("Ca"); // check for self-assignment if(&rhs == this) return *this; a = rhs.a; return *this; } T& operator=(T&& rhs) { report("Ma"); // check for self-assignment if(&rhs == this) return *this; a = std::exchange(rhs.a, 0); return *this; } ~T() { report("D"); } int a = 1; }; void func_start() { cout << "|"; } T Fprv() { return T(4); } const T Fcprv() { return T(5); } T gs; void t(T s) { func_start(); cout << sa; } void ct(const T s) { func_start(); cout << sa; } void tr(T& s) { func_start(); cout << sa; } void ctr(const T& s) { func_start(); cout << sa; } void trr(T&& s) { func_start(); cout << sa; } void ctrr(const T&& s) { func_start(); cout << sa; } template<typename Z> void pf(Z&& s) { func_start(); gs = forward<Z>(s); cout << gs.a; } void print_col(const string &st, int width) { cout << endl << left << setw(width) << st; } void test_pass(string lto, string lfrom) { from = lfrom; to = lto; res[from][to] = ""; if (!from.empty() && !to.empty()) { if (!contains(froms, from)) froms.push_back(from); if (!contains(tos, to)) tos.push_back(to); } print_col(lto + "(" + lfrom + "): ", 20); } #define EVAL(x) #x #define TEST_PASS(t, v) { \ test_pass(EVAL(t), #v); \ t(v); \ cout << "-"; \ } void test_conversion() { ready = 1; T LV; const T CLV; T& LR = LV; const T& CLR = LV; //T&& XV = T(); -- actually LR //const T&& CXV = T(); -- actually LR auto &&fr = T(); #undef DT #define DT t TEST_PASS(DT, 2); TEST_PASS(DT, Fprv()); TEST_PASS(DT, Fcprv()); TEST_PASS(DT, LV); TEST_PASS(DT, CLV); TEST_PASS(DT, LR); TEST_PASS(DT, CLR); TEST_PASS(DT, move(LV)); TEST_PASS(DT, move(CLV)); TEST_PASS(DT, fr); #undef DT #define DT ct TEST_PASS(DT, 2); TEST_PASS(DT, Fprv()); TEST_PASS(DT, Fcprv()); TEST_PASS(DT, LV); TEST_PASS(DT, CLV); TEST_PASS(DT, LR); TEST_PASS(DT, CLR); TEST_PASS(DT, move(LV)); TEST_PASS(DT, move(CLV)); TEST_PASS(DT, fr); #undef DT #define DT tr //TEST_PASS(DT, 2); //TEST_PASS(DT, Fprv()); //TEST_PASS(DT, Fcprv()); TEST_PASS(DT, LV); //TEST_PASS(DT, CLV); TEST_PASS(DT, LR); //TEST_PASS(DT, CLR); //TEST_PASS(DT, move(LV)); //TEST_PASS(DT, move(CLV)); TEST_PASS(DT, fr); #undef DT #define DT ctr TEST_PASS(DT, 2); TEST_PASS(DT, Fprv()); TEST_PASS(DT, Fcprv()); TEST_PASS(DT, LV); TEST_PASS(DT, CLV); TEST_PASS(DT, LR); TEST_PASS(DT, CLR); TEST_PASS(DT, move(LV)); TEST_PASS(DT, move(CLV)); TEST_PASS(DT, fr); #undef DT #define DT trr TEST_PASS(DT, 2); TEST_PASS(DT, Fprv()); //TEST_PASS(DT, Fcprv()); //TEST_PASS(DT, LV); //TEST_PASS(DT, CLV); //TEST_PASS(DT, LR); //TEST_PASS(DT, CLR); TEST_PASS(DT, move(LV)); //TEST_PASS(DT, move(CLV)); //TEST_PASS(DT, fr); #undef DT #define DT ctrr TEST_PASS(DT, 2); TEST_PASS(DT, Fprv()); TEST_PASS(DT, Fcprv()); //TEST_PASS(DT, LV); //TEST_PASS(DT, CLV); //TEST_PASS(DT, LR); //TEST_PASS(DT, CLR); TEST_PASS(DT, move(LV)); TEST_PASS(DT, move(CLV)); //TEST_PASS(DT, fr); #undef DT #define DT pf TEST_PASS(DT, 2); TEST_PASS(DT, Fprv()); TEST_PASS(DT, Fcprv()); TEST_PASS(DT, LV); TEST_PASS(DT, CLV); TEST_PASS(DT, LR); TEST_PASS(DT, CLR); TEST_PASS(DT, move(LV)); TEST_PASS(DT, move(CLV)); TEST_PASS(DT, fr); cout << endl; const int twidth = 10; cout << left << setw(twidth) << "From:"; for (const auto& lto : tos) { cout << left << setw(twidth) << lto; } cout << endl; for (const auto& lfrom : froms) { cout << left << setw(twidth) << lfrom; for (const auto& lto : tos) { if (!res.count(lfrom) || !res[lfrom].count(lto)) { cout << left << setw(twidth) << "-"; } else if (res[lfrom][lto].empty()) { cout << left << setw(twidth) << "+"; } else { cout << left << setw(twidth) << res[lfrom][lto]; } } cout << endl; } cout << endl; } int main() { test_conversion(); cout << endl; return 0; } /* Output: Dc Dc Dc t(2): Pc |2D - t(Fprv()): Pc |4D - t(Fcprv()): Pc |5D - t(LV): Cc |1D - t(CLV): Cc |1D - t(LR): Cc |1D - t(CLR): Cc |1D - t(move(LV)): Mc |1D - t(move(CLV)): Cc |1D - t(fr): Cc |1D - ct(2): Pc |2D - ct(Fprv()): Pc |4D - ct(Fcprv()): Pc |5D - ct(LV): Cc |0D - ct(CLV): Cc |1D - ct(LR): Cc |0D - ct(CLR): Cc |0D - ct(move(LV)): Mc |0D - ct(move(CLV)): Cc |1D - ct(fr): Cc |1D - tr(LV): |0- tr(LR): |0- tr(fr): |1- ctr(2): Pc |2D - ctr(Fprv()): Pc |4D - ctr(Fcprv()): Pc |5D - ctr(LV): |0- ctr(CLV): |1- ctr(LR): |0- ctr(CLR): |0- ctr(move(LV)): |0- ctr(move(CLV)): |1- ctr(fr): |1- trr(2): Pc |2D - trr(Fprv()): Pc |4D - trr(move(LV)): |0- ctrr(2): Pc |2D - ctrr(Fprv()): Pc |4D - ctrr(Fcprv()): Pc |5D - ctrr(move(LV)): |0- ctrr(move(CLV)): |1- pf(2): |Va 2- pf(Fprv()): Pc |Ma 4D - pf(Fcprv()): Pc |Ca 5D - pf(LV): |Ca 0- pf(CLV): |Ca 1- pf(LR): |Ca 0- pf(CLR): |Ca 0- pf(move(LV)): |Ma 0- pf(move(CLV)): |Ca 1- pf(fr): |Ca 1- From: t ct tr ctr trr ctrr pf 2 PcD PcD - PcD PcD PcD Va Fprv() PcD PcD - PcD PcD PcD PcMaD Fcprv() PcD PcD - PcD - PcD PcCaD LV CcD CcD + + - - Ca CLV CcD CcD - + - - Ca LR CcD CcD + + - - Ca CLR CcD CcD - + - - Ca move(LV) McD McD - + + + Ma move(CLV) CcD CcD - + - + Ca fr CcD CcD + + - - Ca DDD */

友情链接：


过载分辨率
const引用与移动语义
按值传递和std ::的优点按引用传递


从函数返回


从函数返回表达式时，返回的表达式FROM_TYPE的类别可能与函数返回类型RETURN_TYPE的类别不匹配，而返回类型RETURN_TYPE可能与分配的变量TO_TYPE的类别不匹配： RETURN_TYPE f() { FROM_TYPE x; return x; } TO_TYPE y = f(); RETURN_TYPE f() { FROM_TYPE x; return x; } TO_TYPE y = f();


在这种情况下，隐式转换可能发生两次：


  • 从FROM_TYPE转换为RETURN TYPE时。 此转换与分配期间的转换相同，只是尝试将PRV或CPRV转换为T &&，const T &&或const T＆（这种转换不会发生，因为临时对象的生存期不能超过函数作用域， link ）。
  • 从RETURN_TYPE转换为TO_TYPE时-通常的分配转换（请参见上面的“分配值类别”）。


脚注：


1-在赋值期间创建的用于保存引用初始化程序的临时对象将一直保留到其引用范围（红色字体）的结尾。


打印已使用的复制/移动构造函数和运算符的选项的示例和测试：


打印每个被调用的构造函数和运算符的示例和测试
 #include <iostream> #include <iomanip> #include <map> #include <vector> #include <string> using namespace std; template<class C, class T> auto contains(const C& v, const T& x) -> decltype(end(v), true) { return end(v) != std::find(begin(v), end(v), x); } template <class... Types> constexpr inline __attribute__((__always_inline__)) int UNUSED(Types&&...) { return 0; }; map<string, map<string, string>> res; vector<string> froms; vector<string> tos; string from; string to; int ready = 0; void report(string st) { if (!from.empty() && !to.empty()) { res[from][to] += st; } if (ready) cout << st << " "; } struct T { T() { report("Dc"); } T(int va) : a(va) { report("Pc"); } T(const T& other) : a(other.a) { report("Cc"); } T(T&& other) : a(std::exchange(other.a, 0)) { report("Mc"); } T& operator=(int va) { report("Va"); a = va; return *this; } T& operator=(const T& rhs) { report("Ca"); // check for self-assignment if(&rhs == this) return *this; a = rhs.a; return *this; } T& operator=(T&& rhs) { report("Ma"); // check for self-assignment if(&rhs == this) return *this; a = std::exchange(rhs.a, 0); return *this; } ~T() { report("D"); } int a = 1; }; T lv; const T clv; T& lr = lv; T&& rr = std::move(lv); const T& clr = clv; const T&& crr = std::move(clv); auto&& arr = std::move(lv); T Fprv() { return T(); } const T Fcprv() { return T(); } void func_start() { cout << "|"; } T prv_t() { func_start(); return T(2); } const T prv_ct() { func_start(); return T(2); } // Prohibited (returning reference to temporary object): // T& prv_tr() { func_start(); return T(2); } // const T& prv_ctr() { func_start(); return T(2); } // T&& prv_trr() { func_start(); return T(2); } // const T&& prv_ctrr() { func_start(); return T(2); } template<typename Z> Z&& prv_fr() { func_start(); return Z(2); } /* Same as prv_ (I tested) T cprv_t() { func_start(); return Fcprv(); } const T cprv_ct() { func_start(); return Fcprv(); } // Prohibited (returning reference to temporary object): // T& cprv_tr() { func_start(); return Fcprv(); } // const T& cprv_ctr() { func_start(); return Fcprv(); } // T&& cprv_trr() { func_start(); return Fcprv(); } // const T&& cprv_ctrr() { func_start(); return Fcprv(); } template<typename Z> Z&& cprv_fr() { func_start(); return Fcprv(); } */ /* Same as prv_ (I tested) T lit_t() { func_start(); return 3; } const T lit_ct() { func_start(); return 3; } //T& lit_tr() { func_start(); return 3; } //const T& lit_ctr() { func_start(); return 3; } //T&& lit_trr() { func_start(); return 3; } //const T&& lit_ctrr() { func_start(); return 3; } template<typename Z> Z&& lit_fr() { func_start(); return 3; } */ T lr_t() { func_start(); return lr; } const T lr_ct() { func_start(); return lr; } T& lr_tr() { func_start(); return lr; } const T& lr_ctr() { func_start(); return lr; } //T&& lr_trr() { func_start(); return lr; } //const T&& lr_ctrr() { func_start(); return lr; } template<typename Z> Z&& lr_fr() { func_start(); return lr; } T clr_t() { func_start(); return clr; } const T clr_ct() { func_start(); return clr; } //T& clr_tr() { func_start(); return clr; } const T& clr_ctr() { func_start(); return clr; } //T&& clr_trr() { func_start(); return clr; } //const T&& clr_ctrr() { func_start(); return clr; } template<typename Z> Z&& clr_fr() { func_start(); return clr; } /* This is the same as xv and cxv (I tested it) T rr_t() { func_start(); return move(rr); } const T rr_ct() { func_start(); return move(rr); } //T& rr_tr() { func_start(); return move(rr); } const T& rr_ctr() { func_start(); return move(rr); } T&& rr_trr() { func_start(); return move(rr); } const T&& rr_ctrr() { func_start(); return move(rr); } template<typename Z> Z&& rr_fr() { func_start(); return move(rr); } T crr_t() { func_start(); return move(crr); } const T crr_ct() { func_start(); return move(crr); } //T& crr_tr() { func_start(); return move(crr); } const T& crr_ctr() { func_start(); return move(crr); } //T&& crr_trr() { func_start(); return move(crr); } const T&& crr_ctrr() { func_start(); return move(crr); } template<typename Z> Z&& crr_fr() { func_start(); return move(crr); } */ /* Same as lr_ (I tested) T arr_t() { func_start(); return arr; } const T arr_ct() { func_start(); return arr; } T& arr_tr() { func_start(); return arr; } const T& arr_ctr() { func_start(); return arr; } //T&& arr_trr() { func_start(); return arr; } //const T&& arr_ctrr() { func_start(); return arr; } template<typename Z> Z&& arr_fr() { func_start(); return arr; } */ T lv_t() { func_start(); return lv; } const T lv_ct() { func_start(); return lv; } T& lv_tr() { func_start(); return lv; } const T& lv_ctr() { func_start(); return lv; } //T&& lv_trr() { func_start(); return lv; } //const T&& lv_ctrr() { func_start(); return lv; } template<typename Z> Z&& lv_fr() { func_start(); return lv; } T xv_t() { func_start(); return move(lv); } const T xv_ct() { func_start(); return move(lv); } //T& xv_tr() { func_start(); return move(lv); } const T& xv_ctr() { func_start(); return move(lv); } T&& xv_trr() { func_start(); return move(lv); } const T&& xv_ctrr() { func_start(); return move(lv); } template<typename Z> Z&& xv_fr() { func_start(); return move(lv); } T clv_t() { func_start(); return clv; } const T clv_ct() { func_start(); return clv; } //T& clv_tr() { func_start(); return clv; } const T& clv_ctr() { func_start(); return clv; } //T&& clv_trr() { func_start(); return clv; } //const T&& clv_ctrr() { func_start(); return clv; } template<typename Z> Z&& clv_fr() { func_start(); return clv; } T cxv_t() { func_start(); return move(clv); } const T cxv_ct() { func_start(); return move(clv); } //T& cxv_tr() { func_start(); return move(clv); } const T& cxv_ctr() { func_start(); return move(clv); } //T&& cxv_trr() { func_start(); return move(clv); } const T&& cxv_ctrr() { func_start(); return move(clv); } template<typename Z> Z&& cxv_fr() { func_start(); return move(clv); } void print_col(const string &st, int width) { cout << endl << left << setw(width) << st; } void test_call(string lto, string lfrom) { from = lfrom; to = lto; res[from][to] = ""; if (!from.empty() && !to.empty()) { if (!contains(froms, from)) froms.push_back(from); if (!contains(tos, to)) tos.push_back(to); } print_col(lto + " = " + lfrom + ": ", 20); } #define EVAL(x) #x #define TEST_CALL(t, v) { \ test_call(EVAL(t), #v); \ ts = v(); \ cout << sa; \ UNUSED(s); \ cout << "-"; \ } void test_return() { ready = 1; cout << endl; #define DT T TEST_CALL(DT, prv_t); TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); TEST_CALL(DT, lr_t); TEST_CALL(DT, lr_ct); TEST_CALL(DT, lr_tr); TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); TEST_CALL(DT, clr_t); TEST_CALL(DT, clr_ct); TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); TEST_CALL(DT, lv_t); TEST_CALL(DT, lv_ct); TEST_CALL(DT, lv_tr); TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); TEST_CALL(DT, xv_t); TEST_CALL(DT, xv_ct); TEST_CALL(DT, xv_ctr); TEST_CALL(DT, xv_trr); TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); TEST_CALL(DT, clv_t); TEST_CALL(DT, clv_ct); TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); TEST_CALL(DT, cxv_t); TEST_CALL(DT, cxv_ct); TEST_CALL(DT, cxv_ctr); TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); #undef DT #define DT const T TEST_CALL(DT, prv_t); TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); TEST_CALL(DT, lr_t); TEST_CALL(DT, lr_ct); TEST_CALL(DT, lr_tr); TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); TEST_CALL(DT, clr_t); TEST_CALL(DT, clr_ct); TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); TEST_CALL(DT, lv_t); TEST_CALL(DT, lv_ct); TEST_CALL(DT, lv_tr); TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); TEST_CALL(DT, xv_t); TEST_CALL(DT, xv_ct); TEST_CALL(DT, xv_ctr); TEST_CALL(DT, xv_trr); TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); TEST_CALL(DT, clv_t); TEST_CALL(DT, clv_ct); TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); TEST_CALL(DT, cxv_t); TEST_CALL(DT, cxv_ct); TEST_CALL(DT, cxv_ctr); TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); #undef DT #define DT T& //TEST_CALL(DT, prv_t); //TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); //TEST_CALL(DT, lr_t); //TEST_CALL(DT, lr_ct); TEST_CALL(DT, lr_tr); //TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); //TEST_CALL(DT, clr_t); //TEST_CALL(DT, clr_ct); //TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); //TEST_CALL(DT, lv_t); //TEST_CALL(DT, lv_ct); TEST_CALL(DT, lv_tr); //TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); //TEST_CALL(DT, xv_t); //TEST_CALL(DT, xv_ct); //TEST_CALL(DT, xv_ctr); //TEST_CALL(DT, xv_trr); //TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); //TEST_CALL(DT, clv_t); //TEST_CALL(DT, clv_ct); //TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); //TEST_CALL(DT, cxv_t); //TEST_CALL(DT, cxv_ct); //TEST_CALL(DT, cxv_ctr); //TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); #undef DT #define DT T&& TEST_CALL(DT, prv_t); //TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); TEST_CALL(DT, lr_t); //TEST_CALL(DT, lr_ct); //TEST_CALL(DT, lr_tr); //TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); TEST_CALL(DT, clr_t); //TEST_CALL(DT, clr_ct); //TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); TEST_CALL(DT, lv_t); //TEST_CALL(DT, lv_ct); //TEST_CALL(DT, lv_tr); //TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); TEST_CALL(DT, xv_t); //TEST_CALL(DT, xv_ct); //TEST_CALL(DT, xv_ctr); TEST_CALL(DT, xv_trr); //TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); TEST_CALL(DT, clv_t); //TEST_CALL(DT, clv_ct); //TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); TEST_CALL(DT, cxv_t); //TEST_CALL(DT, cxv_ct); //TEST_CALL(DT, cxv_ctr); //TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); #undef DT #define DT const T&& TEST_CALL(DT, prv_t); TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); TEST_CALL(DT, lr_t); TEST_CALL(DT, lr_ct); //TEST_CALL(DT, lr_tr); //TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); TEST_CALL(DT, clr_t); TEST_CALL(DT, clr_ct); //TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); TEST_CALL(DT, lv_t); TEST_CALL(DT, lv_ct); //TEST_CALL(DT, lv_tr); //TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); TEST_CALL(DT, xv_t); TEST_CALL(DT, xv_ct); //TEST_CALL(DT, xv_ctr); TEST_CALL(DT, xv_trr); TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); TEST_CALL(DT, clv_t); TEST_CALL(DT, clv_ct); //TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); TEST_CALL(DT, cxv_t); TEST_CALL(DT, cxv_ct); //TEST_CALL(DT, cxv_ctr); TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); #undef DT #define DT const auto&& TEST_CALL(DT, prv_t); TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); TEST_CALL(DT, lr_t); TEST_CALL(DT, lr_ct); //TEST_CALL(DT, lr_tr); //TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); TEST_CALL(DT, clr_t); TEST_CALL(DT, clr_ct); //TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); TEST_CALL(DT, lv_t); TEST_CALL(DT, lv_ct); //TEST_CALL(DT, lv_tr); //TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); TEST_CALL(DT, xv_t); TEST_CALL(DT, xv_ct); //TEST_CALL(DT, xv_ctr); TEST_CALL(DT, xv_trr); TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); TEST_CALL(DT, clv_t); TEST_CALL(DT, clv_ct); //TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); TEST_CALL(DT, cxv_t); TEST_CALL(DT, cxv_ct); //TEST_CALL(DT, cxv_ctr); TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); #undef DT #define DT auto& //TEST_CALL(DT, prv_t); TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); //TEST_CALL(DT, lr_t); TEST_CALL(DT, lr_ct); TEST_CALL(DT, lr_tr); TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); //TEST_CALL(DT, clr_t); TEST_CALL(DT, clr_ct); TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); //TEST_CALL(DT, lv_t); TEST_CALL(DT, lv_ct); TEST_CALL(DT, lv_tr); TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); //TEST_CALL(DT, xv_t); TEST_CALL(DT, xv_ct); TEST_CALL(DT, xv_ctr); //TEST_CALL(DT, xv_trr); TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); //TEST_CALL(DT, clv_t); TEST_CALL(DT, clv_ct); TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); //TEST_CALL(DT, cxv_t); TEST_CALL(DT, cxv_ct); TEST_CALL(DT, cxv_ctr); TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); #undef DT #define DT const T& TEST_CALL(DT, prv_t); TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); TEST_CALL(DT, lr_t); TEST_CALL(DT, lr_ct); TEST_CALL(DT, lr_tr); TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); TEST_CALL(DT, clr_t); TEST_CALL(DT, clr_ct); TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); TEST_CALL(DT, lv_t); TEST_CALL(DT, lv_ct); TEST_CALL(DT, lv_tr); TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); TEST_CALL(DT, xv_t); TEST_CALL(DT, xv_ct); TEST_CALL(DT, xv_ctr); TEST_CALL(DT, xv_trr); TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); TEST_CALL(DT, clv_t); TEST_CALL(DT, clv_ct); TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); TEST_CALL(DT, cxv_t); TEST_CALL(DT, cxv_ct); TEST_CALL(DT, cxv_ctr); TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); #undef DT #define DT const auto& TEST_CALL(DT, prv_t); TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); TEST_CALL(DT, lr_t); TEST_CALL(DT, lr_ct); TEST_CALL(DT, lr_tr); TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); TEST_CALL(DT, clr_t); TEST_CALL(DT, clr_ct); TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); TEST_CALL(DT, lv_t); TEST_CALL(DT, lv_ct); TEST_CALL(DT, lv_tr); TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); TEST_CALL(DT, xv_t); TEST_CALL(DT, xv_ct); TEST_CALL(DT, xv_ctr); TEST_CALL(DT, xv_trr); TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); TEST_CALL(DT, clv_t); TEST_CALL(DT, clv_ct); TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); TEST_CALL(DT, cxv_t); TEST_CALL(DT, cxv_ct); TEST_CALL(DT, cxv_ctr); TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); #undef DT #define DT auto&& TEST_CALL(DT, prv_t); TEST_CALL(DT, prv_ct); //TEST_CALL(DT, prv_fr); TEST_CALL(DT, lr_t); TEST_CALL(DT, lr_ct); TEST_CALL(DT, lr_tr); TEST_CALL(DT, lr_ctr); //TEST_CALL(DT, lr_fr); TEST_CALL(DT, clr_t); TEST_CALL(DT, clr_ct); TEST_CALL(DT, clr_ctr); //TEST_CALL(DT, clr_fr); TEST_CALL(DT, lv_t); TEST_CALL(DT, lv_ct); TEST_CALL(DT, lv_tr); TEST_CALL(DT, lv_ctr); //TEST_CALL(DT, lv_fr); TEST_CALL(DT, xv_t); TEST_CALL(DT, xv_ct); TEST_CALL(DT, xv_ctr); TEST_CALL(DT, xv_trr); TEST_CALL(DT, xv_ctrr); //TEST_CALL(DT, xv_fr); TEST_CALL(DT, clv_t); TEST_CALL(DT, clv_ct); TEST_CALL(DT, clv_ctr); //TEST_CALL(DT, clv_fr); TEST_CALL(DT, cxv_t); TEST_CALL(DT, cxv_ct); TEST_CALL(DT, cxv_ctr); TEST_CALL(DT, cxv_ctrr); //TEST_CALL(DT, cxv_fr); cout << endl; const int twidth = 9; cout << left << setw(twidth) << "From:"; for (const auto& lto : tos) { cout << left << setw(twidth) << lto; } cout << endl; for (const auto& lfrom : froms) { cout << left << setw(twidth) << lfrom; for (const auto& lto : tos) { if (!res.count(lfrom) || !res[lfrom].count(lto)) { cout << left << setw(twidth) << "-"; } else if (res[lfrom][lto].empty()) { cout << left << setw(twidth) << "+"; } else { cout << left << setw(twidth) << res[lfrom][lto]; } } cout << endl; } } int main() { test_return(); return 0; } /* Output: T = prv_t: |Pc 2-D T = prv_ct: |Pc 2-D T = lr_t: |Cc 1-D T = lr_ct: |Cc 1-D T = lr_tr: |Cc 1-D T = lr_ctr: |Cc 1-D T = clr_t: |Cc 1-D T = clr_ct: |Cc 1-D T = clr_ctr: |Cc 1-D T = lv_t: |Cc 1-D T = lv_ct: |Cc 1-D T = lv_tr: |Cc 1-D T = lv_ctr: |Cc 1-D T = xv_t: |Mc 1-D T = xv_ct: |Mc 0-D T = xv_ctr: |Cc 0-D T = xv_trr: |Mc 0-D T = xv_ctrr: |Cc 0-D T = clv_t: |Cc 1-D T = clv_ct: |Cc 1-D T = clv_ctr: |Cc 1-D T = cxv_t: |Cc 1-D T = cxv_ct: |Cc 1-D T = cxv_ctr: |Cc 1-D T = cxv_ctrr: |Cc 1-D const T = prv_t: |Pc 2-D const T = prv_ct: |Pc 2-D const T = lr_t: |Cc 0-D const T = lr_ct: |Cc 0-D const T = lr_tr: |Cc 0-D const T = lr_ctr: |Cc 0-D const T = clr_t: |Cc 1-D const T = clr_ct: |Cc 1-D const T = clr_ctr: |Cc 1-D const T = lv_t: |Cc 0-D const T = lv_ct: |Cc 0-D const T = lv_tr: |Cc 0-D const T = lv_ctr: |Cc 0-D const T = xv_t: |Mc 0-D const T = xv_ct: |Mc 0-D const T = xv_ctr: |Cc 0-D const T = xv_trr: |Mc 0-D const T = xv_ctrr: |Cc 0-D const T = clv_t: |Cc 1-D const T = clv_ct: |Cc 1-D const T = clv_ctr: |Cc 1-D const T = cxv_t: |Cc 1-D const T = cxv_ct: |Cc 1-D const T = cxv_ctr: |Cc 1-D const T = cxv_ctrr: |Cc 1-D T& = lr_tr: |0- T& = lv_tr: |0- T&& = prv_t: |Pc 2-D T&& = lr_t: |Cc 0-D T&& = clr_t: |Cc 1-D T&& = lv_t: |Cc 0-D T&& = xv_t: |Mc 0-D T&& = xv_trr: |0- T&& = clv_t: |Cc 1-D T&& = cxv_t: |Cc 1-D const T&& = prv_t: |Pc 2-D const T&& = prv_ct: |Pc 2-D const T&& = lr_t: |Cc 0-D const T&& = lr_ct: |Cc 0-D const T&& = clr_t: |Cc 1-D const T&& = clr_ct: |Cc 1-D const T&& = lv_t: |Cc 0-D const T&& = lv_ct: |Cc 0-D const T&& = xv_t: |Mc 0-D const T&& = xv_ct: |Mc 0-D const T&& = xv_trr: |0- const T&& = xv_ctrr: |0- const T&& = clv_t: |Cc 1-D const T&& = clv_ct: |Cc 1-D const T&& = cxv_t: |Cc 1-D const T&& = cxv_ct: |Cc 1-D const T&& = cxv_ctrr: |1- const auto&& = prv_t: |Pc 2-D const auto&& = prv_ct: |Pc 2-D const auto&& = lr_t: |Cc 0-D const auto&& = lr_ct: |Cc 0-D const auto&& = clr_t: |Cc 1-D const auto&& = clr_ct: |Cc 1-D const auto&& = lv_t: |Cc 0-D const auto&& = lv_ct: |Cc 0-D const auto&& = xv_t: |Mc 0-D const auto&& = xv_ct: |Mc 0-D const auto&& = xv_trr: |0- const auto&& = xv_ctrr: |0- const auto&& = clv_t: |Cc 1-D const auto&& = clv_ct: |Cc 1-D const auto&& = cxv_t: |Cc 1-D const auto&& = cxv_ct: |Cc 1-D const auto&& = cxv_ctrr: |1- auto& = prv_ct: |Pc 2-D auto& = lr_ct: |Cc 0-D auto& = lr_tr: |0- auto& = lr_ctr: |0- auto& = clr_ct: |Cc 1-D auto& = clr_ctr: |1- auto& = lv_ct: |Cc 0-D auto& = lv_tr: |0- auto& = lv_ctr: |0- auto& = xv_ct: |Mc 0-D auto& = xv_ctr: |0- auto& = xv_ctrr: |0- auto& = clv_ct: |Cc 1-D auto& = clv_ctr: |1- auto& = cxv_ct: |Cc 1-D auto& = cxv_ctr: |1- auto& = cxv_ctrr: |1- const T& = prv_t: |Pc 2-D const T& = prv_ct: |Pc 2-D const T& = lr_t: |Cc 0-D const T& = lr_ct: |Cc 0-D const T& = lr_tr: |0- const T& = lr_ctr: |0- const T& = clr_t: |Cc 1-D const T& = clr_ct: |Cc 1-D const T& = clr_ctr: |1- const T& = lv_t: |Cc 0-D const T& = lv_ct: |Cc 0-D const T& = lv_tr: |0- const T& = lv_ctr: |0- const T& = xv_t: |Mc 0-D const T& = xv_ct: |Mc 0-D const T& = xv_ctr: |0- const T& = xv_trr: |0- const T& = xv_ctrr: |0- const T& = clv_t: |Cc 1-D const T& = clv_ct: |Cc 1-D const T& = clv_ctr: |1- const T& = cxv_t: |Cc 1-D const T& = cxv_ct: |Cc 1-D const T& = cxv_ctr: |1- const T& = cxv_ctrr: |1- const auto& = prv_t: |Pc 2-D const auto& = prv_ct: |Pc 2-D const auto& = lr_t: |Cc 0-D const auto& = lr_ct: |Cc 0-D const auto& = lr_tr: |0- const auto& = lr_ctr: |0- const auto& = clr_t: |Cc 1-D const auto& = clr_ct: |Cc 1-D const auto& = clr_ctr: |1- const auto& = lv_t: |Cc 0-D const auto& = lv_ct: |Cc 0-D const auto& = lv_tr: |0- const auto& = lv_ctr: |0- const auto& = xv_t: |Mc 0-D const auto& = xv_ct: |Mc 0-D const auto& = xv_ctr: |0- const auto& = xv_trr: |0- const auto& = xv_ctrr: |0- const auto& = clv_t: |Cc 1-D const auto& = clv_ct: |Cc 1-D const auto& = clv_ctr: |1- const auto& = cxv_t: |Cc 1-D const auto& = cxv_ct: |Cc 1-D const auto& = cxv_ctr: |1- const auto& = cxv_ctrr: |1- auto&& = prv_t: |Pc 2-D auto&& = prv_ct: |Pc 2-D auto&& = lr_t: |Cc 0-D auto&& = lr_ct: |Cc 0-D auto&& = lr_tr: |0- auto&& = lr_ctr: |0- auto&& = clr_t: |Cc 1-D auto&& = clr_ct: |Cc 1-D auto&& = clr_ctr: |1- auto&& = lv_t: |Cc 0-D auto&& = lv_ct: |Cc 0-D auto&& = lv_tr: |0- auto&& = lv_ctr: |0- auto&& = xv_t: |Mc 0-D auto&& = xv_ct: |Mc 0-D auto&& = xv_ctr: |0- auto&& = xv_trr: |0- auto&& = xv_ctrr: |0- auto&& = clv_t: |Cc 1-D auto&& = clv_ct: |Cc 1-D auto&& = clv_ctr: |1- auto&& = cxv_t: |Cc 1-D auto&& = cxv_ct: |Cc 1-D auto&& = cxv_ctr: |1- auto&& = cxv_ctrr: |1- From: T const T T& T&& const T&&const auto&&auto& const T& const auto&auto&& prv_t PcD PcD - PcD PcD PcD - PcD PcD PcD prv_ct PcD PcD - - PcD PcD PcD PcD PcD PcD lr_t CcD CcD - CcD CcD CcD - CcD CcD CcD lr_ct CcD CcD - - CcD CcD CcD CcD CcD CcD lr_tr CcD CcD + - - - + + + + lr_ctr CcD CcD - - - - + + + + clr_t CcD CcD - CcD CcD CcD - CcD CcD CcD clr_ct CcD CcD - - CcD CcD CcD CcD CcD CcD clr_ctr CcD CcD - - - - + + + + lv_t CcD CcD - CcD CcD CcD - CcD CcD CcD lv_ct CcD CcD - - CcD CcD CcD CcD CcD CcD lv_tr CcD CcD + - - - + + + + lv_ctr CcD CcD - - - - + + + + xv_t McD McD - McD McD McD - McD McD McD xv_ct McD McD - - McD McD McD McD McD McD xv_ctr CcD CcD - - - - + + + + xv_trr McD McD - + + + - + + + xv_ctrr CcD CcD - - + + + + + + clv_t CcD CcD - CcD CcD CcD - CcD CcD CcD clv_ct CcD CcD - - CcD CcD CcD CcD CcD CcD clv_ctr CcD CcD - - - - + + + + cxv_t CcD CcD - CcD CcD CcD - CcD CcD CcD cxv_ct CcD CcD - - CcD CcD CcD CcD CcD CcD cxv_ctr CcD CcD - - - - + + + + cxv_ctrr CcD CcD - - + + + + + + */

从函数返回时，如果可以使用返回值优化（RVO）按值返回本地对象，则不应将std :: move或std :: forward应用于本地对象-因为将LV转换为XV将阻止编译器应用RVO和移动对象。 在应用时，RVO更为有效，因为它避免了调用任何其他复制/移动构造函数或运算符的过程。


另一方面，不同的编译器不能在不同情况下应用RVO，但是在大多数情况下，编译器将至少能够应用移动分配运算符。 如果存在调用复制构造函数而不是RVO的风险，则可以使用std :: move或std :: forward intead按值返回对象，但是据我所知，唯一可能发生的情况是使用返回语句中的三元运算符，例如return param ? a : b return param ? a : b ，通常可以很容易地用if语句替换。 您可以找到有关哪些编译器在以下情况下应用RVO的更多信息：https：//这里和https：//这里。


在Ubuntu上使用最新的Clang测试RVO（2019年）：


打印每个被调用的构造函数和运算符的示例和测试
 #include <iostream> #include <string> #include <iomanip> using namespace std; void report(string st) { cout << st << " "; } struct T { T() { report("Dc"); } T(int va) : a(va) { report("Pc"); } T(const T& other) : a(other.a) { report("Cc"); } T(T&& other) : a(std::exchange(other.a, 0)) { report("Mc"); } T& operator=(int va) { report("Va"); a = va; return *this; } T& operator=(const T& rhs) { report("Ca"); // check for self-assignment if(&rhs == this) return *this; a = rhs.a; return *this; } T& operator=(T&& rhs) { report("Ma"); // check for self-assignment if(&rhs == this) return *this; a = std::exchange(rhs.a, 0); return *this; } ~T() { report("D"); } int a = 1; }; T urvo_single() { //const bool param = true; return T(); } T urvo_two() { const bool param = true; if(param) return T(); else return T(); } T urvo_two_with_param(bool param) { if(param) return T(); else return T(); } T urvo_with_exception_1(bool param) { if(!param) throw std::exception(); return T(); } T urvo_with_exception_2(bool param) { if(param) return T(); else throw std::exception(); } T urvo_with_exception_3() { const bool param = true; if(param) return T(); else throw std::exception(); } static T make_X() { return T(); } T rrvo_single() { //const bool param = true; return make_X(); } T rrvo_two() { const bool param = true; if(param) return make_X(); else return make_X(); } T rrvo_two_with_param(bool param) { if(param) return make_X(); else return make_X(); } T rrvo_with_exception_1(bool param) { if(!param) throw std::exception(); return make_X(); } T rrvo_with_exception_2(bool param) { if(param) return make_X(); else throw std::exception(); } T rrvo_with_exception_3() { const bool param = true; if(param) return make_X(); else throw std::exception(); } T nrvo_single_1() { T a; return a; } T nrvo_single_2() { { T a; return a; } } T nrvo_single_with_exception_1(bool param) { T a; if(!param) throw std::exception(); return a; } T nrvo_single_with_exception_1a(bool param) { if(!param) throw std::exception(); T a; return a; } T nrvo_single_with_exception_2(bool param) { T a; if(param) return a; else throw std::exception(); // Silence compilation error, does not count as an additional // return statement as it is unreachable code return a; } T nrvo_single_with_exception_2a(bool param) { if(param) { T a; return a; } else throw std::exception(); } T nrvo_single_with_exception_3() { const bool param = true; T a; if(param) return a; else throw std::exception(); } T nrvo_single_with_exception_3a() { const bool param = true; if(param) { T a; return a; } else throw std::exception(); } T nrvo_two_different_tern() { const bool param = true; T a, b; return param ? a : b; } T nrvo_two_different_if() { const bool param = true; T a, b; if(param) return a; else return b; } T nrvo_two_different_if_2() { const bool param = true; if(param) { T a; return a; } else { T b; return b; } } T nrvo_two_different_with_param_tern(bool param) { T a, b; return param ? a : b; } T nrvo_two_different_with_param_if(bool param) { T a, b; if(param) return a; else return b; } T nrvo_two_different_with_param_if_2(bool param) { if(param) { T a; return a; } else { T b; return b; } } T nrvo_two_equal_tern() { const bool param = true; T a; return param ? a : a; } T nrvo_two_equal_if() { const bool param = true; T a; if(param) return a; else return a; } T nrvo_two_equal_with_param_tern(bool param) { T a; return param ? a : a; } T nrvo_two_equal_with_param_if(bool param) { T a; if(param) return a; else return a; } T nrvo_urvo_mixed_static() { static const bool param = true; if (param) return T(); T a; return a; } T nrvo_urvo_mixed_dynamic(bool param) { if (param) return T(); T a; return a; } void print_col(const string &st, int width) { cout << endl << left << setw(width) << st; } #define CHECK_COPIES(stmt) { \ print_col(#stmt ": ", 20); \ try { \ stmt; \ } \ catch(...) { \ } \ } \ int main() { CHECK_COPIES( T a = urvo_single()); CHECK_COPIES( T a = urvo_two()); CHECK_COPIES( T a = urvo_two_with_param(true)); CHECK_COPIES( T a = urvo_with_exception_1(true)); CHECK_COPIES( T a = urvo_with_exception_2(true)); CHECK_COPIES( T a = urvo_with_exception_3()); cerr << " "; CHECK_COPIES( T a = rrvo_single()); CHECK_COPIES( T a = rrvo_two()); CHECK_COPIES( T a = rrvo_two_with_param(true)); CHECK_COPIES( T a = rrvo_with_exception_1(true)); CHECK_COPIES( T a = rrvo_with_exception_2(true)); CHECK_COPIES( T a = rrvo_with_exception_3()); cerr << " "; CHECK_COPIES( T a = nrvo_single_1()); CHECK_COPIES( T a = nrvo_single_2()); CHECK_COPIES( T a = nrvo_single_with_exception_1(true)); CHECK_COPIES( T a = nrvo_single_with_exception_1a(true)); CHECK_COPIES( T a = nrvo_single_with_exception_2(true)); CHECK_COPIES( T a = nrvo_single_with_exception_2a(true)); CHECK_COPIES( T a = nrvo_single_with_exception_3()); CHECK_COPIES( T a = nrvo_single_with_exception_3a()); cerr << " "; CHECK_COPIES( T a = nrvo_two_different_tern()); CHECK_COPIES( T a = nrvo_two_different_if()); CHECK_COPIES( T a = nrvo_two_different_if_2()); CHECK_COPIES( T a = nrvo_two_different_with_param_tern(true)); CHECK_COPIES( T a = nrvo_two_different_with_param_if(true)); CHECK_COPIES( T a = nrvo_two_different_with_param_if_2(true)); CHECK_COPIES( T a = nrvo_two_equal_tern()); CHECK_COPIES( T a = nrvo_two_equal_if()); CHECK_COPIES( T a = nrvo_two_equal_with_param_tern(true)); CHECK_COPIES( T a = nrvo_two_equal_with_param_if(true)); cerr << " "; CHECK_COPIES( T a = nrvo_urvo_mixed_static()); CHECK_COPIES( T a = nrvo_urvo_mixed_dynamic(true)); } /* Output: T a = urvo_single(): Dc D T a = urvo_two(): Dc D T a = urvo_two_with_param(true): Dc D T a = urvo_with_exception_1(true): Dc D T a = urvo_with_exception_2(true): Dc D T a = urvo_with_exception_3(): Dc D T a = rrvo_single(): Dc D T a = rrvo_two(): Dc D T a = rrvo_two_with_param(true): Dc D T a = rrvo_with_exception_1(true): Dc D T a = rrvo_with_exception_2(true): Dc D T a = rrvo_with_exception_3(): Dc D T a = nrvo_single_1(): Dc D T a = nrvo_single_2(): Dc D T a = nrvo_single_with_exception_1(true): Dc D T a = nrvo_single_with_exception_1a(true): Dc D T a = nrvo_single_with_exception_2(true): Dc D T a = nrvo_single_with_exception_2a(true): Dc D T a = nrvo_single_with_exception_3(): Dc D T a = nrvo_single_with_exception_3a(): Dc D T a = nrvo_two_different_tern(): Dc Dc Cc DDD T a = nrvo_two_different_if(): Dc Dc Mc DDD T a = nrvo_two_different_if_2(): Dc D T a = nrvo_two_different_with_param_tern(true): Dc Dc Cc DDD T a = nrvo_two_different_with_param_if(true): Dc Dc Mc DDD T a = nrvo_two_different_with_param_if_2(true): Dc D T a = nrvo_two_equal_tern(): Dc Cc DD T a = nrvo_two_equal_if(): Dc D T a = nrvo_two_equal_with_param_tern(true): Dc Cc DD T a = nrvo_two_equal_with_param_if(true): Dc D T a = nrvo_urvo_mixed_static(): Dc D T a = nrvo_urvo_mixed_dynamic(true): Dc D */

Links:


Copy elision
Copy elision revisited
Compare clang and gcc copy elision (tests with similar results are commented out)


Passing and returning trivially copyable objects of small size


Trivially copyable struct or class:


  • has no virtual members or virtual base classes
  • has default copy and move constructors and operators

Trivially copyable objects of small size (up to 16 bytes) can be passed in CPU registers when passed to function by value — like scalar types.


Also, when passing it by value, compiler has additional guarantee that object will not change during the course of the function even if other functions are called — this is why compiler can generate more effective code in this situations. If passed by const T&, there is no such guarantee, because object passed by constant reference can still be changed from other function, or multiple passed objects can use shared memory.


For the same reason, std::string_view should be passed by value.


Links:


Trivially copyable


Returning from a function with a reference parameter


A reference to an object can be passed to a function so that function can change source object:


 T& change(T& X) { X.change(); return X; }

This method should not be used if temporary object can be created inside function and returned by value, because returning by value can take advantage of return value optimization (see above).


Passing multiple parameters and returning one of them from function


If all passed parameters are lvalues, you can pass and return by const T&. This gives high performance, because no copy/move constructors or operators are called:


 const T& get(const T& a, const T& b) { if ( /* something */ ) return a; else return b; }

prvalue cannot be passed to get function, because this would result in returning a reference to a temporary object. If prvalue has to be passed,


If both parameters are temporary objects (prvalues), T&& can be returned, which also gives high performance (no copy/move constructors or operators are called):


 T&& get(T&& a, T&& b) { if ( /* something */ ) return std::move(a); else return std::move(b); }

If some of parameters are temporary objects (prvalues) and other are lvalues, it is difficult to high performance without complicating the function interface. In the following example if lvalue and prvalue are passed as A and B arguments into the function, copy constructor will be called if A is chosen and move constructor if B is chosen:


 T get(const T& a, T&& b) { if ( /* something */ ) return a; else return std::move(b); }

Passing to constructor or setter


In this section I describe passing an object of type T to an object of type C during construction of type C or by calling a setter:


 class C { C(TX) : x(X) {} void set(TX) { x = X; } T x; }; T y; C c(y); c.set(y);

  • For copying an lvalue to x member variable in an object of class C:
    • with class C constructor: any of the following variants is good ("T move" variant is a bit less efficient due to calling an additional move constructor)
    • with class C set(T):
    • if T is copy-on-write (like CString in MFC/ATL) and x will not change later, any of the following variants is good ("T move" variant is a bit less efficient due to calling an additional move constructor)
    • if T is not copy-on write or x can change later, "T move" variant becomes inefficient, because it does not allow to avoid deallocation, even if already allocated memory in existing object x is enough for new object and class can reuse allocated memory (like std::string)
  • For moving an xvalue or prvalue to x member variable in an object of class C with C's constructor or C's set member function any of the following variants is good except "const T&" (because it requires copying). Perfect forwarding has a slight performance advantage due to avoiding calling move constructor when converting from different type, instead forwarding parameter of a different type to a parametrized constructor

Variants:


  • const T& — optimal if we do not need to move objects or store them in dynamically growing containers
     class C { C(const T& X) : x(X) {} void set(const T& X) { x = X; } T x; };
  • const T& + T&& move — optimized variant if we need to support both lvalue copy and rvalue move. Can lead to code duplication, especially if constructors with different combinations of & and && are needed. Provides simple implementation of noexcept for effective use with containers.
     class C { C(const T& X) : x(X) {} C(T&& X) : x(std::move(X)) {} void set(const T& X) { x = X; } void set(T&& X) { x = std::move(X); } T x; };
  • T move — looks simple, but not very intuitive (can avoid copying and change object, passed with std::move as xvalue). Less efficient than perfect forwarding and &+&& in some situations (does not allow to avoid deallocation, even if already allocated memory in existing object x is enough for new object and class can reuse allocated memory, calls excessive moves and destructors)
     class C { C(TX) : x(std::move(X)) {} void set(TX) { x = std::move(X); } T x; };
  • Perfect forwarding (PF) — most effective variant (especially when forwarding literals or other types, which can be converted to class with parametrized constructors), but it is more difficult to write, which increases risk of mistakes. Does not support virtual functions and has to be implemented in header file.
     class C { template<typename Z> C(Z&& X) : x(std::forward<Z>(X)) {} template<typename Z> void set(Z&& X) { x = std::forward<Z>(X); } T x; };
  • Strict perfect forwarding (PF Strict). Has same qualities as perfect forwarding, but is more safe, because it does not allow to convert types and thus cannot receive literals (actually, it is the only variant among discussed in this section, which does not allow to convert types).
     class C { template<class Z, class=std::enable_if_t<std::is_same<std::decay_t<Z>, T>::value>> C(Z&& X) : x(std::forward<Z>(X)) {} template<class Z, class=std::enable_if_t<std::is_same<std::decay_t<Z>, T>::value>> void set(Z&& X) { x = std::forward<Z>(X); } T x; };


In the comparison tables below constructor properties are highlighted with green, setter member function properties are highlighted with yellow.



Annotations:
1 — Becomes combinatorial if constructor has multiple parameters
2 — Function is technically noexcept, but copy before function call can result in exception
3 — Requires noexcept(std::is_nothrow_assignable<Type&, T>::value)
4 — Does not accept conversion
5 — Unconditional deallocation leads to inability to reuse allocated storage (std::string). Not a problem for copy-on-write implementations (Tstring)
6 — If you do not have overloaded assignment operator for value type in object class, this will be: VcMaD


Examples and tests of variants with printing of used copy/move constructors and operators:


Examples and tests with printing of each called constructor and operator
 #include <iostream> using namespace std; struct TStruct { TStruct() { cout << "Dc"; } TStruct(int va) : a(va) { cout << "Vc"; } TStruct(const TStruct& other) : a(other.a) { cout << "Cc"; } TStruct(TStruct&& other) : a(std::exchange(other.a, 0)) { cout << "Mc"; } TStruct& operator=(int va) { cout << "Va"; a = va; return *this; } TStruct& operator=(const TStruct& rhs) { cout << "Ca"; // check for self-assignment if(&rhs == this) return *this; a = rhs.a; return *this; } TStruct& operator=(TStruct&& rhs) { cout << "Ma"; // check for self-assignment if(&rhs == this) return *this; a = std::exchange(rhs.a, 0); return *this; } ~TStruct() { cout << "D"; } int a = 1; }; struct TRef { TRef(TStruct& tsv) : ts(tsv) {} void set(TStruct& tsv) { ts = tsv; } TStruct ts; }; struct TConstRef { TConstRef(const TStruct& tsv) : ts(tsv) {} void set(const TStruct& tsv) { ts = tsv; } TStruct ts; }; struct TConstRefAndRvalueRef { TConstRefAndRvalueRef(const TStruct& tsv) : ts(tsv) { cout << "Lr"; } TConstRefAndRvalueRef(TStruct&& tsv) : ts(std::move(tsv)) { cout << "Rr"; } void set(const TStruct& tsv) { cout << "Sl"; ts = tsv; } void set(TStruct&& tsv) { cout << "Sr"; ts = std::move(tsv); } TStruct ts; }; struct TConstRvalueRef { TConstRvalueRef(const TStruct&& tsv) : ts(std::move(tsv)) { } void set(const TStruct&& tsv) { ts = std::move(tsv); } TStruct ts; }; struct TValueMove { TValueMove(TStruct tsv) : ts(std::move(tsv)) {} void set(TStruct tsv) { ts = std::move(tsv); } TStruct ts; }; struct TConstValueMove { TConstValueMove(const TStruct tsv) : ts(std::move(tsv)) {} void set(const TStruct tsv) { ts = std::move(tsv); } TStruct ts; }; struct TPerfectForward { template<class T> TPerfectForward(T&& tsv) : ts(std::forward<T>(tsv)) {} template<class T> void set(T&& tsv) { ts = std::forward<T>(tsv); } TStruct ts; }; struct TConstPerfectForward { template<class T> TConstPerfectForward(const T&& tsv) : ts(std::forward<T>(tsv)) {} template<class T> void set(const T&& tsv) { ts = std::forward<T>(tsv); } TStruct ts; }; struct TPerfectForwardStrict { template<class T, class=std::enable_if_t<std::is_same<std::decay_t<T>, TStruct>::value>> TPerfectForwardStrict(T&& tsv) : ts(std::forward<T>(tsv)) {} template<class T, class=std::enable_if_t<std::is_same<std::decay_t<T>, TStruct>::value>> void set(T&& tsv) { ts = std::forward<T>(tsv); } TStruct ts; }; struct TMultiVariant { TMultiVariant(const TStruct& tsv) : ts(tsv) { cout << "Lr"; } TMultiVariant(TStruct&& tsv) : ts(std::move(tsv)) { cout << "Rr"; } void set(const TStruct& tsv) { cout << "Sl"; ts = tsv; } void set(TStruct&& tsv) { cout << "Sr"; ts = std::move(tsv); } template<class T> TMultiVariant(T&& tsv) : ts(std::forward<T>(tsv)) { cout << "Pf"; } template<class T> void set(T&& tsv) { cout << "Sp"; ts = std::forward<T>(tsv); } TStruct ts; }; struct TMultiVariantStrict { TMultiVariantStrict(const TStruct& tsv) : ts(tsv) { cout << "Lr"; } TMultiVariantStrict(TStruct&& tsv) : ts(std::move(tsv)) { cout << "Rr"; } void set(const TStruct& tsv) { cout << "Sl"; ts = tsv; } void set(TStruct&& tsv) { cout << "Sr"; ts = std::move(tsv); } template<class T, class=std::enable_if_t<std::is_same<std::decay_t<T>, TStruct>::value>> TMultiVariantStrict(T&& tsv) : ts(std::forward<T>(tsv)) { cout << "Pf"; } template<class T, class=std::enable_if_t<std::is_same<std::decay_t<T>, TStruct>::value>> void set(T&& tsv) { cout << "Sp"; ts = std::forward<T>(tsv); } TStruct ts; }; TStruct get_st() { return TStruct(7); } void detect_change(int& param, int val) { if (param != val) { cout << "!"; param = val; } else { cout << "-"; } } void test_passing() { cout << "TStruct st(100): "; TStruct st(100); // Test constructing with object cout << endl << "TRef (st): "; TRef rf(st); detect_change(st.a, 100); cout << endl << "TConstRef (st): "; TConstRef crf(st); detect_change(st.a, 100); cout << endl << "TConstRefAndRvalueRef (st): "; TConstRefAndRvalueRef crf_rvf(st); detect_change(st.a, 100); // Will not compile // cout << endl << "TConstRvalueRef (st): "; // TConstRvalueRef crvf(st); // detect_change(st.a, 100); cout << endl << "TValueMove (st): "; TValueMove vm(st); detect_change(st.a, 100); cout << endl << "TConstValueMove (st): "; TConstValueMove cvm(st); detect_change(st.a, 100); cout << endl << "TPerfectForward (st): "; TPerfectForward pf(st); detect_change(st.a, 100); // Will not compile // cout << endl << "TConstPerfectForward (st): "; // TConstPerfectForward cpf(st); // detect_change(st.a, 100); cout << endl << "TPerfectForwardStrict (st): "; TPerfectForwardStrict pft(st); detect_change(st.a, 100); cout << endl << "TMultiVariant (st): "; TMultiVariant mv(st); detect_change(st.a, 100); cout << endl << "TMultiVariantStrict (st): "; TMultiVariantStrict mvt(st); detect_change(st.a, 100); // Test constructing with moved object // Will not compile because of std::move // cout << endl << "TRef (std::move(st)): "; // TRef rf4(std::move(st)); // detect_change(st.a, 100); cout << endl << "TConstRef (std::move(st)): "; TConstRef crf4(std::move(st)); detect_change(st.a, 100); cout << endl << "TConstRefAndRvalueRef (std::move(st)): "; TConstRefAndRvalueRef crf_rvf4(std::move(st)); detect_change(st.a, 100); cout << endl << "TConstRvalueRef (std::move(st)): "; TConstRvalueRef crvf4(std::move(st)); detect_change(st.a, 100); cout << endl << "TValueMove (std::move(st)): "; TValueMove vm4(std::move(st)); detect_change(st.a, 100); cout << endl << "TConstValueMove (std::move(st)): "; TConstValueMove cvm4(std::move(st)); detect_change(st.a, 100); cout << endl << "TPerfectForward (std::move(st)): "; TPerfectForward pf4(std::move(st)); detect_change(st.a, 100); // Will not compile // cout << endl << "TConstPerfectForward (std::move(st)): "; // TConstPerfectForward cpf4(std::move(st)); // detect_change(st.a, 100); cout << endl << "TPerfectForwardStrict (std::move(st)): "; TPerfectForwardStrict pft4(std::move(st)); detect_change(st.a, 100); cout << endl << "TMultiVariant (std::move(st)): "; TMultiVariant mv4(std::move(st)); detect_change(st.a, 100); cout << endl << "TMultiVariantStrict (std::move(st)): "; TMultiVariantStrict mvt4(std::move(st)); detect_change(st.a, 100); // Test constructing with temporary object // Will not compile // cout << endl << "TRef (TStruct(6)): "; // TRef rf3(TStruct(6)); cout << endl << "TConstRef (TStruct(6)): "; TConstRef crf3(TStruct(6)); cout << endl << "TConstRefAndRvalueRef (TStruct(6)): "; TConstRefAndRvalueRef crf_rvf3(TStruct(6)); cout << endl << "TConstRvalueRef (TStruct(6)): "; TConstRvalueRef crvf3(TStruct(6)); cout << endl << "TValueMove (TStruct(6)): "; TValueMove vm3(TStruct(6)); cout << endl << "TConstValueMove (TStruct(6)): "; TConstValueMove cvm3(TStruct(6)); cout << endl << "TPerfectForward (TStruct(6)): "; TPerfectForward pf3(TStruct(6)); // Will not compile //cout << endl << "TConstPerfectForward (TStruct(6)): "; //TConstPerfectForward cpf3(TStruct(6)); cout << endl << "TPerfectForwardStrict (TStruct(6)): "; TPerfectForwardStrict pft3(TStruct(6)); cout << endl << "TMultiVariant (TStruct(6)): "; TMultiVariant mv3(TStruct(6)); cout << endl << "TMultiVariantStrict (TStruct(6)): "; TMultiVariantStrict mvt3(TStruct(6)); // Test constructing with RVO object // Will not compile //cout << endl << "TRef (get_st()): "; //TRef rf5(get_st()); cout << endl << "TConstRef (get_st()): "; TConstRef crf5(get_st()); cout << endl << "TConstRefAndRvalueRef (get_st()): "; TConstRefAndRvalueRef crf_rvf5(get_st()); cout << endl << "TConstRvalueRef (get_st()): "; TConstRvalueRef crvf5(get_st()); cout << endl << "TValueMove (get_st()): "; TValueMove vm5(get_st()); cout << endl << "TConstValueMove (get_st()): "; TConstValueMove cvm5(get_st()); cout << endl << "TPerfectForward (get_st()): "; TPerfectForward pf5(get_st()); //cout << endl << "TConstPerfectForward (get_st()): "; //TConstPerfectForward cpf5(get_st()); cout << endl << "TPerfectForwardStrict (get_st()): "; TPerfectForwardStrict pft5(get_st()); cout << endl << "TMultiVariant (get_st()): "; TMultiVariant mv5(get_st()); cout << endl << "TMultiVariantStrict (get_st()): "; TMultiVariantStrict mvt5(get_st()); // Test constructing with literal // Will not compile //cout << endl << "TRef (5): "; //TRef rf2(5); cout << endl << "TConstRef (5): "; TConstRef crf2(5); cout << endl << "TConstRefAndRvalueRef (5): "; TConstRefAndRvalueRef crf_rvf2(5); cout << endl << "TConstRvalueRef (5): "; TConstRvalueRef crvf2(5); cout << endl << "TValueMove (5): "; TValueMove vm2(5); cout << endl << "TConstValueMove (5): "; TConstValueMove cvm2(5); cout << endl << "TPerfectForward (5): "; TPerfectForward pf2(5); // Will not compile //cout << endl << "TConstPerfectForward (5): "; //TConstPerfectForward cpf2(5); // Will not compile due to SFINAE test //cout << endl << "TPerfectForwardStrict (5): "; //TPerfectForwardStrict pft2(5); cout << endl << "TMultiVariant (5): "; TMultiVariant mv2(5); cout << endl << "TMultiVariantStrict (5): "; TMultiVariantStrict mvt2(5); // Test setting with object st.a = 100; cout << endl << "TRef set(st): "; rf.set(st); detect_change(st.a, 100); cout << endl << "TConstRef set(st): "; crf.set(st); detect_change(st.a, 100); cout << endl << "TConstRefAndRvalueRef set(st): "; crf_rvf.set(st); detect_change(st.a, 100); // Will not compile // cout << endl << "TConstRvalueRef set(st): "; // crvf2.set(st); // detect_change(st.a, 100); cout << endl << "TValueMove set(st): "; vm.set(st); detect_change(st.a, 100); cout << endl << "TConstValueMove set(st): "; cvm.set(st); detect_change(st.a, 100); cout << endl << "TPerfectForward set(st): "; pf.set(st); detect_change(st.a, 100); cout << endl << "TPerfectForwardStrict set(st): "; pft.set(st); detect_change(st.a, 100); cout << endl << "TMultiVariant set(st): "; mv.set(st); detect_change(st.a, 100); cout << endl << "TMultiVariantStrict set(st): "; mvt.set(st); detect_change(st.a, 100); // Test setting with moved object // Will not compile //cout << endl << "TRef set(std::move(st)): "; //rf.set(std::move(st)); //detect_change(st.a, 100); cout << endl << "TConstRef set(std::move(st)): "; crf.set(std::move(st)); detect_change(st.a, 100); cout << endl << "TConstRefAndRvalueRef set(std::move(st)): "; crf_rvf.set(std::move(st)); detect_change(st.a, 100); cout << endl << "TConstRvalueRef set(std::move(st)): "; crvf2.set(std::move(st)); detect_change(st.a, 100); cout << endl << "TValueMove set(std::move(st)): "; vm.set(std::move(st)); detect_change(st.a, 100); cout << endl << "TConstValueMove set(std::move(st)): "; cvm.set(std::move(st)); detect_change(st.a, 100); cout << endl << "TPerfectForward set(std::move(st)): "; pf.set(std::move(st)); detect_change(st.a, 100); detect_change(st.a, 100); cout << endl << "TPerfectForwardStrict set(std::move(st)): "; pft.set(std::move(st)); detect_change(st.a, 100); cout << endl << "TMultiVariant set(std::move(st)): "; mv.set(std::move(st)); detect_change(st.a, 100); cout << endl << "TMultiVariantStrict set(std::move(st)): "; mvt.set(std::move(st)); detect_change(st.a, 100); // Test setting with constructed temporary object // Will not compile //cout << endl << "TRef set(TStruct(8)): "; //rf.set(TStruct(8)); cout << endl << "TConstRef set(TStruct(8)): "; crf.set(TStruct(8)); cout << endl << "TConstRefAndRvalueRef set(TStruct(8)): "; crf_rvf.set(TStruct(8)); cout << endl << "TConstRvalueRef set(TStruct(8)): "; crvf2.set(TStruct(8)); cout << endl << "TValueMove set(TStruct(8)): "; vm.set(TStruct(8)); cout << endl << "TConstValueMove set(TStruct(8)): "; cvm.set(TStruct(8)); cout << endl << "TPerfectForward set(TStruct(8)): "; pf.set(TStruct(8)); cout << endl << "TPerfectForwardStrict set(TStruct(8)): "; pft.set(TStruct(8)); cout << endl << "TMultiVariant set(TStruct(8)): "; mv.set(TStruct(8)); cout << endl << "TMultiVariantStrict set(TStruct(8)): "; mvt.set(TStruct(8)); // Test setting with RVO object // Will not compile //cout << endl << "TRef set(get_st()): "; //rf.set(get_st()); cout << endl << "TConstRef set(get_st()): "; crf.set(get_st()); cout << endl << "TConstRefAndRvalueRef set(get_st()): "; crf_rvf.set(get_st()); cout << endl << "TConstRvalueRef set(get_st()): "; crvf2.set(get_st()); cout << endl << "TValueMove set(get_st()): "; vm.set(get_st()); cout << endl << "TConstValueMove set(get_st()): "; cvm.set(get_st()); cout << endl << "TPerfectForward set(get_st()): "; pf.set(get_st()); cout << endl << "TPerfectForwardStrict set(get_st()): "; pft.set(get_st()); cout << endl << "TMultiVariant set(get_st()): "; mv.set(get_st()); cout << endl << "TMultiVariantStrict set(get_st()): "; mvt.set(get_st()); cout << endl << "st = 3: "; st = 3; // Test setting with literal // Will not compile //cout << endl << "TRef set(4): "; //rf.set(4); cout << endl << "TConstRef set(4): "; crf.set(4); cout << endl << "TConstRefAndRvalueRef set(4): "; crf_rvf.set(4); cout << endl << "TConstRvalueRef set(4): "; crvf2.set(4); cout << endl << "TValueMove set(4): "; vm.set(4); cout << endl << "TConstValueMove set(4): "; cvm.set(4); cout << endl << "TPerfectForward set(4): "; pf.set(4); // Will not compile due to SFINAE test //cout << endl << "TPerfectForwardStrict set(st): "; //pft.set(4); cout << endl << "TMultiVariant set(4): "; mv.set(4); cout << endl << "TMultiVariantStrict set(4): "; mvt.set(4); cout << endl; } int main() { test_passing(); return 0; } /* Output: TStruct st(100): Vc TRef (st): Cc- TConstRef (st): Cc- TConstRefAndRvalueRef (st): CcLr- TValueMove (st): CcMcD- TConstValueMove (st): CcCcD- TPerfectForward (st): Cc- TPerfectForwardStrict (st): Cc- TMultiVariant (st): CcPf- TMultiVariantStrict (st): CcPf- TConstRef (std::move(st)): Cc- TConstRefAndRvalueRef (std::move(st)): McRr! TConstRvalueRef (std::move(st)): Cc- TValueMove (std::move(st)): McMcD! TConstValueMove (std::move(st)): McCcD! TPerfectForward (std::move(st)): Mc! TPerfectForwardStrict (std::move(st)): Mc! TMultiVariant (std::move(st)): McRr! TMultiVariantStrict (std::move(st)): McRr! TConstRef (TStruct(6)): VcCcD TConstRefAndRvalueRef (TStruct(6)): VcMcRrD TConstRvalueRef (TStruct(6)): VcCcD TValueMove (TStruct(6)): VcMcD TConstValueMove (TStruct(6)): VcCcD TPerfectForward (TStruct(6)): VcMcD TPerfectForwardStrict (TStruct(6)): VcMcD TMultiVariant (TStruct(6)): VcMcRrD TMultiVariantStrict (TStruct(6)): VcMcRrD TConstRef (get_st()): VcCcD TConstRefAndRvalueRef (get_st()): VcMcRrD TConstRvalueRef (get_st()): VcCcD TValueMove (get_st()): VcMcD TConstValueMove (get_st()): VcCcD TPerfectForward (get_st()): VcMcD TPerfectForwardStrict (get_st()): VcMcD TMultiVariant (get_st()): VcMcRrD TMultiVariantStrict (get_st()): VcMcRrD TConstRef (5): VcCcD TConstRefAndRvalueRef (5): VcMcRrD TConstRvalueRef (5): VcCcD TValueMove (5): VcMcD TConstValueMove (5): VcCcD TPerfectForward (5): Vc TMultiVariant (5): VcPf TMultiVariantStrict (5): VcMcRrD TRef set(st): Ca- TConstRef set(st): Ca- TConstRefAndRvalueRef set(st): SlCa- TValueMove set(st): CcMaD- TConstValueMove set(st): CcCaD- TPerfectForward set(st): Ca- TPerfectForwardStrict set(st): Ca- TMultiVariant set(st): SpCa- TMultiVariantStrict set(st): SpCa- TConstRef set(std::move(st)): Ca- TConstRefAndRvalueRef set(std::move(st)): SrMa! TConstRvalueRef set(std::move(st)): Ca- TValueMove set(std::move(st)): McMaD! TConstValueMove set(std::move(st)): McCaD! TPerfectForward set(std::move(st)): Ma!- TPerfectForwardStrict set(std::move(st)): Ma! TMultiVariant set(std::move(st)): SrMa! TMultiVariantStrict set(std::move(st)): SrMa! TConstRef set(TStruct(8)): VcCaD TConstRefAndRvalueRef set(TStruct(8)): VcSrMaD TConstRvalueRef set(TStruct(8)): VcCaD TValueMove set(TStruct(8)): VcMaD TConstValueMove set(TStruct(8)): VcCaD TPerfectForward set(TStruct(8)): VcMaD TPerfectForwardStrict set(TStruct(8)): VcMaD TMultiVariant set(TStruct(8)): VcSrMaD TMultiVariantStrict set(TStruct(8)): VcSrMaD TConstRef set(get_st()): VcCaD TConstRefAndRvalueRef set(get_st()): VcSrMaD TConstRvalueRef set(get_st()): VcCaD TValueMove set(get_st()): VcMaD TConstValueMove set(get_st()): VcCaD TPerfectForward set(get_st()): VcMaD TPerfectForwardStrict set(get_st()): VcMaD TMultiVariant set(get_st()): VcSrMaD TMultiVariantStrict set(get_st()): VcSrMaD st = 3: Va TConstRef set(4): VcCaD TConstRefAndRvalueRef set(4): VcSrMaD TConstRvalueRef set(4): VcCaD TValueMove set(4): VcMaD TConstValueMove set(4): VcCaD TPerfectForward set(4): Va TMultiVariant set(4): SpVa TMultiVariantStrict set(4): VcSrMaD DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD */

Links:
CppCon 2014: Herb Sutter "Back to the Basics! Essentials of Modern C++ Style"
PDF presentation
Meaning of acronym SSO in the context of std::string
Why is value taking setter member functions not recommended in Herb Sutter's CppCon 2014 talk (Back to Basics: Modern C++ Style)?
What's the correct enable_if constraint on perfect forwarding setter?
Beware of Perfect Forwarding Constructors


Why other variants of passing to constructor or setter are less used


  • T& — cannot accept rvalue. Is the same as "const T&" variant, but less safe.
     class T { C(T& X) : x(X) {} void set(T& X) { x = X; } T x; };
  • const T&& move — does not allow move due to prohibiting change of passed object. Not used.
     class C { C(const T&& X) : x(std::move(X)) {} void set(const T&& X) { x = std::move(X); } T x; };
  • const T move — does not allow move due to prohibiting change of passed object.
     class C { C(const TX) : x(std::move(X)) {} void set(const TX) { x = std::move(X); } T x; };
  • const Perfect forwarding (PF) — will not compile
     class C { template<typename Z> C(const Z&& X) : x(std::forward<Z>(X)) {} template<typename Z> void set(const Z&& X) { x = std::forward<Z>(X); } T x; };



Working with smart pointers


  • Do not pass shared_ptr (or other countref alternatives) by value, if you do not need to count references (reduces performance due to atomic operations with counter) — prefer passing objects by * or & as usual.
  • Do not pass shared_ptr (or other countref alternatives) by reference or const reference, if you do not need to count references — prefer passing objects by * or & as usual.
  • If fabric has to return a polymorphic type (and thus cannot return by value), return unique_ptr, which can be converted to shared_ptr if needed (or return shared_ptr if you are sure that it will always be needed intead of unique_ptr)
  • If a function will need to decide if it wants to copy a shared_ptr or not — you can pass a const shared_ptr& to it
  • Never dereference or call a method of non-local shared_ptr, because this puts object outside of shared_ptr control (instead, first make a local copy of shared_ptr). Example:



Links:


CppCon 2014: Herb Sutter "Back to the Basics! Essentials of Modern C++ Style"
PDF presentation
What is an 'aliased local shared_ptr' in this example?


Some of the used images were taken from the linked articles



Go to Part 1

Source: https://habr.com/ru/post/zh-CN479358/

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