模板类型的推断
下面的函数f是个模板函数，typename t。下表是，根据调用测的实参，推断出来的t的类型。
请注意下表的红字部分， f(t&& t)看起来是右值引用，但其实它会根据实参的类型，来决定t的类型，如果实参是左值，则它是左值，如果实参是右值，则它是右值。
所以可以看出来，t&可以变成const& ，f(t&& t)也可以变成const&。
| f(t t) | f(const t t) | f(t& t) | f(const t& t) | f(t&& t) | f(const t&& t) | |
|---|---|---|---|---|---|---|
| f(1) | f<int> | f<int> | error cannot bind non-const lvalue reference of type ‘int&’ to an rvalue of type ‘int’ | f<int> | f<int> | f<int> |
| int i = 1; f(i); |
f<int> | f<int> | f<int> | f<int> | f<int&> | error cannot bind rvalue reference of type ‘const int&&’ to lvalue of type ‘int’ |
| int& ri = i; f(ri); |
f<int> | f<int> | f<int> | f<int> | f<int&> | cannot bind rvalue reference of type ‘const int&&’ to lvalue of type ‘int’ |
| const int ci = 2; f(ci); |
f<int> | f<int> | f<const int> | f<int> | f<const int&> | cannot bind rvalue reference of type ‘const int&&’ to lvalue of type ‘const int’ |
| const int& rci = ci; f(rci); |
f<int> | f<int> | f<const int> | f<int> | f<const int&> | cannot bind rvalue reference of type ‘const int&&’ to lvalue of type ‘const int’ |
实验代码
#include <iostream> template<typename t> void f1(t t){ t = 99; } template<typename t> void f4(const t t){ //t = 99; } template<typename t> void f2(t& t){ //t = 99; } template<typename t> void f3(const t& t){} template<typename t> void f5(t&& t){} template<typename t> void f6(const t&& t){} int main(){ /* //void f1(t t) f1(1);//int int i = 1; f1(i);//int int& ri = i; f1(ri);//int std::cout << ri << std::endl; const int ci = 2; f1(ci);//int const int& rci = ci; f1(rci);//int */ /* //void f4(const t t) f4(1);//int int i = 1; f4(i);//int int& ri = i; f4(ri);//int std::cout << ri << std::endl; const int ci = 2; f4(ci);//int const int& rci = ci; f4(rci);//int */ /* //void f2(t& t) //f2(1);//error cannot bind non-const lvalue reference of type ‘int&’ to an rvalue of type ‘int’ int i = 1; f2(i);//int int& ri = i; f2(ri);//int const int ci = 2; f2(ci);//const int const int& rci = ci; f2(rci);//const int */ /* //void f3(const t& t) f3(1);//int int i = 1; f3(i);//int int& ri = i; f3(ri);//int const int ci = 2; f3(ci);//int const int& rci = ci; f3(rci);//int */ /* //void f5(t&& t) f5(1);//int int i = 1; f5(i);//int& int& ri = i; f5(ri);//int& std::cout << ri << std::endl; const int ci = 2; f5(ci);//const int& const int& rci = ci; f5(rci);//const int& */ /* //void f6(const t&& t) f6(1);//int int i = 1; //f6(i);//error cannot bind rvalue reference of type ‘const int&&’ to lvalue of type ‘int’ int& ri = i; //f6(ri);//error cannot bind rvalue reference of type ‘const int&&’ to lvalue of type ‘int’ std::cout << ri << std::endl; const int ci = 2; //f6(ci);//error cannot bind rvalue reference of type ‘const int&&’ to lvalue of type ‘const int’ const int& rci = ci; //f6(rci);//error cannot bind rvalue reference of type ‘const int&&’ to lvalue of type ‘const int’ */ }
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