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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