问题描述
我一直在尝试使用模板成员函数在我的类中设置一个值。我想使用通用引用，以便我可以接受正确类型的任何变体（例如 T ， T& ， T&&& ， const T ， const T& ， const T&&& ）
I've been trying to use a template member function to set a value inside of my class. I wanted to use a universal reference so that I could accept any variant of the correct type (e.g. T, T&, T&&, const T, const T&, const T&&)
然而，似乎我的成员函数只接受rvalues一个自由函数接受一个通用引用。
However, it seems that my member function will only accept rvalues, unlike a free function accepting a universal reference.
template <typename T>
class Foo{
public:
void memberURef(T&& t){
val = std::forward<T>(t);
}
private:
T val;
};
template <typename T>
void freeURef(T&& t){
}
int main() {
int lval = 1;
const int clval = 1;
freeURef(lval); // fine
freeURef(clval); // fine
Foo<int> foo;
foo.memberURef(2);
foo.memberURef(lval); //error: cannot bind 'int' lvalue to 'int&&'
foo.memberURef(clval); //error: no matching function for call to 'Foo<int>::memberURef(const int&)'
return 0;
}
推荐答案
code> Foo 被实例化为 Foo 。一旦发生这种情况，类模板将被实例化如下：
In the code given, Foo is instantiated as Foo<int>. Once this happens, the class template is instantiated as shown:
class Foo{
public:
void memberURef(int&& t){ // Uh oh!
val = std::forward<int>(t);
}
private:
int val;
};
请注意成员函数不再是模板，因此不再接受通用引用，右值引用。为了使成员函数接受一个通用引用， Foo 类需要修改如下：
Notice how the member function is no longer a template, and therefore no longer accepts a universal reference, but an rvalue reference. In order to make a member function that accepts a universal reference, the Foo class would need to be modified as follows:
template <typename T>
class Foo{
public:
template <typename L>
void memberURef(L&& t){
val = std::forward<L>(t);
}
private:
T val;
};
这样， memberURef em>函数模板，因此它仍然接受一个通用引用。
That way, memberURef is still a function template after the class template has been instantiated, and it thus still accepts a universal reference.
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