[英]Member function template with universal reference won't accept lvalues
I've been trying to use a template member function to set a value inside of my class. 我一直在尝试使用模板成员函数在我的类中设置一个值。 I wanted to use a universal reference so that I could accept any variant of the correct type (eg
T
, T&
, T&&
, const T
, const T&
, const T&&
) 我想使用通用引用,以便我可以接受任何正确类型的变体(例如
T
, T&
, T&&
, const T
, const T&
, const T&&
)
However, it seems that my member function will only accept rvalues, unlike a free function accepting a universal reference. 但是,似乎我的成员函数只接受rvalues,而不像接受通用引用的自由函数。
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;
}
In the code given, Foo
is instantiated as Foo<int>
. 在给出的代码中,
Foo
被实例化为Foo<int>
。 Once this happens, the class template is instantiated as shown: 一旦发生这种情况,就会实例化类模板,如下所示:
class Foo{
public:
void memberURef(int&& t){ // Uh oh!
val = std::forward<int>(t);
}
private:
int val;
};
Notice how the member function is no longer a template, and therefore no longer accepts a universal reference, but an rvalue reference. 注意成员函数不再是模板,因此不再接受通用引用,而是接受rvalue引用。 In order to make a member function that accepts a universal reference, the
Foo
class would need to be modified as follows: 为了使成员函数接受通用引用,需要修改
Foo
类,如下所示:
template <typename T>
class Foo{
public:
template <typename L>
void memberURef(L&& t){
val = std::forward<L>(t);
}
private:
T val;
};
That way, memberURef
is still a function template after the class template has been instantiated, and it thus still accepts a universal reference. 这样,在实例化类模板之后,
memberURef
仍然是一个函数模板 ,因此它仍然接受通用引用。
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