c++ - 如何在C++中使用/访问类的内部 friend 功能？

Question

我们可以在类内声明并定义一个 friend 函数，如下所示

class C
{
    friend void F() {cout<<"inner friend func"<<endl;}
};

如何访问/使用此内部 friend 功能？

C::F() //error: 'F' is not a member of 'C'
C().F()//error: 'class C' has no member named 'F'

Answer 1

friend decalration声明F()为非成员函数，因此C::F()和C().F()均不起作用。而且，您必须在命名空间范围内添加一个声明以进行调用，因为在名称查找中该声明不可见(ADL除外，但F()不接受任何参数，因此ADL对此不起作用)。

A name first declared in a friend declaration within a class or class template X becomes a member of the innermost enclosing namespace of X, but is not visible for lookup (except argument-dependent lookup that considers X) unless a matching declaration at the namespace scope is provided

和

Names introduced by friend declarations within a non-local class X become members of the innermost enclosing namespace of X, but they do not become visible to ordinary name lookup (neither unqualified nor qualified) unless a matching declaration is provided at namespace scope, either before or after the class definition. Such name may be found through ADL which considers both namespaces and classes.

例如。

void F();
class C
{
    friend void F() {cout<<"inner friend func"<<endl;}
};

然后像这样称呼它

F();

LIVE