在类中定义friend

问题描述：

我可以把friend函数/类的定义放在另一个类中吗？我的意思是这样：

Can i put definition of friend function / class inside another class? I mean something like this:

class Foo
{
    friend void foo() {} // 1
    friend class Bar {}; // 2
};

gcc编译好友函数，但无法编译好友类。

gcc compiles friend function, but can't compile friend class.

答

您可以在朋友声明中定义朋友具有无法以任何其他方式获得的有趣的行为（在封装类型是模板的情况下）。

You can define a friend function in the friend declaration, and it has interesting behavior that cannot be obtained any other way (in the case of the enclosing type being a template).

您不能在朋友声明中定义朋友类，需要。如果您要创建具有完全访问权限的新类型 inline ，则只需创建嵌套类型即可。作为一个成员，它将完全访问封闭类型。唯一的区别是，类型不会在命名空间级别找到，但是如果需要可以添加typedef（或者，在命名空间级别定义类，并只声明类中的友谊）。

You cannot define a friend class in the friend declaration, and there is no need for that. If you want to create a new type inline with full access, you can just create a nested type. Being a member it will have full access to the enclosing type. The only difference is that the type will not be found at namespace level, but you can add a typedef if needed (or alternatively, define the class at namespace level and just declare the friendship inside the class).

class Outer {
   int x;
   class Inner {
      static void f( Outer& o ) { o.x = 5; } // fine
   };
};

在类中定义friend

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