让内部类成为 C++ 中的朋友

发布于 2024-11-15 10:56:16 字数 292 浏览 4 评论 0原文

我想让一个内部类成为一个不相关类的友元，但这似乎不起作用（至少在 gcc 4.1.2 中）：

class A {
    int i;
    friend class B; // fine
    friend class B::C; // not allowed?
};

class B {
    int geti(A* ap) { return ap->i; }

    class C {
        int geti(A* ap) { return ap->i; }
    };
};

原文

I want to make an inner class a friend of an unrelated class but this doesn't seem to work (at least in gcc 4.1.2):

class A {
    int i;
    friend class B; // fine
    friend class B::C; // not allowed?
};

class B {
    int geti(A* ap) { return ap->i; }

    class C {
        int geti(A* ap) { return ap->i; }
    };
};

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萝莉病 2024-11-22 10:56:16

在使用它之前，您必须声明B::C。以下可能有效。

更新：忽略所要求的可用演示，这里有一种可以工作的构造方法（减去成员函数的定义），但请记住，一切都是私有的。

class A;

class B
{
  int geti(A * ap);

public:
  class C
  {
    int geti(A * ap);
  };
};

class A
{
  friend class B;    // fine
  friend class B::C; // fine too
  int i;
};

然后在别处定义 getter 函数：

int B::geti(A * ap) { ... }
int B::C::geti(A * ap) { ... }

替代方案：前向声明嵌套类 B::C 并保存一个外部定义：

class A;

class B
{
  int geti(const A * ap) const; // we cannot use A yet!

public:
  class C;
};

class A
{
  friend class B;    // fine
  friend class B::C; // fine too
  int i;
};

int B::geti(const A * ap) const { return ap->i; }

class B::C
{
  inline int geti(const A * ap) const { return ap->i; }
};

You have to declare B::C before using it. The following might work.

Update: Ignoring a usable demonstration as requested, here's a way of structuring this (minus the definitions of member functions) that could work, but bear in mind that everything is private as it stands.

class A;

class B
{
  int geti(A * ap);

public:
  class C
  {
    int geti(A * ap);
  };
};

class A
{
  friend class B;    // fine
  friend class B::C; // fine too
  int i;
};

Then define the getter functions elsewhere:

int B::geti(A * ap) { ... }
int B::C::geti(A * ap) { ... }

Alternative: forward-declare the nested class B::C and save one external definition:

class A;

class B
{
  int geti(const A * ap) const; // we cannot use A yet!

public:
  class C;
};

class A
{
  friend class B;    // fine
  friend class B::C; // fine too
  int i;
};

int B::geti(const A * ap) const { return ap->i; }

class B::C
{
  inline int geti(const A * ap) const { return ap->i; }
};

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