如何用C ++中的友人声明解决循环依赖关系? [英] How to resolve circular dependency with friend declarations in C++?

问题描述

为什么以下代码无法编译，我该如何解决?我得到的错误是:

Why doesn't the following code compile and how can I fix it? The error I get is:

使用未声明的标识符"Foo"

Use of undeclared identifier 'Foo'

尽管在错误发生的位置(在 Bar 中的 friend 声明中)明确声明并定义了 Foo .

although Foo is clearly declared and defined at the point where the error occurs (at the friend declaration in Bar).

foo.h :

#ifndef FOO_H
#define FOO_H

#include "bar.h" // needed for friend declaration in FooChild

class Foo {
public:
  void Func(const Bar&) const;
};

class FooChild : public Foo {
  friend void Bar::Func(FooChild*);
};

#endif

foo.cpp :

#include "foo.h"

void Foo::Func(const Bar& B) const {
  // do stuff with B.X
}

bar.h :

#ifndef BAR_H
#define BAR_H

#include "foo.h"

class Bar {
public:
  void Func(FooChild*) {}
private:
  int X;
  friend void Foo::Func(const Bar&) const; // "use of undeclared identifier 'Foo'"
};

#endif

此处是上述代码的可编译在线版本.

Here's a compilable online version of the above code.

推荐答案

将 FooChild 放入其自己的头文件中.

Put FooChild in a header file of its own.

foo.h:

#ifndef FOO_H
#define FOO_H

class Bar;

class Foo {
public:
  void Func(const Bar&) const;
};

#endif

foochild.h:

foochild.h:

#ifndef FOOCHILD_H
#define FOOCHILD_H

#include "foo.h"
#include "bar.h"

class FooChild : public Foo {
  friend void Bar::Func(FooChild*);
};

#endif

这篇关于如何用C ++中的友人声明解决循环依赖关系?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！