C++ 中相互包含的标头 [英] Headers Including Each Other in C++

问题描述

我是 C++ 新手，但我无法在网上找到这个(很可能是微不足道的)问题的答案.我在编译一些两个类相互包含的代码时遇到了一些麻烦.首先，我的#include 语句应该放在宏的内部还是外部?在实践中，这似乎并不重要.但是，在这种特殊情况下，我遇到了麻烦.将#include 语句放在宏之外会导致编译器递归并给我#include 嵌套太深"的错误.这对我来说似乎很有意义，因为在调用 #include 之前没有完全定义任何类.然而，奇怪的是，当我尝试将它们放入其中时，我无法声明其中一个类的类型，因为它无法识别.本质上，这就是我要编译的内容:

I'm a C++ newbie, but I wasn't able to find the answer to this (most likely trivial) question online. I am having some trouble compiling some code where two classes include each other. To begin, should my #include statements go inside or outside of my macros? In practice, this hasn't seemed to matter. However, in this particular case, I am having trouble. Putting the #include statements outside of the macros causes the compiler to recurse and gives me "#include nested too deeply" errors. This seems to makes sense to me since neither class has been fully defined before #include has been invoked. However, strangely, when I try to put them inside, I am unable to declare a type of one of the classes, for it is not recognized. Here is, in essence, what I'm trying to compile:

啊.h

#ifndef A_H_
#define A_H_

#include "B.h"

class A
{
    private:
        B b;

    public:
        A() : b(*this) {}
};

#endif /*A_H_*/

B.h

#ifndef B_H_
#define B_H_

#include "A.h"

class B
{
    private:
            A& a;

    public:
        B(A& a) : a(a) {}
 };

#endif /*B_H_*/

main.cpp

#include "A.h"

int main()
{
    A a;
}

如果有什么不同，我使用的是 g++ 4.3.2.

If it makes a difference, I am using g++ 4.3.2.

为了清楚起见，一般来说，#include 语句应该放在哪里?我一直看到它们超出了宏的范围，但我清楚地描述的场景似乎打破了这个原则.提前感谢任何帮助者！如果我犯了任何愚蠢的错误，请允许我澄清我的意图！

And just to be clear, in general, where should #include statements go? I have always seen them go outside of the macros, but the scenario I described clearly seems to break this principle. Thanks to any helpers in advance! Please allow me to clarify my intent if I have made any silly mistakes!

推荐答案

我认为宏"是指#ifndef 包含警卫?如果是这样，#includes 肯定应该进去.这是包含守卫存在的主要原因之一，因为否则您很容易像您注意到的那样以无限递归结束.

By "the macros" I assume you mean the #ifndef include guards? If so, #includes should definitely go inside. This is one of the major reasons why include guards exists, because otherwise you easily end up with an infinite recursion as you noticed.

无论如何，问题是在您使用 A 和 B 类(在另一个类中)时，它们还没有被声明.看看#includes 被处理后的代码是什么样子的:

Anyway, the problem is that at the time you use the A and B classes (inside the other class), they have not yet been declared. Look at what the code looks like after the #includes have been processed:

//#include "A.h" start
#ifndef A_H_
#define A_H_

//#include "B.h" start
#ifndef B_H_
#define B_H_

//#include "A.h" start
#ifndef A_H_ // A_H_ is already defined, so the contents of the file are skipped at this point
#endif /*A_H_*/

//#include "A.h" end

class B
{
    private:
            A& a;

    public:
            B(A& a) : a(a) {}
 };

#endif /*B_H_*/

//#include "B.h" end

class A
{
    private:
            B b;

    public:
            A() : b(*this) {}
};

#endif /*A_H_*/
//#include "A.h" end

int main()
{
    A a;
}

现在阅读代码.B 是编译器遇到的第一个类，它包含一个 A& 成员.A 是什么?编译器还没有遇到任何A的定义，所以它会发出错误.

Now read the code. B is the first class the compiler encounters, and it includes an A& member. What is A? The compiler hasn't encountered any definition of A yet, so it issues an error.

解决方法是对A进行前向声明.在B的定义之前的某个时刻，添加一行class A;

The solution is to make a forward declaration of A. At some point before the definition of B, add a line class A;

这为编译器提供了必要的信息，即 A 是一个类.我们对此一无所知，但由于 B 只需要包含对它的引用，这就足够了.在 A 的定义中，我们需要 B 类型的成员(不是引用)，所以这里 B 的整个定义必须是可见的.幸运的是，它是.

This gives the compiler the necessary information, that A is a class. We don't know anything else about it yet, but since B only needs to include a reference to it, this is good enough. In the definition of A, we need a member of type B (not a reference), so here the entire definition of B has to be visible. Which it is, luckily.

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