問題描述

我是 C++ 新手，但我無法在網上找到這個(很可能是微不足道的)問題的答案.我在編譯一些包含兩個類的代碼時遇到了一些麻煩.首先，我的#include 語句應該放在宏的內部還是外部?在實踐中，這似乎無關緊要.但是，在這種特殊情況下，我遇到了麻煩.將#include 語句放在宏之外會導致編譯器遞歸并給我#include 嵌套太深"錯誤.這對我來說似乎很有意義，因為在調用 #include 之前這兩個類都沒有被完全定義.然而，奇怪的是，當我嘗試將它們放入其中時，我無法聲明其中一個類的類型，因為它不被識別.本質上，這就是我要編譯的內容:

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

#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;
}

如果有區別，我使用的是 g++ 4.3.2.

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

為了清楚起見，一般來說，#include 語句應該放在哪里?我一直看到它們在宏之外，但我清楚地描述的場景似乎打破了這個原則.提前感謝任何幫助者！如果我犯了任何愚蠢的錯誤，請允許我澄清我的意圖！

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!

推薦答案

我認為宏"是指 #ifndef 包含守衛?如果是這樣，#includes 絕對應該進入.這是包含守衛存在的主要原因之一，否則您很容易像您注意到的那樣以無限遞歸結束.

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.

無論如何，問題是當你使用A和B類(在另一個類中)時，它們還沒有被聲明.看看處理完#includes 之后的代碼是什么樣子的:

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;
}

現在閱讀代碼.B 是編譯器遇到的第一個類，它包含一個 A& 成員.什么是A?編譯器還沒有遇到 A 的任何定義，所以它會發出錯誤.

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.

解決辦法是對A做一個前向聲明，在B的定義之前的某個點，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;

這為編譯器提供了必要的信息，即 A 是一個類.我們對此一無所知，但由于 B 只需要包含對它的引用，這就足夠了.在 A 的定義中，我們需要一個 B 類型的成員(不是引用)，所以這里 B 的整個定義必須是可見的.幸運的是.

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.

這篇關于C++ 中相互包含的頭文件的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！