类名未在C ++中命名类型 [英] Class name does not name a type in C++

问题描述

我刚刚开始用C ++编程，我试图创建2个类，其中一个包含另一个类。

I just started programming in C++, and I've tried to create 2 classes where one will contain the other.

文件 Ah ：

#ifndef _A_h
#define _A_h

class A{
    public:
        A(int id);
    private:
        int _id;
        B _b; // HERE I GET A COMPILATION ERROR: B does not name a type
};

#endif

文件 A.cpp ：

#include "A.h"
#include "B.h"
#include <cstdio>

A::A(int id): _id(id), _b(){
    printf("hello\n the id is: %d\n", _id);
}

文件 Bh ：

#ifndef _B_h
#define _B_h

class B{
    public:
        B();
};
#endif

文件 B.cpp ：

#include "B.h"
#include <cstdio>

B::B(){
    printf("this is hello from B\n");
}

我先编译B类，然后编译A类，但随后得到错误消息：

I first compile the B class and then the A class, but then I get the error message:

Ah：9：错误： B未命名类型

A.h:9: error: ‘B’ does not name a type

如何解决此问题？

推荐答案

预处理器插入内容 Ah 和 Bh 文件中的 include 语句的确切位置发生（这实际上只是复制/粘贴）。当编译器然后解析 A.cpp 时，它会在知道类<$ c之前找到类 A 的声明。 $ c> B 。这会导致您看到错误。有两种解决方法：

The preprocessor inserts the contents of the files A.h and B.h exactly where the include statement occurs (this is really just copy/paste). When the compiler then parses A.cpp, it finds the declaration of class A before it knows about class B. This causes the error you see. There are two ways to solve this:

1. 在<$ c $中包含 Bh c> Ah 。通常，将头文件包含在需要它们的文件中是一个好主意。如果您依赖于通过另一个标头间接包含或在编译单元（cpp文件）中包含特殊顺序，则只会在项目变大时使您和其他人感到困惑。



2. 如果在类 A 中使用类型为 B 的成员变量，则编译器需要知道确切且完整的信息声明 B ，因为它需要为 A 创建内存布局。另一方面，如果您使用的是指向 B 的指针或引用，则前向声明就足够了，因为编译器需要为指针或引用保留内存与类定义无关。看起来像这样：

1. Include B.h in A.h. It is generally a good idea to include header files in the files where they are needed. If you rely on indirect inclusion though another header, or a special order of includes in the compilation unit (cpp-file), this will only confuse you and others as the project gets bigger.

2. If you use member variable of type B in class A, the compiler needs to know the exact and complete declaration of B, because it needs to create the memory-layout for A. If, on the other hand, you were using a pointer or reference to B, then a forward declaration would suffice, because the memory the compiler needs to reserve for a pointer or reference is independent of the class definition. This would look like this:

class B; // forward declaration        
class A {
public:
    A(int id);
private:
    int _id;
    B & _b;
};

这对于避免标题之间的循环依赖非常有用。

This is very useful to avoid circular dependencies among headers.

我希望这会有所帮助。

这篇关于类名未在C ++中命名类型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！