从C ++入门班进入循环依赖 [英] Classes from C++ Primer going into Cyclic Dependency

问题描述

我下面这本书 - C ++入门学习C ++。我在章介绍类中间，我被困在解决头文件包括作为例子中有两类。

I am following the book - C++ Primer for learning C++. I'm in middle of the chapter introducing classes, and I'm stuck in resolving the header files include of two classes taken as example there.

下面是两个类和头文件：

Here are the two classes, and the header files:

#ifndef SCREENCLS_H
#define SCREENCLS_H

#include <iostream>

#include "WindowManager.h"

using namespace std;

class ScreenCls {
    friend void WindowManager::clear(ScreenIndex);

public:
    typedef string::size_type pos;
    ScreenCls() { }
    ScreenCls(pos height, pos width, char c): height(height), width(width), contents(height * width, c) { }

    ScreenCls &set(char);
    ScreenCls &set(pos, pos, char);

    char get() const { return contents[cursor]; }  // Implicitly inline

private:
    pos cursor;
    pos height;
    pos width;
    string contents;
};

#endif // SCREENCLS_H

ScreenCls.cpp

#include "ScreenCls.h"

char ScreenCls::get(pos r, pos c) const {
    pos row = r * width;
    return contents[row + c];
}

ScreenCls &ScreenCls::set(char ch) {
    contents[cursor] = ch;
    return *this;
}

ScreenCls &ScreenCls::set(pos r, pos c, char ch) {
    contents[r * width + c] = ch;
    return *this;
}

WindowManager.h

#ifndef WINDOWMANAGER_H
#define WINDOWMANAGER_H

#include <iostream>
#include <vector>
#include "ScreenCls.h"

using namespace std;

class WindowManager {

public:
    // location ID for each screen on window
    using ScreenIndex = vector<ScreenCls>::size_type;
    // reset the Screen at the given position to all blanks
    void clear(ScreenIndex);

private:
    vector<ScreenCls> screens{ Screen(24, 80, ' ') };
};

#endif // WINDOWMANAGER_H

WindowManager.cpp

#include "WindowManager.h"
#include "ScreenCls.h"

void WindowManager::clear(ScreenIndex index) {
    ScreenCls &s = screens[i];
    s.contents = string(s.height * s.width, ' ');
}

这是我的项目结构：

/src/ScreenCls.cpp 
/src/WindowManager.cpp 
/include/ScreenCls.h 
/include/WindowManager.h

我使用的 code :: Blocks的的IDE。我已经添加了 / src目录和 /包括在编译器文件夹中的搜索目录的设置。我还添加项目根目录到搜索目录。

I'm using Code::Blocks IDE. I've added /src and /include folder in the Search Directory in the compiler Settings. I've also added the project root directory to the search directory.

现在，当我试图构建项目，它显示了以下错误：

Now, when I'm trying to build the project, it shows following errors:

'ScreenCls' was not declared in this scope (WindowManager.h)  
'ScreenIndex' has not been declared (WindowManager.h)  
'ScreenIndex' has not been declared (ScreenCls.h)  

和我不知道这里发生了什么。我通过一些资源在网上搜索，发现的此链接。而且它没有帮助在这里。有人可能需要一些时间来看看，并提出​​一个解决方案？

And I've no idea what's happening here. I searched through some resources online, and found this link. And it's not helping here. Can someone take some time to look and suggest a solution?

推荐答案

这很简单：

您的程序＃包括ScreenCls.h又包括WindowManager.h。但ScreenCls.h中的WindowManager.h的＃包括什么也不做，因为它已经被列入到现在WindowManager.h不知道ScreenCls是什么。

Your program #includes "ScreenCls.h" which in turn includes "WindowManager.h". But the #include of "ScreenCls.h" in "WindowManager.h" does nothing because it has already been included and now WindowManager.h does not know what ScreenCls is.

您需要一个前锋，而不是申报，这意味着你要么声明尽可能多类为必要或使用指针。

You need a forward declaration instead which means you either declare as much of your class as necessary or use pointers.

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