ex7_32.h为啥写到一个文件中？写到两个文件中不是更合理吗？我写到两个文件中但是一直出错！！

Screen.h <div class="highlight highlight-source-c++ notranslate position-relative

编译： clang++ Window_mgr.h Screen.h -std=c++11 <p dir="au

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ex7_32.h中能不能把Window_mgr和Screen类拆分到两个文件中，clear是Window_mgr的成员，是Screen的友元？ about cpp-primer HOT 5 CLOSED

mooophy commented on May 23, 2024

ex7_32.h中能不能把Window_mgr和Screen类拆分到两个文件中，clear是Window_mgr的成员，是Screen的友元？

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Comments (5)

pezy commented on May 23, 2024

写到一个文件中，是因为两个类总放到一起讲，关系比较密切，且都非常短小，也为了更精简吧。

但你如果要分别写到两个头文件中，当然也是可以的。如果出错，能否提供错误提示呢？

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qinf commented on May 23, 2024

Screen.h

#ifndef _screen_h
#define _screen_h

#include <string>
#include <iostream>

class Screen {
public:
    friend void Window_mgr::clear(ScreenIndex);
    //friend class Window_mgr;
    using pos = std::string::size_type;
    Screen() = default;
    Screen(pos ht, pos wd): height(ht), width(wd), contents(ht*wd, ' ') { }
    Screen(pos ht, pos wd, char c): height(ht), width(wd), contents(ht*wd, c) { }
    char get() const {
        return contents[cursor];
    }
    inline char get(pos ht, pos wd) const;
    Screen &move(pos r, pos c);
    Screen &set(char);
    Screen &set(pos, pos, char);
    void some_member() const;
    Screen &display(std::ostream &os) {
        do_display(os);
        return *this;
    }
    const Screen &display(std::ostream &os) const {
        do_display(os);
        return *this;
    }
private:
    mutable size_t access_ctr;
    pos cursor = 0;
    pos height = 0, width = 0;
    std::string contents;
    void do_display(std::ostream &os) const {
        os << contents;
    }
};

inline Screen &Screen::set(char c) {
    contents[cursor] = c;
    return *this;
}
inline Screen &Screen::set(pos row, pos col, char ch) {
    contents[row*width+col] = ch;
    return *this;
}
#endif

Window_mgr.h

#ifndef _Window_mgr_h
#define _Window_mgr_h
#include <string>
#include <vector>
#include "Screen.h"
class Window_mgr {
public:
    using ScreenIndex = std::vector<Screen>::size_type;
    void clear(ScreenIndex);
private:
    std::vector<Screen> screens{Screen(24, 80, ' ')};
};
inline void Window_mgr::clear(ScreenIndex index) {
    Screen &s = screens[index];
    s.contents = std::string(s.height * s.width, ' ');
}

#endif

编译： clang++ Window_mgr.h Screen.h -std=c++11
错误信息：

clang: warning: treating 'c-header' input as 'c++-header' when in C++ mode, this behavior is deprecated
clang: warning: treating 'c-header' input as 'c++-header' when in C++ mode, this behavior is deprecated
In file included from Window_mgr.h:5:
./Screen.h:9:17: error: use of undeclared identifier 'Window_mgr'
    friend void Window_mgr::clear(ScreenIndex);
                ^
./Screen.h:9:35: error: unknown type name 'ScreenIndex'
    friend void Window_mgr::clear(ScreenIndex);
                                  ^
Window_mgr.h:15:7: error: 'contents' is a private member of 'Screen'
    s.contents = std::string(s.height * s.width, ' ');
      ^
./Screen.h:35:17: note: declared private here
    std::string contents;
                ^
Window_mgr.h:15:32: error: 'height' is a private member of 'Screen'
    s.contents = std::string(s.height * s.width, ' ');
                               ^
./Screen.h:34:9: note: declared private here
    pos height = 0, width = 0;
        ^
Window_mgr.h:15:43: error: 'width' is a private member of 'Screen'
    s.contents = std::string(s.height * s.width, ' ');
                                          ^
./Screen.h:34:21: note: declared private here
    pos height = 0, width = 0;
                    ^
5 errors generated.
Screen.h:9:17: error: use of undeclared identifier 'Window_mgr'
    friend void Window_mgr::clear(ScreenIndex);
                ^
Screen.h:9:35: error: unknown type name 'ScreenIndex'
    friend void Window_mgr::clear(ScreenIndex);
                                  ^
2 errors generated.

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pezy commented on May 23, 2024

编译： clang++ Window_mgr.h Screen.h -std=c++11

抛开错误，我们先看看编译器的编译顺序，按你代码的写法，编译器先找到Window_mgr.h，里面发现有这么一句:

#include "Screen.h"

于是二话不说，就去找Screen.h，这才正式进入 Screen 类，赫然发现有一个友元函数叫这个：

Window_mgr::clear(ScreenIndex);

里面有两个东西没见过，不认识：Window_mgr 和 ScreenIndex。

注意咱们的顺序，编译器在进入 Window_mgr 类之前，先进入的是 Screen 类，所以在 Screen 类里，它并不认识 Window_mgr 和 ScreenIndex。所以报错：

./Screen.h:9:17: error: use of undeclared identifier 'Window_mgr'
    friend void Window_mgr::clear(ScreenIndex);
                ^
./Screen.h:9:35: error: unknown type name 'ScreenIndex'
    friend void Window_mgr::clear(ScreenIndex);
                                  ^

到这一步，编译器显然放弃将 clear 函数视为友元了，无怪乎你在 clear 函数中想调用 Screen 类的私有方法，就会报错了：

Window_mgr.h:15:7: error: 'contents' is a private member of 'Screen'
    s.contents = std::string(s.height * s.width, ' ');
      ^
./Screen.h:35:17: note: declared private here
    std::string contents;
                ^
Window_mgr.h:15:32: error: 'height' is a private member of 'Screen'
    s.contents = std::string(s.height * s.width, ' ');
                               ^
./Screen.h:34:9: note: declared private here
    pos height = 0, width = 0;
        ^
Window_mgr.h:15:43: error: 'width' is a private member of 'Screen'
    s.contents = std::string(s.height * s.width, ' ');
                                          ^
./Screen.h:34:21: note: declared private here
    pos height = 0, width = 0;
                    ^

那么如何修正呢？

很容易，调整顺序即可。

Screen.h

#ifndef _screen_h
#define _screen_h

#include <string>
#include <iostream>

#include "Window_mgr.h"

class Screen {
public:
    friend void Window_mgr::clear(ScreenIndex);
    //friend class Window_mgr;
    using pos = std::string::size_type;
    Screen() = default;
    Screen(pos ht, pos wd): height(ht), width(wd), contents(ht*wd, ' ') { }
    Screen(pos ht, pos wd, char c): height(ht), width(wd), contents(ht*wd, c) { }
    char get() const {
        return contents[cursor];
    }
    inline char get(pos ht, pos wd) const;
    Screen &move(pos r, pos c);
    Screen &set(char);
    Screen &set(pos, pos, char);
    void some_member() const;
    Screen &display(std::ostream &os) {
        do_display(os);
        return *this;
    }
    const Screen &display(std::ostream &os) const {
        do_display(os);
        return *this;
    }
private:
    mutable size_t access_ctr;
    pos cursor = 0;
    pos height = 0, width = 0;
    std::string contents;
    void do_display(std::ostream &os) const {
        os << contents;
    }
};

inline Screen &Screen::set(char c) {
    contents[cursor] = c;
    return *this;
}
inline Screen &Screen::set(pos row, pos col, char ch) {
    contents[row*width+col] = ch;
    return *this;
}

inline void Window_mgr::clear(ScreenIndex index) {
    Screen &s = screens[index];
    s.contents = std::string(s.height * s.width, ' ');
}
#endif

Window_mgr.h

#ifndef _Window_mgr_h
#define _Window_mgr_h
#include <string>
#include <vector>

class Screen;

class Window_mgr {
public:
    using ScreenIndex = std::vector<Screen>::size_type;
    void clear(ScreenIndex);
private:
    std::vector<Screen> screens;
};

#endif

在调整顺序的过程中，我略加修正的地方：

Window_mgr 是 Screen 的友元，所以在 Screen.h 中 #include "Window_mgr.h"
Window_mgr 也需要用到 Screen 类，所以增加前置声明 class Screen;
去掉对Window_mgr 私有成员screens 的类内初始化，因为此时Screen类并未初始化。
将Window_mgr 的 clear 方法的实现移至 Screen.h 中，因为它是 Screen 的友元函数，需要用到 Screen 类的各种私有信息，显然要等 Screen 初始化完成，故移至Screen.h 的最后。

其余未作任何调整。

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Mooophy commented on May 23, 2024

@pezy awesome

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qinf commented on May 23, 2024

@pezy 感谢

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ex7_32.h中能不能把Window_mgr和Screen类拆分到两个文件中，clear是Window_mgr的成员，是Screen的友元？ about cpp-primer HOT 5 CLOSED

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