在所有子标头中包含基本标头时出现问题

Question

我有一个基类，它在下面的头文件中声明：

#pragma once
#include "StateHandler.hpp"

namespace ta {
    class GameState {
    public:
        // ...
    };
} /* ta */

然后，我有两个孩子，看起来像这样：

#pragma once
#include "../GameState.hpp"

namespace ta {
    class DefaultState: public ta::GameState {
    public:
        // ...
    };
} /* ta */

和

#pragma once
#include "../GameState.hpp"

namespace ta {
    class WorldMapState: public ta::GameState {
    public:
        // ...
    };
} /* ta */

当我试图编译它时，我得到了错误，GameState没有在我包含的两个孩子中的第二个中声明。当我从GameState.hpp中删除#pragma once时，它告诉我GameSate得到了重新定义。我知道为什么会这样，但我找不到解决的办法。

更新:使用include-guards也不起作用。在使用#pragma once或include-guards时，我得到以下错误：

In file included from /[...]/include/statemachine/gamestates.hpp:2,
                 from /[...]/include/common.hpp:4,
                 from /[...]/include/statemachine/gamestates/../StateHandler.hpp:5,
                 from /[...]/include/statemachine/gamestates/../GameState.hpp:4,
                 from /[...]/include/statemachine/gamestates/DefaultState.hpp:4,
                 from /[...]/include/statemachine/gamestates.hpp:1,
                 from /[...]/include/common.hpp:4,
                 from /[...]/source/main.cpp:1:
/[...]/include/statemachine/gamestates/WorldMapState.hpp:7:47: error: expected class-name before '{' token
     class WorldMapState: public ta::GameState {
                                               ^

这是我在GameState.hpp中没有使用include-guards或#pragma once时得到的错误(这个错误出现了3次)：

In file included from /[...]/include/common.hpp:5,
                 from /[...]/source/main.cpp:1:
/[...]/include/statemachine/GameState.hpp:4:11: error: redefinition of 'class ta::GameState'
     class GameState {
           ^~~~~~~~~
In file included from /[...]/include/statemachine/gamestates/WorldMapState.hpp:4,
                 from /[...]/include/statemachine/gamestates.hpp:2,
                 from /[...]/include/common.hpp:4,
                 from /[...]/include/statemachine/gamestates/../StateHandler.hpp:5,
                 from /[...]/include/statemachine/gamestates/../GameState.hpp:1,
                 from /[...]/include/statemachine/gamestates/DefaultState.hpp:4,
                 from /[...]/include/statemachine/gamestates.hpp:1,
                 from /[...]/include/common.hpp:4,
                 from /[...]/source/main.cpp:1:
/[...]/include/statemachine/gamestates/../GameState.hpp:4:11: note: previous definition of 'class ta::GameState'
     class GameState {

Answer 1

根据this answer的说法，#pragma once有无法修复的错误。它永远不应该被使用。

您可以使用标题保护，如下所示

#ifndef HEADER_H
#define HEADER_H

// Header file code

#endif

Answer 2

#pragma once不是一个标准，即使它被许多编译器支持，也不应该用来保护头文件，而应该使用#ifndef。因为没有标准行为，所以你不应该假设所有编译器的行为都是一样的。