Question

我尝试了向前宣布，但没有成功。

我有两个类：SDL_CTexture和SDL_CRenderer。它们不会将任何实例，指针或引用保存为成员变量，但它们都具有成员函数，该函数采用const＆amp;另一种类型的参数分别作为参数。请参阅下面的代码。

这是SDL_CTexture.hpp。注意第20行的前向声明和第31行的函数。

  # ifndef SDL_CTEXTURE_HPP
10 # define SDL_CTEXTURE_HPP
11 
12 # include <SDL2/SDL.h>
13 # include "SDL_CRenderer.hpp"
14 # include <string>
15 
16 namespace sdl_cpp{
17   class SDL_CTexture;
18 }
19 
20 class sdl_cpp::SDL_CRenderer;
21 
22 class sdl_cpp::SDL_CTexture{
23 public:
24   SDL_CTexture();
25   ~SDL_CTexture();
26 
27   /* Create a texture from image file.                                     
28    * It calls SDL_LoadBMP internally, and takes care of freeing            
29    * temporary SDL_Surface.                                                
30    */
31   bool load(const char* filename, const sdl_cpp::SDL_CRenderer &renderer);
32 
33   SDL_Texture* get_texture() const;
34   int get_width() const;
35   int get_height() const;
36   static std::string CLASS;
37 
38 protected:
39   SDL_Texture* m_texture;
40   int m_w;/* texture width */
41   int m_h;/* texture height */
42 };

同样，SDL_CRenderer.hpp。见第21行的前向声明和第35行的功能。

# ifndef SDL_CRENDERER_HPP
10 # define SDL_CRENDERER_HPP
11 
12 # include <SDL2/SDL.h>
13 # include "SDL_CWindow.hpp"
14 # include "SDL_CTexture.hpp"
15 # include <string>
16 
17 namespace sdl_cpp{
18   class SDL_CRenderer;
19 }
20 
21 class sdl_cpp::SDL_CTexture;
22 
23 class sdl_cpp::SDL_CRenderer{
24 public:
25   SDL_CRenderer();
26   ~SDL_CRenderer();
27 
28   /* wrappers */
29   bool create(const sdl_cpp::SDL_CWindow &window, int index = -1, Uint32 f\
   lags = 0);
30   void set_color(Uint8 r=0, Uint8 g=0, Uint8 b=0, Uint8 a=255);
31   void clear();
32   void present();
33 
34   /* copy the whole texture as it is, to destined coordinator */
35   bool copy(const sdl_cpp::SDL_CTexture &texture, int x, int y);
36 
37   SDL_Renderer* get_renderer() const;
38 
39   static std::string CLASS;
40 
41 protected:
42   SDL_Renderer* m_renderer;
}

当我尝试编译它时生成了错误：

g++ SDL_CTexture.cpp -Wall -c -std=c++11 `sdl2-config --cflags` -o SDL_CTexture.o
In file included from SDL_CTexture.hpp:13:0,
                 from SDL_CTexture.cpp:10:
SDL_CRenderer.hpp:21:16: error: ‘SDL_CTexture’ in namespace ‘sdl_cpp’ does not name a type
 class sdl_cpp::SDL_CTexture;
                ^
SDL_CRenderer.hpp:35:19: error: ‘SDL_CTexture’ in namespace ‘sdl_cpp’ does not name a type
   bool copy(const sdl_cpp::SDL_CTexture &texture, int x, int y);
                   ^
SDL_CRenderer.hpp:35:42: error: ISO C++ forbids declaration of ‘texture’ with no type [-fpermissive]
   bool copy(const sdl_cpp::SDL_CTexture &texture, int x, int y);
                                          ^
In file included from SDL_CTexture.cpp:10:0:
SDL_CTexture.hpp:20:16: warning: declaration ‘class sdl_cpp::SDL_CRenderer’ does not declare anything [enabled by default]
 class sdl_cpp::SDL_CRenderer;
                ^
make: *** [SDL_CTexture.o] Error 1

我该怎么办？

PS。我必须将每个.cpp编译成它自己的.o文件，所以我必须保持包含它，以及那些包含警卫。目前我通过将两个标头合并为一个来实现“工作”，但很明显，这看起来并不像是一个好的设计:)所以，我仍然会因为一个很好的解决方案而受到赞赏。

Answer 1

如果您转发声明SDL_CREnderer（第20行），则不必再包含标题（第13行）。这就是你使用前向声明的原因。
对于SDL_Ctexture第14行和第21行

如果你删除了包含，我相信它应该可行。

Answer 2

不能从不同的名称空间转发声明类。

namespace A {
  class X; // okay
};

class A::Y; // error

这正是你在这里尝试的：

namespace sdl_cpp{
  class SDL_CTexture;
}

class sdl_cpp::SDL_CRenderer;

你可以通过将它们组合到

来解决它

namespace sdl_cpp {
  class SDL_CRenderer;
  class SDL_CTexture;
}

如何解决C ++中的相互依赖？

2 个答案: