为什么我不能转发声明指针结构？

Question

I am using a slightly obscure sound library called SDL_mixer. GCC is complaining about me forward declaring a pointer struct? What am I doing wrong?

Mix_Music **music; // music[2] must be a pointer to fit any random file
music = new Mix_Music[3];
music[2] = Mix_LoadMUS("fire.ogg");

GCC returns:

     ||=== Build file: "no target" in "no project" (compiler: unknown) ===|
    xxx.cpp||In function 'int SDL_main(int, char**)':|
    xxx.cpp|28|error: invalid use of incomplete type 'Mix_Music {aka struct _Mix_Music}'|
   SDL_mixer.h|131|error: forward declaration of 'Mix_Music {aka struct _Mix_Music}'|
    ||=== Build failed: 2 error(s), 0 warning(s) (0 minute(s), 0 second(s)) ===|

edit: I need the struct to be dynamically allocated, as thousands of files might be loaded. Stack memory won't do.

Answer 1

In new Mix_Music[3] the compiler needs to know how big each object is in order to know how much space to allocate. A forward declaration doesn't give it that information.

You probably want new Mix_Music*[3] . Although stack allocation might work, and std::array or std::vector would make things better.