SDL_DisplayFormatAlpha(c++)出现问题

正如我在 this question ,我正在使用SDL开发一款小游戏。现在我在SDL_DisplayFormatAlpha上遇到了问题。我试图从PNG图像中创建一个带有alpha通道的曲面。它以前是有效的,但现在我做了一些轻微的重构,有些东西坏了。我把它缩小到这个构造函数:

Surface::Surface( tfilename file ) {
    // initialize the surface data member to the image indicated by filename
    SDL_Surface *tempSurface;
    tempSurface = IMG_Load( file.c_str() );
    if ( !tempSurface ) {
        surface = NULL;
        exit(1);
    }
    else {
        surface = SDL_DisplayFormatAlpha( tempSurface );
        //surface = tempSurface;
    }
    SDL_FreeSurface( tempSurface );
}

这编译得很好,但当我运行它时,我遇到了分段错误。 gdb报告的错误:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb79c16c0 (LWP 8089)]
0xb7e8b9a3 in SDL_DisplayFormatAlpha () from /usr/lib/libSDL-1.2.so.0

堆栈跟踪如下:

#0  0xb7e8b9a3 in SDL_DisplayFormatAlpha () from /usr/lib/libSDL-1.2.so.0
#1  0x0804987e in Surface (this=0x804d060, file=@0xbfb20760) at Surface.cpp:16
#2  0x0804a159 in Image (this=0x804d038, x=0, y=0, file=@0xbfb207a0)
    at Image.cpp:16
#3  0x0804a3de in Object (this=0x804d028, imageFile=@0xbfb207dc)
    at Object.cpp:4
#4  0x080491cb in Application (this=0xbfb20810) at Application.cpp:8
#5  0x08048e0d in main () at main.cpp:5

如果我发表评论 surface = SDL_DisplayFormatAlpha( tempSurface ); 和 SDL_FreeSurface( tempSurface ); 并取消注释 surface = tempSurface; 像这样:

Surface::Surface( tfilename file ) {
    // initialize the surface data member to the image indicated by filename
    SDL_Surface *tempSurface;
    tempSurface = IMG_Load( file.c_str() );
    if ( !tempSurface ) {
        surface = NULL;
        exit(1);
    }
    else {
        //surface = SDL_DisplayFormatAlpha( tempSurface );
        surface = tempSurface;
    }
    //SDL_FreeSurface( tempSurface );
}

那么它似乎工作得很好。有人能告诉我发生了什么事吗?实际上,当我评论SDL_DisplayFormatAlpha时,透明度似乎也起作用了。该功能是否仅适用于尚未具有alpha通道的图像?