C ++怪異函數指針行為

Question

這是一個簡單的opengl / sdl程序。 通常會有一個巨大的雙端隊列，程序會在該雙端隊列中保存顯示數據，例如，將像這樣從該雙端隊列中插入/讀取/讀取一個三角形（GL_Begin .... Vertice 1 .... Vertice 2 ... Vertice 3 .. 。GL_End）

項目結構

enum Shapes{
    LINE,
    POLYGON,
    TRIANGLE
};

struct Item{
    int id;
    int type;
    Shapes shape;
    double x;
    double y;
    double z;
};

...

void GEngine::CC2D(int id,double x,double y){ // Change Coordinates ... Move the Item

    for(int i=0;i<Items.size();i++){
        Item* item = &Items[i];                  // Pointer to the item INSIDE the deque because we need to edit it, we can not create a new instance of Item

        if(item->id == id && item->type == 2){
            item->x += x;
            item->y += y;
        }
    }

    DrawScene();
}

void GEngine::PollEvents( void (*Event)() ){
    int mbpressed = 0; // Mouse button pressed flag
    int mouse_xpre = 0;
    int mouse_ypre = 0;
    int move_id = 0; // TESTING
    while(1){
        while( SDL_PollEvent( &event ) ){

            switch( event.type ){
                case SDL_MOUSEMOTION:{
                    if(mbpressed == 1){
                        mouse_x = event.motion.x; // X2
                        mouse_y = event.motion.y; // Y2
                    //  (*Event)();

                    //  CC2D(  (   X2  -   X1     ),(   Y2  -   Y1     )
                        CC2D(move_id,(mouse_x-mouse_xpre),(mouse_y-mouse_ypre));      // The difference between the current and the previous mouse position is equal to the DX and DY which will be used to move the vertices.
                        mouse_xpre = mouse_x;     // X1
                        mouse_ypre = mouse_y;     // Y1

                        SDL_Delay(20);

                    }
                    break;

現在的問題。 如果我使用CC2D（move_id，（mouse_x-mouse_xpre），（mouse_y-mouse_ypre））; 它可以正常工作，但是如果我將CC2D函數添加到事件1中，則算法會失敗，並且不是使用鼠標移動項目而是將其從屏幕移開。

換一種說法。

case SDL_MOUSEMOTION:{
                        if(mbpressed == 1){
                            mouse_x = event.motion.x; // X2
                            mouse_y = event.motion.y; // Y2
                        //  (*Event)();

                        //  CC2D(  (   X2  -   X1     ),(   Y2  -   Y1     )
                        CC2D(move_id,(mouse_x-mouse_xpre),(mouse_y-mouse_ypre));      // The difference between the current and the previous mouse position is equal to the DX and DY which will be used to move the vertices.
                        mouse_xpre = mouse_x;     // X1
                        mouse_ypre = mouse_y;     // Y1

                        SDL_Delay(20);

                    }
                    break;
}

^效果很好。

但

GEngine gEngine;

void Event(){
            gEngine.CC2D(0,(gEngine.mouse_x-gEngine.mouse_xpre),(gEngine.mouse_y-gEngine.mouse_ypre));
    }

int main(int argc, char *argv[]){

... code ...

gEngine.PollEvents(&Event);

return 0; // NEVER REACHED
}

與

case SDL_MOUSEMOTION:{
                    if(mbpressed == 1){
                        mouse_x = event.motion.x; // X2
                        mouse_y = event.motion.y; // Y2
                        (*Event)();

                        mouse_xpre = mouse_x;     // X1
                        mouse_ypre = mouse_y;     // Y1

                        SDL_Delay(20);

                    }
                    break;
}

才不是 ...

為了進一步簡化：

Case SDL_MOUSEMOTION: ...... code ..... CC2D( params) ........

工作正常，但

Event(){ CC2D( params ) } // Main.cpp
Case SDL_MOUSEMOTION: ...... code ..... (*Event)() ........ // GEngine.cpp

無法按預期工作

Answer 1

您在此處在PollEvents中聲明局部變量：

int mouse_xpre = 0;
int mouse_ypre = 0;

請注意，它們分別名為mouse_xpre和mouse_ypre 。 然后在回調中

gEngine.CC2D(0,(gEngine.mouse_x-gEngine.mouse_xpre),(gEngine.mouse_y-gEngine.mouse_ypre));
//                                      ^^^^^^^^^^                           ^^^^^^^^^^

訪問成員變量。 您需要刪除局部變量的聲明，以便它們不會隱藏成員變量，並且您將在兩個地方都使用成員變量。