c - 需要指针帮助

Question

typedef struct
{
    int mPosX,mPosY;//X & Y coordinates
    int mVelX,mVelY;//Velocity
    SDL_Rect *mColliders;//Dot's Collision Boxes
}dot;

void scale(SDL_Rect* r,size_t capacity)
{
    r=(SDL_Rect*)calloc(capacity,sizeof(SDL_Rect));
}
void rescale(SDL_Rect* r,size_t newcapacity)
{
    r=(SDL_Rect*)realloc(r,sizeof(SDL_Rect)*newcapacity);
}
void gc(SDL_Rect* r)
{
    free(r);
    r=NULL;
}


void dot_init(dot *d,int x,int y)
{
    //Initialize the Offsets
    d->mPosX=x;
    d->mPosY=y;
    scale(d->mColliders,11);
    //Initialize the velocity
    d->mVelX=0;
    d->mVelY=0;

SDL_Rectis 是一个结构，包含 x、y、w、h< 等字段 所有 int。现在如何访问这些字段？

例如。类似的东西

d->mColliders[2].h=1;
d->mColliders[3].w=16;//This ain't working

我很困惑

Answer 1

在此:

void scale(SDL_Rect* r,size_t capacity)
{
    r=(SDL_Rect*)calloc(capacity,sizeof(SDL_Rect));
}

void dot_init(dot *d,int x,int y)
{
    //Initialize the Offsets
    d->mPosX=x;
    d->mPosY=y;

    d->mColliders = NULL;
    scale(d->mColliders,11);
    // here d->mColliders is still NULL
}

调用 scale 后，d->mColliders 仍为 NULL，因为指针传递给函数的方式使其只能在本地修改.

查看并运行一个最小的示例来说明这一点:https://ideone.com/rIjoOC

您应该编写(需要 C++):

void scale(SDL_Rect*& r,size_t capacity) // not the reference to pointer with &
{
    r=(SDL_Rect*)calloc(capacity,sizeof(SDL_Rect));
}

void dot_init(dot *d,int x,int y)
{
    //Initialize the Offsets
    d->mPosX=x;
    d->mPosY=y;

    d->mColliders = NULL;
    scale(d->mColliders,11);
    // here d->mColliders is not NULL
}

在 C 中，使用双指针而不是引用，然后执行以下操作:

void scale(SDL_Rect** r,size_t capacity)
{
    *r=(SDL_Rect*)calloc(capacity,sizeof(SDL_Rect));
}

void dot_init(dot *d,int x,int y)
{
    //Initialize the Offsets
    d->mPosX=x;
    d->mPosY=y;

    d->mColliders = NULL;
    scale(&(d->mColliders),11);
    // here d->mColliders is not NULL
}

或者(适用于 C 和 C++):

SDL_Rect* scale(size_t capacity) // simply return the allocated array
{
    return (SDL_Rect*)calloc(capacity,sizeof(SDL_Rect));
}

void dot_init(dot *d,int x,int y)
{
    //Initialize the Offsets
    d->mPosX=x;
    d->mPosY=y;

    d->mColliders = NULL;
    d->mColliders = scale(11);
    // here d->mColliders is not NULL
}