指针函数从const函数指向的对象的调用运算符

Question

define operator

    RGBAPixel * operator()(size_t x, size_t y);
    RGBAPixel const * operator()(size_t x, size_t y) const;

How should I call it in a const function? (that png on the bottom)

PNG Quadtree::decompress() const
{
    PNG * png = new PNG((size_t) res, (size_t) res);
    decompressHelper(png,0,0,res/2,root);
    return * png;
}

void Quadtree::decompressHelper(PNG * png, int x, int y, int setresolution, QuadtreeNode * subRoot) const
{
    if (subRoot->nwChild == NULL) {
        png((size_t) x, (size_t) y) = subRoot->element;

Answer 1

Just define the operator overload as const as well:

RGBAPixel * operator()(size_t x, size_t y) const;
                                           ^~~~~

Since you're passing by pointer, you should dereference it before calling (don't forget to dereference):

*(*png)((size_t) x, (size_t) y) = subRoot->element;
 ^^   ^

It's not really a function pointer, but a pointer to a class object that can be used as a function. That's a subtle difference, though.

I also recommend changing this function to prevent a potential memory leak:

PNG Quadtree::decompress() const
{
    PNG png((size_t) res, (size_t) res);
    decompressHelper(&png, 0, 0, res/2, root);
    return png;
}

You're not delete -ing what you've allocated with new .