How to read element from a vector pointer?

Question

I need to access a vector pointer elements, I have the following code for my animation structures (simplified here, unnecessary variables cut off):

struct framestruct {
    int w,h;
};
struct animstruct {
    vector<framestruct> *frames;
};

vector<framestruct> some_animation; // this will be initialized with some frames data elsewhere.

animstruct test; // in this struct we save the pointer to those frames.

void init_anim(){
    test.frames = (vector<framestruct> *)&some_animation; // take pointer.
}

void test_anim(){
    test.frames[0].w; // error C2039: 'w' : is not a member of 'std::vector<_Ty>'
}

The array works, I tested it by: test.frames->size() and it was 7 as I planned.

So how can I access the vector elements (w and h) at N'th index from the vector?

Answer 1

You need to dererference the pointer before accessing the array. Just like you are doing with the -> operator to get the size.

(*test.frames)[0].w;

You could use the -> operator to also access the [] operator method, but the syntax isn't as nice:

test.frames->operator[](0).w;

If you want to be able to use [] directly like a true vector syntactically, then you can either allow the frames member to take a copy of the vector, you it can reference the vector. Or, you can overload [] on the animstruct itself to use the [] syntax on your test variable.

Copy:

struct animstruct { vector<framestruct> frames; };
animstruct test;
void init_anim(){ test.frames = some_animation; }

test.frames[0].w;

Reference:

struct animstruct { vector<framestruct> &frames;
                    animstruct (vector<framestruct> &f) : frames(f) {} };
animstruct test(some_animation);
void init_anim(){}

test.frames[0].w;

Overload:

struct animstruct { vector<framestruct> *frames;
                    framestruct & operator[] (int i) { return (*frames)[i]; } };
animstruct test;
void init_anim(){ test.frames = &some_animation; }

test[0].w;