Question

I would like to create a struct and use it inside an other struct as an array. My problem is that I don’t know how big array I would like to allocate, I will only know once I am in a function. I mean I would like to use [] instead of a pre-determined constant, like 10000.

I think if you look at my code it would be self-explanatory. Can you help me how to make this code work? Moreover it would help me a lot if you could tell me what is the name of the topic I am asking about (is it dynamic arrays?) and that where can I find articles/tutorials about this topic.

Here is the code with my broken way of thinking about arrays in structs.

#include <iostream>

using namespace std;

struct keyframe {
    bool a;
    int b;
    int c;
};


struct keyframe_file {
    const int num_views;
    const int num_keyframes;
    keyframe keyframes[];
};


int main() {

    keyframe_file my_file;

    my_file.num_views = 1;
    my_file.num_keyframes = 6;

    my_file.keyframes = new keyframe[my_file.num_keyframes];

    my_file.keyframes[0].a = true;
    my_file.keyframes[0].b = 5;
    my_file.keyframes[0].c = 9;

    return 0;

}

Answer 1

If it suits your purpose, an STL container (std::vector) is easily one of the best options – the less memory management you have to worry about, the better.

In any case, look at the struct definition Nawaz posted above – that’s exactly how it should be. Dynamic arrays in C++ are simply pointers. You have, however, allocated the memory properly in your code, but you haven’t freed it (so it’s leaking). Since you allocated with new [] you will need to

delete [] my_file.keyframes;

in order to free the memory properly.

Resizing is another issue: with a smart implementation, array resizing can be an amortized O(1) operation which is nice. When you resize, it will always take you O(n) since you need to copy all the elements into a new array of different size, but if you do it half as much, it becomes O(1). That is, double the array each time you need to resize. Here is a very quick example

void resize()
{
  if(numOfElementsInArray == sizeOfArray)
  {
    ArrayType * arr = new ArrayType[sizeOfArray*2]; // Allocate a double size array
    for(int i=0;i<sizeOfArray;++i)
      currentArray[i] = arr[i];
    delete [] currentArray; // Free memory in old array
    currentArray = arr; // Set the array to our new one
    sizeOfArray *= 2; // Double the size
  }
}

NOTE: The example above does not take into account space complexity; that said, if you have 5000 elements, and remove all but 5, this method with not shrink it (which is probably what you will want to do for all practical purposes)