How to determine the size a struct array was initialized to?

Question

In C++, how do I check the allocated value/size a struct array has been initialized to?

int main()
{
    struct A
    {
      int x;
      int y;
    };
    A* a = new A[20];
}

sizeof(a) / sizeof(a[0]);

returns a value of 1. So this is not what I was expecting.

Is there any way I can do this so that I can get the size (20) allocated for a?

Where I am going to use it is in a case like this:

void fillArray(A valueToFill, int matrixSize, A *a)
{
    for (int i = 0; i < matrixSize; i++)
    {
        a[i] = valueToFill;
    }
}

I intend to check if size of a is actually matrixSize. Otherwise it will crash if matrixSize is bigger than a.

C? `new` is 100% C++. And if you are doing C++, better opt for a container like `std::vector` to handle it. — StoryTeller - Unslander Monica, Aug 14 '19 at 07:17
There is no inbuilt way to do this. You have to store the size yourself. But the good news is `std::vector` does all that for you and also takes care of properly implementing copies, moves and all that fun stuff that gives you headaches with manual memory management. — Max Langhof, Aug 14 '19 at 07:21
I have updated my question with an example on what I actually intend to do. In ```fillArray()``` function above, can you suggest a way to check size of ```a``` vs ```matrixSize```? I can't really change it to vector though since I have to preserve the function signature (which is written the C way). — kzaiwo, Aug 14 '19 at 07:23
If you expected to get the size of an allocation from a pointer then you are going to be disappointed. You'll be much much happier if you use `std::vector` instead. `std::vector` has been part of C++ for well over 20 years, yet people are still being taught to use `new`, it's incredible. — john, Aug 14 '19 at 07:26
@keechan Sorry but what you want to do is impossible. But this being C++ you could provide an *overload* of your function which uses a vector. The overload could even call the original function. That way you have the safety of a vector and still use the original function. `void fillArray(A valueToFill, vector& a) { fillArray(valueToFill, a.size(), a.data(); }` It actually very straightforward to use vectors with legacy code. The designers did think of that. — john, Aug 14 '19 at 07:34
There's even better news. If you are using C++ you don't even need `fillArray` at all. There's a standard [`std::fill`](https://en.cppreference.com/w/cpp/algorithm/fill) function template that works with any range. It's also highly optimized, likely to unroll part of the loop. — StoryTeller - Unslander Monica, Aug 14 '19 at 07:38
Thanks so much everyone! I think I will go with suggestion from @john thanks! — kzaiwo, Aug 14 '19 at 08:12

pmg · Answer 1 · 2019-08-14T07:21:43.453

1

Introduce a new variable (I used na below)

int main()
{
    struct A
    {
      int x;
      int y;
    };
    size_t na = 20;
    A* a = new A[na]; // invalid C
}

then, when you need the size, just refer to (the possibly updated) na

printf("current number of elements: %d\n", (int)na);

edited Aug 14 '19 at 07:21

answered Aug 14 '19 at 07:15

pmg

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How to determine the size a struct array was initialized to?

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