What is the overhead cost of an empty vector?

Question

What is the memory overhead of having an empty vector vs having a pointer to a vector?

Option A:

std::vector<int> v;

Option B:

std::vector<int> *v = NULL;

I believe that option B takes 1 32 bit pointer (assuming 32 bit here) How much memory does the empty 'v' take up?

Solution 1

As for the question as asked: It depends on the implementation. With MSVC 7.1 this:

std:: cout << sizeof(std::vector<int>) << std::endl;

gives me 16 (bytes). (3 pointers: begin, end, and end of capacity, plus an allocator)

However it should be noted that the pointer-to-vector gives it a larger overhead:

• in both time and space in the non-empty case
• in complexity in all cases.

Solution 2

It's completely implementation-dependent and you should neither assume nor rely on the details. For what it's worth it's 20-bytes using VC.

Solution 3

std::vector v; takes up sizeof(v) space. It might vary by implementation, so run it and find out how much it takes for you.

Solution 4

In Visual Studio Community 2017 (Version 15.2), running this code:

#include <iostream>
#include <vector>

using namespace std;

void main()
{
    vector<float> test;
    vector<float>* test2 = &test;
    cout << sizeof(test) << "\n";
    cout << sizeof(test2) << "\n";

    cout << "\n";
    system("pause");
}

Running in 32 bit (x86), I get 16 bytes for the vector and 4 bytes for the vector pointer.

Running in 64 bit (x64), I get 32 bytes for the vector and 8 bytes for the vector pointer.

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