Question

I’m trying to understand the amount of bytes occupied by an instance of std::vector. The following code:

    vector <int>uno;
    uno.push_back(1);
    uno.push_back(1);
    cout <<"1: "<< sizeof(uno)<<" bytes"<<endl;
    cout << endl;

    vector <bool>unos;
    unos.push_back(true);
    cout <<"2: "<< sizeof(unos)<<" bytes"<<endl;
    cout << endl;

gives me this output:

1: 12 bytes

2: 20 bytes

Can someone explain me why the size of the vector<bool> has more bytes than the vector<int>?. And what is the correct way to measure the size in bytes of an vector instance… by adding the size of every member in the vector?

Answer 1

In C++, the sizeof operator is always evaluated at compile time, so every vector<T> will have the same size (unless vector<T> is a specialization like vector<bool>).

In case you are wondering about the 12 bytes, that is probably the size of 3 pointers, or alternatively, the size of a pointer and an element count and a capacity. The real data is never stored inside the vector.

If you want to know how much memory is used total, a good approximation is:

sizeof(std::vector<T>) + my_vector.capacity() * sizeof(T)

This is only an approximation because it does not take into account book-keeping data on the heap.