capacity of the vector changes after push_back()

c++, c++11, stdvector

Solution

Because taking a vector `A` and copying it to vector `B` does not guarantee that vector `B` will have the same capacity as vector `A`. Typically, the new vector will allocate only enough memory to hold the elements being copied into it.

In fact, there's an old trick that makes use of this, called the reduce-capacity trick:

int main()
{
   vector<int> v { 1,2,3,4,5 };
   v.clear();                   // capacity still non-zero

   vector<int>(v).swap(v);      // capacity now zero (maybe)
}

… though, technically, whether this actually works is entirely implementation-dependent.

If you move the vector in, rather than copying it, then there are no re-allocations, the buffer is actually the same buffer, and the capacity does not change:

#include <iostream>
#include <vector>

using namespace std;

struct Cell
{
    vector<int> vtx;
};

int main()
{
    vector <Cell> cells;
    Cell tmp;
    tmp.vtx.reserve(5);
    cout << tmp.vtx.capacity() << endl;
    cells.push_back (std::move(tmp));
    cout << cells[0].vtx.capacity() << endl;
    return 0;
}

// 5
// 5

(Notice that I had to move the first `cout` call to before the move, otherwise I'd be `cout`ing something that is in an unknown state.)

Problem

Original source

Related problems

• Can we rely on the reduce-capacity trick?