What is the capacity()
of an std::vector
which is created using the default constuctor? I know that the size()
is zero. Can we state that a default constructed vector does not call heap memory allocation?
This way it would be possible to create an array with an arbitrary reserve using a single allocation, like std::vector<int> iv; iv.reserve(2345);
. Let's say that for some reason, I do not want to start the size()
on 2345.
For example, on Linux (g++ 4.4.5, kernel 2.6.32 amd64)
#include <iostream>
#include <vector>
int main()
{
using namespace std;
cout << vector<int>().capacity() << "," << vector<int>(10).capacity() << endl;
return 0;
}
printed 0,10
. Is it a rule, or is it STL vendor dependent?
The standard doesn't specify what the initial capacity
of a container should be, so you're relying on the implementation. A common implementation will start the capacity at zero, but there's no guarantee. On the other hand there's no way to better your strategy of std::vector<int> iv; iv.reserve(2345);
so stick with it.