Initial capacity of vector in C++

Question

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?

Answer 1

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.

Answer 2

Storage implementations of std::vector vary significantly, but all the ones I've come across start from 0.

The following code:

#include <iostream> #include <vector>  int main() {   using namespace std;      vector<int> normal;   cout << normal.capacity() << endl;      for (unsigned int loop = 0; loop != 10; ++loop)   {       normal.push_back(1);       cout << normal.capacity() << endl;   }      cin.get();   return 0; } 

Gives the following output:

0 1 2 4 4 8 8 8 8 16 16 

under GCC 5.1, 11.2 - Clang 12.0.1 and:

0 1 2 3 4 6 6 9 9 9 13 

under MSVC 2013.