Comparison of performance of adding elements to preallocated containers in c++11

MillionItemList.cpp

#include <vector>
#include <list>
#include <iostream>
#include <ctime>

// FIXME: v(1000000) creates a vector of 1000000 elements. At the end of the function there are 2000000 elements. Instead, it should be constructed empty and then v.reserve(1000000) should be called just after.
void createPreallocatedMillionVector()
{

        std::vector<int> v(1000000);
        for (int i = 0; i < 1000000; ++i)
        {
                v.push_back(i);
        }
}

void createMillionVector()
{
        std::vector<int> v;
        for (int i = 0; i < 1000000; ++i)
        {
                v.push_back(i);
        }
}

// FIXME: This actually creates a list of 2000000 elements. Should instead use a pool allocator.
void createPreallocatedMillionList()
{
        std::list<int> L(1000000, 0);
        for (int i = 0; i < 1000000; ++i)
        {
                L.push_back(i);
        }
}

void createMillionList()
{
        std::list<int> L;
        for (int i = 0; i < 1000000; ++i)
        {
                L.push_back(i);
        }
}
 
int main()
{
        clock_t startTime = clock();
 
        createMillionVector();
 
        clock_t postVectorTime = clock();

	createPreallocatedMillionVector();

	clock_t postAllocVectorTime = clock();
 
        createMillionList();
 
        clock_t postListTime = clock();

        createPreallocatedMillionList();
 
        clock_t postAllocListTime = clock();
 
        std::cout << "Vector took " << (postVectorTime - startTime) / (float)CLOCKS_PER_SEC << " seconds." << std::endl;
        std::cout << "Preallocated Vector took " << (postAllocVectorTime - postVectorTime) / (float)CLOCKS_PER_SEC << " seconds." << std::endl;
 
        std::cout << "List took " << (postListTime - postAllocVectorTime) / (float)CLOCKS_PER_SEC << " seconds." << std::endl;
        std::cout << "Preallocated List took " << (postAllocListTime - postListTime) / (float)CLOCKS_PER_SEC << " seconds." << std::endl;
}

output with g MillionItemList.cpp -stdc0x -O0

Vector took 0.04 seconds.
Preallocated Vector took 0.04 seconds.
List took 0.24 seconds.
Preallocated List took 0.46 seconds.

output with g MillionItemList.cpp -stdc0x -O2

Vector took 0.01 seconds.
Preallocated Vector took 0.02 seconds.
List took 0.13 seconds.
Preallocated List took 0.25 seconds.

output with g++ MillionItemList.cpp -std=c++0x -O0

Vector took 0.04 seconds.
Preallocated Vector took 0.04 seconds.
List took 0.24 seconds.
Preallocated List took 0.46 seconds.

output with g++ MillionItemList.cpp -std=c++0x -O2

Vector took 0.01 seconds.
Preallocated Vector took 0.02 seconds.
List took 0.13 seconds.
Preallocated List took 0.25 seconds.