Is std::vector so much slower than plain arrays?

Question

I\'ve always thought it\'s the general wisdom that std::vector is \"implemented as an array,\" blah blah blah. Today I went down and tested it, and it seems to

Answer 1

I Have to say I'm not an expert in C++. But to add some experiments results:

compile: gcc-6.2.0/bin/g++ -O3 -std=c++14 vector.cpp

machine:

Intel(R) Xeon(R) CPU E5-2690 v2 @ 3.00GHz 

OS:

2.6.32-642.13.1.el6.x86_64

Output:

UseArray completed in 0.167821 seconds
UseVector completed in 0.134402 seconds
UseConstructor completed in 0.134806 seconds
UseFillConstructor completed in 1.00279 seconds
UseVectorPushBack completed in 6.6887 seconds
The whole thing completed in 8.12888 seconds

Here the only thing I feel strange is that "UseFillConstructor" performance compared with "UseConstructor".

The code:

void UseConstructor()
{
    TestTimer t("UseConstructor");

    for(int i = 0; i < 1000; ++i)
    {
        int dimension = 999;

        std::vector pixels(dimension*dimension);
        for(int i = 0; i < dimension * dimension; ++i)
        {
            pixels[i].r = 255;
            pixels[i].g = 0;
            pixels[i].b = 0;
        }
    }
}


void UseFillConstructor()
{
    TestTimer t("UseFillConstructor");

    for(int i = 0; i < 1000; ++i)
    {
        int dimension = 999;

        std::vector pixels(dimension*dimension, Pixel(255,0,0));
    }
}

So the additional "value" provided slows down performance quite a lot, which I think is due to multiple call to copy constructor. But...

Compile:

gcc-6.2.0/bin/g++ -std=c++14 -O vector.cpp

Output:

UseArray completed in 1.02464 seconds
UseVector completed in 1.31056 seconds
UseConstructor completed in 1.47413 seconds
UseFillConstructor completed in 1.01555 seconds
UseVectorPushBack completed in 6.9597 seconds
The whole thing completed in 11.7851 seconds

So in this case, gcc optimization is very important but it can't help you much when a value is provided as default. This, is against my tuition actually. Hopefully it helps new programmer when choose which vector initialization format.