std::array vs C-array vs std:vector

Question

i thought to replace std::vector with std::array in my program, so i went on testing:

template<typename T> class A{
    public:
    void sub_run(T w){
        w[0] = 0.5;
        w[1] = 1.5;
        w[2] = 2.5;
        w[3] = 0.0;

        for (int i = 0; i < 100000000; i++)
        {
            w[0] = i *0.5;
            w[1] = i *1.5;
            w[2] = i *2.5;

            w[3] = w[0] + w[1]*w[2];
        }
    }
};

int main(){

    // Vectors    
    /*
    A<vector<double> > inst_a;
    vector<double> w(4);
    inst_a.sub_run(w);
    */
    // 1.71 sec

    // C-array
    /*
    A<double *> inst_a;
    double w[4];
    inst_a.sub_run(w);
    */
    // 1.03 sec

    // std::array

    A<array<double,4> > inst_a;
    array<double,4> w;
    inst_a.sub_run(w);

    // 3.31 sec

    return 0;
 }

Well, the output is odd enough - the std::array is slower than C-array in 3 times and even slower than vector. No optimization was applied, the only flag is -std=c++11. What could be wrong?

Answer 1

From the GCC documentation about Optimization:

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code.

Without the -O option, GCC does not care about performance. It cares about debugging using gdb or other similar tool. To actually compile programs that runs as fast as possible you should use the -O options. For example, -O2.