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?

No optimization was applied, the only flag is -std=c++11. What could be wrong?. Did you check when optimization was applied? Unoptimized C++ is not trying to beat "C-like" performance in any way shape or form, so if you're going to be talking about performance of the final product you must allow the C++ compiler leeway to optimize...because otherwise, it's going to leave in a lot of things that assist in debugging/etc. Not a fair comparison. — HostileFork says dont trust SE
– HostileFork says dont trust SE, Commented Oct 20, 2014 at 11:49
""No optimization was applied" is exactly what's wrong." - Well this is somehow unapparent, since the cppreference.com just says "...combines the performance and accessibility of a C-style array with the benefits of a standard container, such as knowing its own size, supporting assignment, random access iterators, etc." — stkubr
– stkubr, Commented Oct 20, 2014 at 11:59
@stkubr it may be unapparent from the documentation, but it is a piece of general knowledge. They won't repeat this every time. That would be akin to warning someone to "not forget the integration constant" each time he performs an integration. — The Paramagnetic Croissant
– The Paramagnetic Croissant, Commented Oct 20, 2014 at 12:09

vz0 · Accepted Answer · 2014-10-20 12:09:14Z

3

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.

answered Oct 20, 2014 at 12:09

vz0

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