Question

I have a strange metafunction behavior in my C++ code and I want to understand why.

#include <iostream>
#include <cmath>

inline double f(double x, double y)
{
    std::cout<<"Marker"<<std::endl;
    return sqrt(x*y);
}

template <int N, class T> inline T metaPow(T x)
{
    return ((N > 0) ? (x*metaPow<((N > 0) ? (N-1) : (0))>(x)) : (1.));
}


int main()
{
    double x;
    double y;
    std::cin>>x;
    std::cin>>y;
    std::cout<<metaPow<5>(f(x, y))<<std::endl;
    return 0;
}

I expected that the line metaPow<5>(f(x, y)) was equivalent to f(x, y)*f(x, y)*f(x, y)*f(x, y)*f(x, y)*1.. But if it was, it would print me five times the “Marker” line in the f function.

The strange thing is that I have the good result at the end (for example 181.019 for x = 2 and y = 4) but I have only 1 “Marker” displayed. How is it possible ? And consequently is it a good option to use that function for compile-time optimization instead of the standard pow() ?

Thank you very much !

Answer 1

I believe that f(x,y) is being evaluated before being passed in to your metaPow function. So the x argument to metaPow is just the value sqrt*(8). metaPow is never calling f(x,y). Hence, f(x,y) is only called once – when you initially call metaPow in your main function.