decorator.cpp

#include <iostream>
#include <cmath>

template <class T, class F>
struct double_decorator
{
    F func;
    double_decorator(F func) : func(func) {}

    T operator()(T x)
    {
        return func(x) * T(2);
    }
};

template <class T, class F>
double_decorator<T, F> decorate(F func)
{
    return double_decorator<T, F>(func);
}

float root(float x)
{
    return std::sqrtf(x);
}

int main(int argc, const char* argv[])
{
    auto d = decorate<float>(root);
    std::cout << d(4.0f) << std::endl;
}