1.cpp

#include <iostream>
#include <cstddef>
#include <cstdint>
#include <type_traits>

using namespace std; //for convinience
struct Fixed
{
    static const float up ;
    static const float down ; // avoid float-point division
    uint32_t d;

    explicit Fixed(float x):
        d(x * (1 << 16))
    {}

    explicit Fixed(const uint32_t x, std::false_type) : d(x) {} //internal use only

    float getFloat()
    {
        return static_cast<float>(d) * down;
    }

    Fixed operator*(const Fixed other)
    {
        return Fixed((uint32_t)(((uint64_t)d * other.d) >> 16), std::false_type());
    }
};

const float Fixed::up = 1 << 16;
const float Fixed::down = 1 / Fixed::up;

int main()
{
    // tested on gcc 5.3.0 -O2 enabled
    // 0.88s
    //Fixed a(2.5), b(2.5);
    //for (int i=0; i<1000000000; ++i)
    //{
        //a = a * b;
    //}
    //cout << a.getFloat() << endl;


    // 1.47s
    float a(2.5), b(2.5);
    for (int i=0; i<1000000000; ++i)
    {
        a = a * b;
    }
    cout << a << endl;
}