Creating a value constant that depends on the template type

Question

I would like to create a C++ template function which has different constants that get used in the implementation depending on the choice of the template type.

#define FLOAT_EPSILON       (0.000001f)
#define DOUBLE_EPSILON      (0.00000000000001)

template <class T> void func(T params)
{
    const T epsilon = ???? // either FLOAT_EPSILON or DOUBLE_EPSILON depending on T
    // do some calculations using epsilon
}

template void func(float params);
template void func(double params);

I can't quite work out how to do this best, although I thought of some half-assed ways that work. Can you help?

Answer 1

You can use a helper template to choose the epsilon.

template <typename T> struct EpsilonChooser;

template <> struct EpsilonChooser<float>
{
    float const value = 0.000001f;
};

template <> struct EpsilonChooser<double>
{
    double const value = 0.00000000000001;
};

template <class T> void func(T params)
{
    const T epsilon = EpsilonChooser<T>::value;
    // do some calculations using epsilon
}