overload_displatch.cpp

#include <string>
#include <iostream>
#include <array>

class Foo
{
public:
    Foo() : i(42), d(44.0) {}
    
    template<typename T>
    T get();
    
	template<typename T, size_t n>
	std::array<T, n> get();
    
private:

	template<typename T, size_t n>
	std::array<T, n> get_impl();

    int i = 0;
    double d = 0.0;
};

template<typename T>
T Foo::get()
{
    if constexpr (std::is_same<int, typename std::remove_cv<T>::type>::value)
    {
        return i;
    }
    else if constexpr (std::is_same<double, typename std::remove_cv<T>::type>::value)
    {
        return d; 
    }
    else
    {
        static_assert(sizeof(T) == -1);
        return T();
    } 
}

template<typename T, size_t n>
std::array<T, n> Foo::get()
{
    if constexpr (std::is_same<int, typename std::remove_cv<T>::type>::value)
    {
        std::array<int, n> arr;
        arr.fill(i);
        return arr;
    }
    else if constexpr (std::is_same<double, typename std::remove_cv<T>::type>::value)
    {
        std::array<double, n> arr;
        arr.fill(d);
        return arr; 
    }
    else
    {
        static_assert(sizeof(T) == -1);
        return std::array<T, n>();
    } 
}

int main() {
    Foo foo;
    
    std::array<int, 4> i = foo.get<int, 4>();
    double f = foo.get<double>();
    
    //char c = foo.get<char>();
    //float f = foo.get<12, float>();
    
    return 0;
}