Template - restrict type

Question

If I have a template function, how can I restrict this function to only two types, without using type_traits ?

#include <iostream>
#include <utility>
#include <type_traits>

template <typename T>
void swap_bytes(T& a, T& b)
{
	std::swap(a, b);
}

template <>
void swap_bytes(int& a, int& b)
{
	std::swap(a, b);
}

template <>
void swap_bytes(float& a, float& b)
{
	std::swap(a, b);
}

int main()
{
	int a{ 3 }, b{ 4 };
	swap_bytes(a, b);
	char c1{ 'q' }, c2{ 'r' };
	swap_bytes(c1, c2);  // works ! and it shouldn't !

	return 0;
}

As you see, I still can use this function with char type.

Answer 1

template <typename T>
void swap_bytes(T& a, T& b)
;

template <>
void swap_bytes(int& a, int& b)
{
	std::swap(a, b);
}

template <>
void swap_bytes(float& a, float& b)
{
	std::swap(a, b);
}

int main()
{
	int a{ 3 }, b{ 4 };
	swap_bytes(a, b);
	char c1{ 'q' }, c2{ 'r' };
	swap_bytes(c1, c2); // fatal error LNK1120

	return 0;
}

I found it, just declare the template, not implement it.