template <class T1, class T2>
void function(T1 a, T1 b, T2 c) { ... }

template <class T1, class T2>
void function(T1 a, T1 b, T2 c = С) { ... }

template<typename T, typename U = int>
void f(T a, T b, U c = U(0))
{
    cout << a << " " << b << endl;
    cout << typeid(c).name() << endl;
    cout << c << endl;

}

int main()
{
    f(0,1,3);
    f(0,1,3.0);
    f(0,1);
}

template <class T1, class T2>
void function(T1 a, T1 b, T2 c)
{
  ...
}

template <class T1>
void function(T1 a, T1 b)
{
  function(a, b, C);
}