Specifying default value for template function argument

Question

Could you explain why the following code doesn't compile? An obvious workaround is to add a 1-argument overload of Apply, is there a simpler one?

template <typename T>
T Identity(const T& i_val)
  {
  return i_val;
  }

template <typename Val, typename Fn>
Val Apply(const Val& v, Fn fn = Identity<Val>)
  {
  return fn(v);
  }

int main() {
  Apply(42);              // error: no matching function for call to 'Apply(int)'
  Apply(42, Identity<int>); // OK
  return 0;
}

Answer 1

Template argument deduction doesn't work that way -- you can't deduce the type of an argument from a defaulted value. In C++11, you can however specify a default template argument:

template <typename Val, typename Fn = Val(&)(Val const &)>
Val Apply(const Val& v, Fn fn = Identity<Val>)
{
   return fn(v);
}