Passing function with default parameter value as template parameter

Question

I am a bit puzzled with the following behavior. I pass a function with two parameter, one having a default value, as a template parameter and call the function with one argument. Why does it fail to compile? And, what is the solution/workaround?

#include <iostream>
using namespace std;

template<typename Function> 
void eval(Function function) {
    function(10);
}

void sum(int i, int j = 0) {
    cout << "Sum is " << i + j;
}

int main() {
    sum(10);    // OK, of course :)
    eval(sum);  // Error!
}

Note that this question is not about calling a templated function with default parameter.

The error message:

prog.cpp: In instantiation of 'void eval(Function) [with Function = void (*)(int, int)]':
prog.cpp:15:10:   required from here
prog.cpp:6:10: error: too few arguments to function
  function(10);
          ^

Answer 1

That's because the optional parameter is part of function declaration. When you call using a function pointer, essentially all the compiler knows is the type of the function pointer. So this line function(10) roughly translates to:

void (*)(int, int) sm = function; // the equivalent happens at the type deduction step 
sm(10); // whoops, where is the second int

The compiler will need the second argument because it has no way of knowing whether sm is pointing to sum which has a default argument, or some other void foo(int a, int b) which doesn't have a default argument.