What if the default parameter value is defined in code not visible at the call site?

Question

I've found some strange code...

//in file ClassA.h:
class ClassA {
public:
    void Enable( bool enable );
};

//in file ClassA.cpp
#include <ClassA.h>
void ClassA::Enable( bool enable = true )
{
   //implementation is irrelevant
}

//in Consumer.cpp
#include <ClassA.h>
....
ClassA classA;
classA.Enable( true );

Obviously since Consumer.cpp only included ClassA.h and not ClassA.cpp the compiler will not be able to see that the parameter has a default value.

When would the declared default value of ClassA::Enable in the signature of the method implementation have any effect? Would this only happen when the method is called from within files that include the ClassA.cpp?

Answer 1

Default values are just a compile time thing. There's no such thing as default value in compiled code (no metadata or things like that). It's basically a compiler replacement for "if you don't write anything, I'll specify that for you." So, if the compiler can't see the default value, it assumes there's not one.

Demo:

// test.h
class Test { public: int testing(int input); };

// main.cpp
#include <iostream>
// removing the default value here will cause an error in the call in `main`:
class Test { public: int testing(int input = 42); };
int f();
int main() {
   Test t;
   std::cout << t.testing()  // 42
             << " " << f()   // 1000
             << std::endl;
   return 0;
}

// test.cpp
#include "test.h"
int Test::testing(int input = 1000) { return input; }
int f() { Test t; return t.testing(); }

Test:

g++ main.cpp test.cpp
./a.out