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
#inclide <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

Put the default value in the declaration, not the definition.

class ClassA {
public:
    void Enable( bool enable = true );
};

Answer 2

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

Answer 3

Would this only happen when the method is called from within files that include the ClassA.cpp?

That's correct. But note that doing so will almost certainly produce multiple definition errors, so the default is only really available from its point of definition within ClassA.cpp.

Answer 4

Let me admit first that this is the first time I have seen this type of code. Putting a default value in header file IS the normal practice but this isn't.

My guess is that this default value can only be used from code written in the same file and this way the programmer who wrote this wanted to put it in some type of easiness in calling the function but he didn't want to disturb the interface (the header file) visible to the outside world.