Question

I have to add a default int& argument (Eg iNewArgument) to an existing function definition in a header file:

What is the syntax to initialize the last argument to make it take a default value?

..
virtual int ExecNew
    (int& param1, int& RowIn,int& RowOut, char* Msg = '0', 
            bool bDebug=true, int& iNewArgument = NULL ) = 0 ;
.
.

NULL is #defined to 0

Error: default argument: cannot convert from int to int&

Answer 1

Your example indicates that you are creating a virtual function. Default arguments in virtual (especially pure) functions are not a good idea, because they are not considered when you call an overridden function.

struct A {
  virtual void f(int = 4711) = 0;
};

struct B : A {
  virtual void f(int) { /* ... */ }
};

Now, if someone calls f on a B object, he will have to provide the argument – the default isn’t used. You would have to duplicate the default argument in the derived class, which creates ugly redundancies. Instead, you can use the non-virtual interface pattern

struct A {
  void f(int a) {
    int out;
    f(a, out);
  }

  void f(int a, int &out) {
    doF(a, out);
  }

protected:
  virtual void doF(int a, int out&) = 0;
};

The user can now call f with one, and with two arguments, from whatever class type he calls the function, and the implementers of A don’t have to worry about the default argument. By using overloading instead of default arguments, you don’t need to break your head around lvalues or temporaries.