Default value to a parameter while passing by reference in C++

Question

Is it possible to give a default value to a parameter of a function while we are passing the parameter by reference. in C++

For eg. when i try to declare a function like

virtual const ULONG Write(ULONG &State = 0, bool sequence = true);

when i do this it gives an error

error C2440: 'default argument' : cannot convert from 'const int' to 'unsigned long &' A reference that is not to 'const' cannot be bound to a non-lvalue

Answer 1

You can do it for a const reference, but not for a non-const one. This is because C++ does not allow a temporary (the default value in this case) to be bound to non-const reference.

One way round this would be to use an actual instance as the default:

static int AVAL = 1;

void f( int & x = AVAL ) {
   // stuff
} 

int main() {
     f();       // equivalent to f(AVAL);
}

but this is of very limited practical use.

Answer 2

I think not, and the reason is that default values are evaluated to constants and values passed by reference must be able to change, unless you also declare it to be constant reference.

Answer 3

No, it's not possible.

Passing by reference implies that the function might change the value of the parameter. If the parameter is not provided by the caller and comes from the default constant, what is the function supposed to change?

Answer 4

It has been said in one of the direct comments to your answer already, but just to state it officially. What you want to use is an overload:

virtual const ULONG Write(ULONG &State, bool sequence);
inline const ULONG Write()
{
  ULONG state;
  bool sequence = true;
  Write (state, sequence);
}

Using function overloads also have additional benefits. Firstly you can default any argument you wish:

class A {}; 
class B {}; 
class C {};

void foo (A const &, B const &, C const &);
void foo (B const &, C const &); // A defaulted
void foo (A const &, C const &); // B defaulted
void foo (C const &); // A & B defaulted etc...

It is also possible to redefine default arguments to virtual functions in derived class, which overloading avoids:

class Base {
public:
  virtual void f1 (int i = 0);  // default '0'

  virtual void f2 (int);
  inline void f2 () {
    f2(0);                      // equivalent to default of '0'
  }
};

class Derived : public Base{
public:
  virtual void f1 (int i = 10);  // default '10'

  using Base::f2;
  virtual void f2 (int);
};

void bar ()
{
  Derived d;
  Base & b (d);
  d.f1 ();   // '10' used
  b.f1 ();   // '0' used

  d.f1 ();   // f1(int) called with '0' 
  b.f1 ();   // f1(int) called with '0
}

There is only one situation where a default really needs to be used, and that is on a constructor. It is not possible to call one constructor from another, and so this technique does not work in that case.

Answer 5

You cannot use a constant literal for a default parameter for the same reason you cannot use one as a parameter to the function call. Reference values must have an address, constant references values need not (ie they can be r-values or constant literals).

int* foo (int& i )
{
   return &i;
}

foo(0); // compiler error.

const int* bar ( const int& i )
{
   return &i;
}

bar(0); // ok.

Ensure that you're default value has an address and you're fine.

int null_object = 0;

int Write(int &state = null_object, bool sequence = true)
{
   if( &state == &null_object )
   {
      // called with default paramter
      return sequence? 1: rand();
   }
   else
   {
      // called with user parameter
      state += sequence? 1: rand();
      return state;
   }
}

I've used this pattern a few times where I had a parameter that could be a variable or null. The regular approach is to have the user pass in a pointer this is case. They pass in a NULL pointer if they don't want you to fill in the value. I like to null object approach. It makes the callers life easier without terribly complicating the callee code.

Answer 6

There are two reasons to pass an argument by reference: (1) for performance (in which case you want to pass by const reference) and (2) because you need the ability to change the value of the argument inside the function.

I highly doubt that passing an unsigned long on modern architecture is slowing you down too much. So I'm assuming that you're intending to change the value of State inside the method. The compiler is complaining because the constant 0 cannot be changed, as it's an rvalue ("non-lvalue" in the error message) and unchangeable (const in the error message).

Simply put, you want a method that can change the argument passed, but by default you want to pass an argument that can't change.

If you don't actually intend to change State inside the method you can simply change it to a const ULONG&. But you're not going to get a big performance benefit from that, so I would recommend changing it to a non-reference ULONG. I notice that you are already returning a ULONG, and I have a sneaky suspicion that its value is the value of State after any needed modifications. In which case I would simply declare the method as so:

// returns value of State
virtual ULONG Write(ULONG State = 0, bool sequence = true);

Of course, I'm not quite sure what you're writing or to where. But that's another question for another time.

Answer 7

This is quite late for an answer (I won't repeat the info in the other sound answers), but there still is the old C way of providing optional arguments: a pointer that can be NULL when not present:

void write( int *optional = 0 ) {
    if (optional) *optional = 5;
}