Can C Functions Have Default Values for Non-Const Reference Parameters?

Default Value for Referenced Parameter in C

When passing a parameter by reference in C , can you specify a default value for it?

Consider the following function declaration:

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

Attempting to compile this code results in 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

The Answer

Default values can only be assigned to constant references, not non-constant references. This is because C prohibits binding a temporary (in this case, the default value) to a non-constant reference.

To work around this constraint, you can use an actual instance as the default value:

static int AVAL = 1;

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

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

However, this method has limited practical applicability.

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