Why Can\'t C Functions Have Default Values for Non-Const Reference Parameters?

Passing Parameters by Reference with Default Values in C

In C , when passing parameters by reference, it is not possible to assign a default value to the parameter. This raises an error when attempting to declare functions with default values for reference parameters. For instance, the following code will result in an error:

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

Error encountered:

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

Resolution

Default values can only be assigned to const references, not non-const references. This is due to C 's restriction that a temporary (such as the default value) cannot be bound to a non-const reference.

As a workaround, 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 approach has limited practical applicability.

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