Can C Reference Parameters Have Default Values?

Default Values for Reference Parameters in C

In C , it is common practice to pass parameters by reference to achieve efficient memory management. However, when passing a parameter by reference, it is often desirable to specify a default value for cases where the function is called without providing a specific argument.

Can We Set Default Values for Reference Parameters?

Unfortunately, C does not directly support providing default values to parameters passed by reference. Attempting to set a default value for a non-const reference parameter, as in the provided example, will result in an error.

The Reason Behind the Restriction

The restriction stems from C 's rule that a reference must be bound to an lvalue (a location in memory that can be modified). A default value, however, is created as a temporary object, which is not an lvalue.

Solution: Using a Const Reference

While it is not possible to provide default values for non-const reference parameters, C allows default values for const references. The const qualifier indicates that the underlying lvalue cannot be modified, making it compatible with temporary objects.

Example:

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

In this example, the const reference parameter State can be assigned the default value 0 upon function invocation without an argument.

Using a Non-Default Declared Instance

Another workaround for non-const reference parameters is to declare an actual instance and use it as the default. However, this approach has limited practical applications.

Example:

static int AVAL = 1;

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

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

While this solution allows specifying a default value for the non-const reference parameter, it requires the creation of an extra instance, which may not always be suitable.

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