How Can I Use Default Values with Pass-by-Reference Parameters in C ?

Passing Parameters by Reference with Default Values in C

In C , function parameters can be passed both by value and by reference. When passing by value, a copy of the parameter's value is made and this copy is passed to the function. When passing by reference, the function operates directly on the memory address of the parameter.

When passing a parameter by reference, it is not possible to specify a default value in the same way as you can for parameters passed by value. This is because a default value is initialized when the function is defined, but a reference parameter is only initialized when the function is called with arguments passed in.

To illustrate, consider the following function declaration:

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

Here, the "State" parameter is declared as a reference to a ULONG and the "sequence" parameter is declared as a boolean. However, assigning a default value to "State" ("= 0") causes 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

This error occurs because a non-const reference cannot be bound to a non-lvalue (i.e., a temporary expression). However, it is possible to specify a default value for a const reference:

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

In this case, the "State" parameter is a const reference to a ULONG and the default value ("= 0") is a const lvalue. This allows the function to be called without having to specify a value for "State".

One exception to this rule is when using a static variable as the default value for a non-const reference:

static int AVAL = 1;

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

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

In this example, the "x" parameter is a non-const reference to an int and the default value ("AVAL") is a static variable. This allows the function to be called without having to specify a value for "x". However, this technique is of limited practical use.

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