default parameters without name in c ++

Question

Consider the functions:

void foo(int a = 3)//named default parameter
void foo1(int = 3)//unnamed default parameter

I understand the need of the first function.(The value of "a" is 3 and it can be used in the program). But the 2nd function(which is not an error) has 3 initialized to nothing. How exactly do i use this value , if i can use this value...

Answer 1

In function declaration/definition, a parameter may have or have not a name, this also applies to a parameter with default value.

But to use a parameter inside a function, a name must be provided.

Normally when declare a function with default parameter

// Unnamed default parameter. 
void foo1(int = 3);

In function definition

void foo1(int a)
{
   std::cout << a << std::endl;
}

Then you can call

foo1();   // the same as call foo1(3)
foo1(2);

Answer 2

I've found the 2nd declaration very useful when declaring an empty virtual method of a base class

virtual void doSomething(bool = false) {};

I don't want to make it pure virtual since in many derived classes I don't want to re-implement it, so I leave it empty in the base class. But I need the default value false when implementing this method in some other derived classes.

virtual void doSomething(bool en = false) {};

This is not wrong but the compiler gives the unused parameter warning