default parameters without name in c ++

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);