Why does C++ allow unnamed function parameters?

Question

The following is a perfectly legal C++ code

void foo (int) {     cout << "Yo!" << endl; }  int main (int argc, char const *argv[]) {     foo(5);      return 0; } 

I wonder, if there a value to ever leave unnamed parameters in functions, given the fact that they can't be referenced from within the function.

Why is this legal to begin with?

Answer 1

Yes, this is legal. This is useful for implementations of virtuals from the base class in implementations that do not intend on using the corresponding parameter: you must declare the parameter to match the signature of the virtual function in the base class, but you are not planning to use it, so you do not specify the name.

The other common case is when you provide a callback to some library, and you must conform to a signature that the library has established (thanks, Aasmund Eldhuset for bringing this up).

There is also a special case for defining your own post-increment and post-decrement operators: they must have a signature with an int parameter, but that parameter is always unused. This convention is bordering on a hack in the language design, though.

Answer 2

Of course not naming a parameter is legal when just declaring the function, but it's also legal in the implementation. This last apparently strange version is useful when the function needs to declare the parameter to have a specific fixed signature, but the parameter is not needed.

This may happen for example for a method in a derived class, for a callback function or for a template parameter.

Not giving the parameter a name makes clear that the parameter is not needed and its value will not be used. Some compilers if you instead name a parameter and then simply don't use it will emit a warning that possibly there is a problem with the function body.