Why is it allowed to have a declaration without a name? [duplicate]

Question

While reading the standard, I was really surprised to see that actually, the name is optional in a declaration:

struct Magic {};
int main() {
  int; // well-formed
  Magic; // well-formed
}

For those interested to read the standard, it's because in a simple-declaration, the init-declarator-list is optional. Here's a demo.

And so I really want to know why this is allowed. Why would I ever want to do that? It really makes no sense to me. My reasoning is that, because if it makes no sense, why allow it?

Answer 1

According to the C++ Standard (7 Declarations)

5 In a simple-declaration, the optional init-declarator-list can be omitted only when declaring a class (Clause 9) or enumeration (7.2), that is, when the decl-specifier-seq contains either a class-specifier, an elaborated-type-specifier with a class-key (9.1), or an enum-specifier....

Thus this code

int main() {
  int; // well-formed
  Magic; // well-formed
}

is ill-formed.

Moreover formally these ill-formed declarations are definitions according to the paragraph #2 of the section 3.1 Declarations and definitions and obviously as such do not make sense.