Typedef declaration in the form `int typedef my_int;`

Question

To declare my_int as a type alias for int we can write:

typedef int my_int;   // (1)

Curiously, the following also seems to define an int alias:

int typedef my_int;   // (2)

I have never seen such syntax before. Why does it work?

Answer 1

My reasoning after reading C++ reference is this: (1) and (2) are declarations of the form

specifiers-and-qualifiers declarators-and-initializers;

with specifiers-and-qualifiers being either typedef int or int typedef.

The order of specifiers and qualifiers doesn't matter, and both (1) and (2) are valid declarations of a type alias. For example, to define an alias for const int we can, in principle, use any of these 6 combinations:

typedef int const my_cint;
typedef const int my_cint;
int typedef const my_cint;
const typedef int my_cint;
int const typedef my_cint;
const int typedef my_cint;

Answer 2

Indeed, your interpretation is correct as explained here:

The typedef specifier, when used in a declaration's decl-specifier-seq, specifies that the declaration is a typedef declaration, and declares typedef-names rather than functions or objects.

Nevertheless, in modern c++, the type alias are better defined with a using clause:

using my_int = int; 

It's not just a matter of style: typedef don't support templatization whereas type alias do:

template <typename T>
using my_list = list<T>;   // not possible with typedef
...
my_list<double> numbers;