Question

Why am I getting an error when I try this in Visual C++?

struct S { };
typedef S *S;

Doesn’t C++ let you typedef a name that was previously only declared as a class or struct?
Or am I misunderstanding what’s going on?

Answer 1

C++ does allow you to typedef an existing class name, but only in a very restricted way. Once you declared your struct S you can do

typedef struct S S;

but you cannot do

typedef struct S *S; // ERROR

or

typedef int S; // ERROR

In the first case the fact that you are redefining S as a pointer is what breaks it.

The language specification says in 7.1.3/2

In a given non-class scope, a typedef specifier can be used to
redefine the name of any type declared in that scope to refer to the
type to which it already refers.

“Already refers” is the key part. In simple words, you can redefine a class name to stand for the same class type, but not for a pointer to the class type or anything else.

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The aforementioned part of C++ standard is also what allows you to write repetitive typedefs like

typedef int T;
typedef int T;
typedef int T;

which would be illegal in C.