is the `synonym` in a `typedef` mandatory?

Question

I encountered this in a code review:

typedef struct C { int i; };

It compiles.

Apart from it being C-style, where structs are in a separate 'namespace', and need to be typedeffed in order to use later on, I found it weird that there is nothing defined with this typedef...

So: doesn't typedef require a type argument and an alias argument?

Answer 1

A quick glance over n3225 doesn't show any requirement that a name is required to be present. The text that comes the most near just says

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.

So your code seems to be valid, but I think it smells bad.

Answer 2

Yes, typedef requires another argument. That typedef won't do anything.

Anyway it's not an error, just a deprecate usage of typedef.

---

C standard (n1256 in 6.7.7) talks about typedef. It doesn't say this usage is deprecated, but compilers report it is, since such statement has no effect.

Answer 3

Thank you for testing your code with Comeau C/C++!
Tell others about http://www.comeaucomputing.com/tryitout !

Your Comeau C/C++ test results are as follows:


Comeau C/C++ 4.3.10.1 (Oct  6 2008 11:28:09) for ONLINE_EVALUATION_BETA2
Copyright 1988-2008 Comeau Computing.  All rights reserved.
MODE:strict errors C++ C++0x_extensions

"ComeauTest.c", line 1: warning: declaration requires a typedef name
  typedef struct C { int i; };
                             ^