Dark corners of C/C++: The typedef keyword doesn’t need to be the first word on the line

Raymond Chen

Here are some strange but legal declarations in C/C++:

int typedef a;
short unsigned typedef b;

By convention, the typedef keyword comes at the beginning of the line, but this is not actually required by the language. The above declarations are equivalent to

typedef int a;
typedef short unsigned b;

The C language (but not C++) also permits you to say typedef without actually defining a type!

typedef enum { c }; // legal in C, not C++

In the above case, the typedef is ignored, and it’s the same as just declaring the enum the plain boring way.

enum { c };

Other weird things you can do with typedef in C:

typedef int;
typedef int short;

None of the above statements do anything, but they are technically legal in pre-C89 versions of the C language. They are just alternate manifestations of the quirk in the grammar that permits you to say typedef without actually defining a type. (In C89, this loophole was closed: Clause 6.7 Constraint 2 requires that “A declaration shall declare at least a declarator, a tag, or the members of an enumeration.”)

That last example of typedef int short; is particularly misleading, since at first glance it sounds like it’s redefining the short data type. But then you realize that int short and short int are equivalent, and this is just an empty declaration of the short int data type. It doesn’t actually widen your shorts. If you need to widen your shorts, go see a tailor.¹

Note that just because it’s legal doesn’t mean it’s recommended. You should probably stick to using typedef the way most people use it, unless you’re looking to enter the IOCCC.

¹ The primary purpose of this article was to tell that one stupid joke. And it’s not even my joke!


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