Question

struct t_empty {
};

This appears to compile properly in C++ but not C. (at least with the TI 28xx DSP compiler, where it emits the error “expected a declaration”) Is this mentioned somewhere in the C standards, or is my compiler broken?

Answer 1

Empty struct is a syntax error in C. The grammar of C language is written so that it prohibits empty structs. I.e. you won’t find it stated in the standard explicitly, it just follows from the grammar.

In C++ empty classes are indeed legal.

P.S. Note, that often you might see the quote from the C standard that says “If the struct-declaration-list contains no named members, the behavior is undefined.”, which is presented as the portion of the document that prohibits empty structs. In reality, empty structs are, again, prohibited by the grammar. So a literally empty struct (as in your question) is a syntax error, not undefined behavior. The above quote from the standard applies to a different situation: a struct with no named members. A struct can end up non-empty, but at the same time with no named members if all members are unnamed bitfields

struct S {
  int : 5;
};

In the above case the behavior is undefined. This is what the above quote is talking about.