It's a unary operator, kind of like ~ or !, which can apply to parenthesis (), until it meets a type, then the parenthesis are mandatory: int x; sizeof

It's a unary operator, kind of like ~ or !, which can apply to parenthesis (), until it meets a type, then the parenthesis are mandatory: int x; sizeof x; // okay sizeof (x); // okay sizeof int; // not okay sizeof (int); // okay It's a weird double rule to parse, but even worse, the strongest binding precedence typically doesn't like an alpha identifier at that point in the recursive descent, so you get weird rules like: read_prefix(): is peak() '!': ... is peak() '~": ... is alpha(peak()) maybe alpha is sizeof? wait, it wasn't sizeof? it was an identifier? long rewind is peak() '+': ... Something like @ would've been a good pick for sizeof. \ramble on (actually, better yet, it should've been used instead of & for taking the address). Imagine: int x; int* p = @x; \ramble off