Why can't you increment/decrement a variable twice in the same expression?

Question

When I try to compile this code

int main() {
    int i = 0;
    ++(++i);
}

I get this error message.

test.c:3:5: error: lvalue required as increment operand
     ++(++i);
     ^

What is the error message saying? Is this something that gets picked up by the parser, or is it only discovered during semantic analysis?

Answer 1

++i will give an rvalue¹ after the evaluation and you can't apply ++ on an rvalue.

§6.5.3.1 (p1):

The operand of the prefix increment or decrement operator shall have atomic, qualified, or unqualified real or pointer type, and shall be a modifiable lvalue.

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_{1. What is sometimes called "rvalue" is in this International Standard described as the "value of an expression". - §6.3.2.1 footnote 64).}

Answer 2

A lvalue is a value you can write to / assign to.

You can apply ++ to i (i is modified) but you cannot apply ++ to the result of the previous ++ operator. I wouldn't have any effect anyway.

Aside: C++ allows that (probably because ++ operator returns a non-const reference on the modified value)