Why ++i++ gives “L-value required error” in C? [duplicate]

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Because postfix operators have higher precedence than prefix operators, so the expression ++i++ is equivalent to ++(i++), that is equivalent to ++( i + 1). The compiler gives l-value error because you are applying ++ on an expression (i++) that is not a modifiable lvalue, so not a valid expression in c according to increment operator definition.

According to Dennis M. Ritchie’s book: “The C Programming Language”:

2.8 Increment and Decrement Operators

(page 44)

The increment and decrement operators can only be applied to variables; an expression like (i + j)++ is illegal. The operand must be a modifiable lvalue of arithmetic or pointer type.

Related: An interesting bug one may like to know about in gcc 4.4.5 is that expression j = ++(i | i); compiles that should produce l-value error. Read: j = ++(i | i); and j = ++(i & i); should an error: lvalue?

Additionally, modifying same variable more then once in an expression without an intervening sequence point causes which is undefined behavior in c and c++. To understand read Explain these undefined behaviors in i = i++ + ++i;.

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