Question

Below code gives error on compilation in a C compiler

++(-i);
error: lvalue required as increment operand

It means that -i returns rvalue.

while code

++(+i);

don’t give any error . Why so? this link says that +i don’t result in lvalue.

Answer 1

It is a glitch in your compiler. In C language all lvalues in expressions are converted to rvalues even before any operators are applied, with the exception of operands of sizeof, &, ++, -- and left-hand sides of . and assignment (see 6.3.2/2)

In other words, in C language +i must produce an rvalue not because unary + supposedly produces an rvalue, but rather because i is converted to rvalue before that unary + even gets a chance to do its thing.

For example, for an int i variable that holds value 42, expression +i is fully equivalent to expression +42. The lvalueness of i is lost and it gets turned into 42 before the semantics of unary + comes into play.

Needless to say, in this case there’s no chance the result of unary + can be an lvalue.