错误：左值需要作为增量操作数

Question

I tried the following code:

int a = ++2;

I am aware that a constant cannot be modified, but I cannot understand the error that the compiler outputs:

*  error: lvalue required as increment operand 

What exactly does it mean?

Answer 1

An expression is considered an "lvalue" when it designates a modifiable location in memory. Operators which modify a value require that their operand is an lvalue.

Assignment operators require that the left-hand side is an lvalue. This is where this jargon originates from. Postfix and prefix increment/decrement operators also have this requirement for their single operand.

An lvalue expression may not be just a variable name, but also a more complex expression, for example:

*(p + 2); // the unary * operator produces an lvalue
*(p + 2) = 5; // so that this is possible
(*(p + 2))++; // or this

Answer 2

What exactly it means is that

1. ++ is by definition an increment operator (an operator that increments)
2. The thing it operates on is called the increment operand
3. This thing is required to be an lvalue (something that amounts to a pointer)
4. In this case 2 is the increment operator and it is not an lvalue .