C language A is not an lvalue, how to interpret it

The assignment expression returns the result, for example, the return value of x=2*y is the same as the value assigned to x.

This self-addition and self-subtraction is actually very simple, consisting of a variable i and a self-addition symbol ++.

In the first case, the variable is in front, such as i++, the value of i is used first, and then 1 is added to the second case: the ++ sign is in front, for example: ++1, the value of i is added by 1 first, and then used.

If it is i++ first, then use the value of i to show 5, then i adds 1 to become 6 and then j --, the value of j used, shows 5, and then subtracts 1 to 4 again.

i: i adds 1 to 7 and then shows 7

j : j -1 becomes 3 before 3 is displayed

And there is a third one:

i++ is the same as the first one, use it first and then add 1, so it shows 7, then adds 1, and becomes 8, but it is not displayed.

j-- is also used first and then subtracted, so 3 is displayed, and then 1 is subtracted to become 2, but it is not displayed.

As for the last two, think about it yourself, the same reason.

In fact, just look at the first symbol, the first one is a variable, use it first and then calculate, the first one is ++ or --, it will be calculated and then used.

lvalue and rvalue precede compilation. In C, two values on either side of the assignment operator are declared, the left is called lvalue, and the right is called rvalue.

Definition: An lvalue is an expression that can be used as an lvalue if it can be referenced to an object, and the object is a piece of memory space that can be inspected and stored.

An rvalue refers to a reference to a piece of data stored in a memory address.

As can be seen from the above two definitions, lvalues actually refer to an object, and an object must have a name in our program or can be accessed by a name, so lvalues can be summarized as: lvalues mean that there must be a specific name in the program that references to this value. The rvalue refers to the content in the address, so the rvalue can be summarized as:

An rvalue means that there is no specific name in the program that references this value.

A, because the return result is the same as the A after the operation, ++A returns the real A, which can be re-assigned, so it can be used as an lvalue. A++ returns a before the operation, and at this time a has +1, the returned data is actually a in the past, it is copied from another place, not the real a, so it cannot be assigned, so it can only be an rvalue.

So a++; The order of execution is to copy the value of a first, perform the overall calculation, and then a=a+1.

The discussion was lively, but I don't think it's that complicated. ++a can be an lvalue because ++a is "a variable" - i.e., variable a after a=a+1. is a variable that acts as an lvalue of the expression.

A++ is not a variable, it is an expression - i.e. a+1, it has not yet calculated the value of a+1 and assigned it to a, so it cannot act as an lvalue - who has seen int a, b; And then a+1=9 or a+b=1000 or b+4=a+cos(b)? The meaning is not clear, so C is not allowed, and the expression cannot be assigned a value ......

A++ is a separate operation, that is, you can't perform the first half of the operation first, then perform other operations, and then execute the second half of the operation, so if A++ returns A, and A increments by 1, the returned A must be the self-incrementing ......

Isn't the execution order of a++ returned a first, and then a=a+1 This is correct, but the value of the expression a++ is the value before a, not the value after a+1, that is, the result of the expression a++ is no longer synchronized with a.

Since it is not synchronized with A, then it is not object A, and its address cannot be obtained, it is just a value, not an object, so it is an rvalue.

The order of operation of a++ is from right to left. That is, the value of a+1 is calculated first and assigned to a.

The so-called rvalue is a value that can only be placed to the right of the equal sign, that is, it can only be assigned to other variables, but cannot be assigned.

A, because the return result is the same as the A after the operation, ++A returns the real A, which can be reassigned.

A++ returns a before the operation, and at this time a is already +1, and the returned data is actually a from the past, which is copied from another place, not the real a, so it cannot be assigned.