将变量分配给前增量变量和后增量变量？

Question

What's the difference between the following two assignments?

#include<iostream>

using namespace std;
int main(){
    int a=10,i=0;
    ++i = a //COMPILES WITHOUT ERROR
    i++ = a //GIVES AN ERROR LVALUE NEEDED
}

Why does the second assignment produce error?

Answer 1

++i returns the new value of i after the incrementation. That value is an lvalue , called i in this case. Modifying i is certainly allowed.

But i++ returns the old value of i before the incrementation. That value is an rvalue , ie an unnamed temporary value. Modifying an rvalue is not allowed in C++.

Answer 2

Pre-increment operation returns its argument ( i ) already incremented by one. The returned thing is a variable and you can assign to it.

Post-increment returns an old value of i - an rvalue , that cannot be assigned to.

See this question for implementation of operator++ in C++.