C++ Help: Error: lvalue required as increment operand

Question

What is wrong here. What does it mean by the error: lvalue required as increment operand? (NOTE: this is a textbook example)

    #include <iostream>
    using namespace std;

   int main()
  {
    int num1 = 0, num2 = 10, result;
    
    num1++;
    
    result = ++(num1 + num2);
    
    cout << num1 << " " << num2 << " " << result;

    return 0;
}

Answer 1

The ++x is called preincrement operator while x++ is called postincrement. Both need a modifiable 'lvalue' variable as the operand. In this case x is the 'lvalue'.

If you have a code y = ++x it has the same semantic meaning as

x = x + 1;
y = x;

So this specific code ++(num1 + num2) is actually making the error because num1 + num2 is not a valid modifiable variable and the semantic of result = ++(num1 + num2) will be:

num1 + num2 = (num1 + num2) + 1; // this is invalid
result = num1 + num2;

You can fix it as:

int x = num1 + num2;
result = ++x;

Or the shorter version, that produces the same result:

++(result = num1 + num2)