#033 – prefix vs postfix increment/decrement operators

Consider

#include <iostream>

int main() {
	int x {9};
	int y {22};
	
	++x;
	y++;
	
	int myArray[3] = {1, 2, 3};
	
	for(int i = 0; i < 3; ++i) {
		std::cout << myArray[i] << " ";
	}
}

Seems fairly harmless. Despite achieving the same result (incrementing the operand), the prefix and postfix versions have slight differences.

In terms of the loop variable i, does it matter if we do i++ vs ++i?

Prefix increment

Prefix increment increments the variable first, then evaluates to the updated value.

#include<iostream>

int main()
{
	int oranges { 12 };

    std::cout<< ++oranges << '\\n';
    std::cout << oranges << '\\n';
}

// Output 
13
13

Standard stuff. This is what we expect to see. Notice how the operand is permanently modified.

Postfix increment

Consider the following

#include <iostream>

int main()
{
    int apples { 10 };

    std::cout << "Sold: " << apples++ << '\\n';
    std::cout << "Remaining: " << apples << '\\n';
}

// Output
Sold: 10
Remaining: 11

?? What happened? The value printed is the old value of apples, even though it’s been incremented.

Well, the postfix version does 3 things:

1. Creates a copy of the operand
2. Increments it
3. Returns the copy

NOTE:: Despite the examples using the increment prefix/postfix operators, the same applies to the prefix/postfix decrement operators

When to use which?

Since postfix increment copies the operand, use it when you specifically need the operand to remain unmodified.

Use prefix increment when you only want to increment the operand. E.g. as a loop variable.

However, prefer the prefix versions, as they are more performant and less likely to cause side effects.