Is there any difference in equality checks on right or left changing value in if condition

Question

Is there any difference between the following two code snippets?

if(x==4)
{

}

and

if(4 == x)
{

}

If so is there any great impact in performance?

Answer is somewhat dependent on the type of `x`, and on whether `==` is overloaded. — , Jul 08 '18 at 13:55
What specifically stops you from writing two simple test programs, and timing how long they take to run? — Sam Varshavchik, Jul 08 '18 at 13:58
@SamVarshavchik: Because that wouldn't answer the question. It would only provide an answer for that specific example. On that specific compiler. Running on that specific machine. — Nicol Bolas, Jul 08 '18 at 14:11
Putting 4 on the left was an old hack to protect against accidentally using = instead ==. Compilers now generate a decent warning for this mistake (try it) so use the syntax that makes most sense to you. — Hans Passant, Jul 08 '18 at 14:17

score 2 · Answer 1 · answered Jul 08 '18 at 14:11

Can there be a difference? Yes; if one of the types involved in the comparison is user-defined, then the writer of that type can make their operator== overload not be commutative.

However, such code should be considered ill-behaved. Users expect equality testing to be commutative. And in C++20, it will be much easier to write a commutative equality test than a non-commutative one.

So while the compiler will not stop you, it is reasonably safe to assume that equality testing will be commutative.

Shlublu · Accepted Answer · 2018-07-08T14:18:31.473

In the most common case (read: "should you not be using an overloaded operator == of yours that would make it different, and should the type of x be comparable to 4") these two snippets have similar performances: both terms are going to be evaluated and then compared.

However there are differences, at least in the approach. For example, should you write by accident:

int x = 2;

if(x = 4) // = instead of ==, typed by mistake
{
    stuff();
}

this will compile (you might have warnings, but no error), and at run time, this will put 4 into x at test time. As x won't be equal to zero, the content of the block will be executed: stuff() will be called. So there will be a bug as your intent was not to change x and not to have the content of the block executed.

But if you write this:

int x = 2;

if(4 = x) // = instead of ==, typed by mistake
{
    stuff();
}

this code will not compile as 4 is a rvalue that cannot be used at the left part of such an assignment.

Is there any difference in equality checks on right or left changing value in if condition

2 Answers2