A 'for' loop that appears to be practically infinite

Question

I'm debugging some code at the moment, and I've come across this line:

for (std::size_t j = M; j <= M; --j) 

(Written by my boss, who's on holiday.)

It looks really odd to me.

What does it do? To me, it looks like an infinite loop.

Answer 1

std::size_t is guaranteed by the C++ standard to be a unsigned type. And if you decrement an unsigned type from 0, the standard guarantees that the result of doing that is the largest value for that type.

That wrapped-around value is always greater than or equal to M¹ so the loop terminates.

So j <= M when applied to an unsigned type is a convenient way of saying "run the loop to zero then stop".

Alternatives such as running j one greater than you want, and even using the slide operator for (std::size_t j = M + 1; j --> 0; ){ exist, which are arguably clearer although require more typing. I guess one disadvantage though (other than the bewildering effect it produces on first inspection) is that it doesn't port well to languages with no unsigned types, such as Java.

Note also that the scheme that your boss has picked "borrows" a possible value from the unsigned set: it so happens in this case that M set to std::numeric_limits<std::size_t>::max() will not have the correct behaviour. In fact, in that case, the loop is infinite. (Is that what you're observing?) You ought to insert a comment to that effect in the code, and possibly even assert on that particular condition.

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¹ Subject to M not being std::numeric_limits<std::size_t>::max().