Objective-C - Is !!BOOL Beneficial

I'm looking over the diffs submitted to a project by another developer, and they have a lot of code that does !!<some BOOL value>. In fact, this seems to be their standard pattern for implementing boolean getters and setters. Th开发者_C百科ey've implemented their code like:

- (BOOL) hasId {
    return !!hasId_;
}
- (void) setHasId:(BOOL) value {
    hasId_ = !!value;
}

I've never seen this pattern before, and am wondering if there is any benefit in using it. Is the double-negation doing anything useful?

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The double boolean operator just makes sure that the value returned is either a 1 or a 0. That's all : )

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! is a logical negation operator. So if setHasId: was passed, eg., 0x2 then the double negation would store 0x1.

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It is equivalent to:

hasId_ = value ? 1 : 0;

It is useful in some cases because if you do this:

BOOL x = y & MY_FLAG;

You might get 0 if MY_FLAG is set, because the result gets truncated to the size of a BOOL (8 bits). This is unexpected. For the same reasons, people sometimes prefer that BOOL is either 0 or 1 (so bit operations work as expected). It is usually unnecessary.

In languages with a built-in bool type such as C (as of C99) and C++, converting an integer to bool does this automatically.

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It makes more sense in some other cases for example where you are returning BOOL but don't want to put an if statement in.

- (BOOL)isMyVarSet
{
    return !!myVar;
}

In this case I can't just return myVar because it's not a BOOL (this is a very contrived example - I can't dig out a decent one from my projects).

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I've used this before and I believe:

if (!!myVar)

is equivalent to:

if (myVar != nil)

Basically, I use it to verify the value of SOMETHING.

I will admit... this is probably not the best practice or most-understood way to accomplish this goal.