Is !! a safe way to convert to bool in C++?

问题:

[This question is related to but not the same as this one.]

If I try to use values of certain types as boolean expressions, I get a warning. Rather than suppress the warning, I sometimes use the ternary operator (?:) to convert to a bool. Using two not operators (!!) seems to do the same thing.

Here's what I mean:

typedef long T;       // similar warning with void * or double
T t = 0;
bool b = t;           // performance warning: forcing 'long' value to 'bool'
b = t ? true : false; // ok
b = !!t;              // any different?

So, does the double-not technique really do the same thing? Is it any more or less safe than the ternary technique? Is this technique equally safe with non-integral types (e.g., with void * or double for T)?

I'm not asking if !!t is good style. I am asking if it is semantically different than t ? true : false.

回答1:

The argument of the ! operator and the first argument of the ternary operator are both implicitly converted to bool, so !! and ?: are IMO silly redundant decorations of the cast. I vote for

b = (t != 0);

No implicit conversions.

回答2:

Alternatively, you can do this: bool b = (t != 0)

回答3:

Careful!

• A boolean is about truth and falseness.
• An integer is about whole numbers.

Those are very distinct concepts:

• Truth and falseness is about deciding stuff.
• Numbers are about counting stuff.

When bridging those concepts, it should be done explicitly. I like Dima's version best:

b = (t != 0);

That code clearly says: Compare two numbers and store the truth-value in a boolean.

回答4:

All valid techniques, all will generate the same code.

Personally, I just disable the warning so I can use the cleanest syntax. Casting to a bool is not something I'm worried about doing accidentally.

回答5:

Yes it is safe.

0 is interpreted as false, everthing else is true,
hence !5 comes out as a false
!0 comes out as true
so !!5 comes out as true

回答6:

I would not use:

bool b = !!t;

That is the least readable way (and thus the hardest to maintain)

The others depend on the situation.
If you are converting to use in a bool expression only.

bool b = t ? true : false;
if (b)
{
    doSomething();
}

Then I would let the language do it for you:

if (t)
{
    doSomething();
}

If you are actually storing a boolean value. Then first I would wonder why you have a long in the first places that requires the cast. Assuming you need the long and the bool value I would consider all the following depending on the situation.

bool  b = t ? true : false;      // Short and too the point.
                                 // But not everybody groks this especially beginners.
bool  b = (t != 0);              // Gives the exact meaning of what you want to do.
bool  b = static_cast<bool>(t);  // Implies that t has no semantic meaning
                                 // except as a bool in this context.

Summary: Use what provides the most meaning for the context you are in.
Try and make it obvious what you are doing

回答7:

I recommend never suppressing that warning, and never using a c cast (bool) to suppress it. The conversions may not always be called as you assume.

There is a difference between an expression that evaluates to true and a boolean of that value.

Both !! and ternary take getting used to, but will do the job similarly, if you do not want to define internal types with overloaded casts to bool.

Dima's approach is fine too, since it assigns the value of an expression to a bool.

回答8:

If you're worried about the warning, you can also force the cast: bool b = (bool)t;

回答9:

I really hate !!t!!!!!!. It smacks of the worst thing about C and C++, the temptation to be too clever by half with your syntax.

bool b(t != 0); // Is the best way IMHO, it explicitly shows what is happening.

回答10:

!! may be compact, but I think it is unnecessarily complicated. Better to disable the warning or use the ternary operator, in my opinion.

回答11:

I would use b = (0 != t) -- at least any sane person can read it easily. If I would see double dang in the code, I would be pretty much surprised.

回答12:

Disable the warning.

Write for clarity first; then profile; then optimize for speed, where required.

回答13:

!! is only useful when you're using a boolean expression in arithmetic fashion, e.g.:

c = 3 + !!extra; //3 or 4

(Whose style is a different discussion.) When all you need is a boolean expression, the !! is redundant. Writing

bool b = !!extra;

makes as much sense as:

if (!!extra) { ... }

回答14:

Comparison to 0 doesn't work so well. Which comes back -- why !! vs. ternary?

class foo { public: explicit operator bool () ; };

foo f;

auto a = f != 0; // invalid operands to binary expression ('foo' and 'int')
auto b = f ? true : false; // ok
auto c = !!f; // ok

回答15:

The double not feels funny to me and in debug code will be very different than in optimized code.

If you're in love with !! you could always Macro it.

#define LONGTOBOOL(x) (!!(x))

(as an aside, the ternary operator is what I favor in these cases)

回答16:

I recommend to use

if (x != 0)

or

if (x != NULL)

instead of if(x); it's more understandable and readable.

回答17:

I would use bool b = t and leave the compile warning in, commenting on this particular line's safety. Disabling the warning may bite you in the butt in another part of the code.