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

Question

[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 su

Answer 1

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.

Answer 2

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

Answer 3

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

Answer 4

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.

Answer 5

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)

Answer 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