What purpose to use "!!" inside the if statement

Question

I saw the code below recently:

std::error_code ec;

// --- some code

if (!!ec)
{
//error reaction
}

For what purpose the !! is used? The only idea is to force bool operator, but for what? May it have sense for some implementations of boost error types?

you can convert an integer to `0` or `1` by `!!`, but in a context where anyhow a `bool` is expected I see no reason for it. Where did you see it? — 463035818_is_not_an_ai, Sep 08 '20 at 17:46
@πάνταῥεῖ I've done exactly this to quench a compiler warning. — Mark Ransom, Sep 08 '20 at 17:48
My guess is that some doofus had the same misguided idea as I did when I posted my (wrong) answer stating that implicit conversion to bool in an if statement is not possible. Admittedly this is not such a common thing in most C++ code I've read so maybe it's also just for clarity. — Peter, Sep 08 '20 at 17:50
@idclev, I saw it in the code of tcp server based on boost asio. I have no possibility to share the code... — brandy, Sep 08 '20 at 17:58
It's a common pattern in JavaScript to convert a variable to a bool. It's largely unnecessary in C++, and fairly uncommon to see in C++. (Likewise with C.) — Eljay, Sep 08 '20 at 18:00
@Peter, possibly your guess about `explicit` is correct. For `if()` it is not required, but may have sense in other cases... — brandy, Sep 08 '20 at 18:04

Amir Kirsh · Answer 1 · 2020-09-08T18:30:30.513

The use of !! inside if statement is redundant, as inside an if a value may be contextually converted to bool even if the casting is marked as explicit.

However, in an assignment this trick may be a substitute for an explicit cast:

std::error_code ec;

// some code

// bool is_error = ec;   // compilation error, operator bool is explicit
bool is_error1 = !!ec;   // ok
bool is_error2 = (bool)ec; // better, one may argue
bool is_error3 = static_cast<bool>(ec); // to avoid C-style cast
bool is_error4 = ec.value() != 0; // most verbose, also possible

It might be that the if(!!ec) is a copy-paste from an assignment expression.

See also: When can I use explicit operator bool without a cast?

A side note: there might be of course a case where this trick is relevant inside if - in case a type implements operator! and lacks operator bool - but this is not the case with std::error_code.

and C++ cast `static_cast(ec)` to avoid C-cast ;) – Jarod42 Sep 08 '20 at 18:29 — Jarod42, Sep 08 '20 at 18:29
@Jarod42 added, for completeness – Amir Kirsh Sep 08 '20 at 18:32 — Amir Kirsh, Sep 08 '20 at 18:32

score 0 · Answer 2 · answered Sep 08 '20 at 18:12

0

It is a double negative. example:

int main()
{
    bool varTrue = true;
    std::cout << "varTrue = " << varTrue << std::endl;
    std::cout << "!varTrue = " << !varTrue << std::endl;
    std::cout << "!!varTrue = " << !!varTrue << std::endl;

}

Output

varTrue = 1
!varTrue = 0
!!varTrue = 1

It seems useless

answered Sep 08 '20 at 18:12

Americain Naif

71
1
3

Its about `std::error_code`... Possibly it may be used like this : `auto b = !!ec;`... – brandy Sep 08 '20 at 18:18
1

nobody said `varTrue` was a `bool`. In fact, it is a `std::error_code` in the question. – Wyck Sep 08 '20 at 18:23

What purpose to use "!!" inside the if statement

2 Answers2