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phpboolean
Matthew Ruddy
Matthew Ruddy

Reputation: 925

PHP converting to boolean using '!!'

Fairly straight-forward question here. Been looking over some code and I've seen a function that seems to convert a given variable to a boolean. It goes like this:

function to_bool( $var ) {
    return !!$var;
}

Pretty simple, but how does it work? Never seen this before, and googling hasn't really gotten me anywhere. Does the extra '!' sort-of flip the result? '!$var' can be used to check if a var is false, so does '!!' turn a 'false' to true, and vice versa?

Upvotes: 6

Views: 3869

Answers (5)

James M. Lay
James M. Lay

Reputation: 2470

You can also look at this from the perspective of type conversion. In PHP, performing a logical expression on a value will give a boolean result. So, the first negation implicitly casts the value as a bool, and the second-evaluated is just to reverse the negation of the first one. You can do the same thing using other logical operators (although I wouldn't recommend it, as some have different precedence):

// using "truthy" 2 as a value //

!!2          // true 
2 ||  false  // true
2 &&  true   // true
2 xor false  // true

// using "falsey" 0 as a value //

!!0         // false
0 ||  false // false
0 &&  true  // false
0 xor false // false

Upvotes: 0

Lews Therin
Lews Therin

Reputation: 10995

Take !!2

!2 evaluates to !(true)

which evaluates to !false end result true. The not boolean basically treats the right operand as a bool.

Upvotes: 3

Thomas Kelley
Thomas Kelley

Reputation: 10302

Hmmm... interesting.

Well, PHP follows a specific set of rules when determining whether a variable is "true" or "false".

Using certain operators will force that decision to be made. For example, when you add a string "a" and an integer 2 with the . operator, you'd get "a2", because the . operator's intent is to concatenate.

Here, the first ! means you're attempting to negate the variable. To negate anything means that you must first be treating it as a boolean, so that's what it does. Then the second ! just negates it back.

It's a weird way of doing it. Frankly, if ($var) would yield the same result as your if (to_bool($var)).

Upvotes: 2

webbiedave
webbiedave

Reputation: 48897

how does it work?

The not operator places the variable into a conditional. Therefore, the result is boolean. The second not flips its value.

It's more clear just to use an explicit cast in your code rather than such a function:

(bool)$var;

Upvotes: 10

Suhail Patel
Suhail Patel

Reputation: 13694

One ! would be an inverted boolean so two !! make a non-inverted boolean. It's a double negation of a variable and returned as a boolean type

Upvotes: 3

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