Английская Википедия:Elvis operator
Шаблон:Short description
Шаблон:About
In certain computer programming languages, the Elvis operator, often written ?:
, is a binary operator that returns the evaluated first operand if that operand evaluates to a value likened to logically true (according to a language-dependent convention, aka. a truthy value), and otherwise returns the evaluated second operand (in which case the first operand evaluated to a value likened to logically false, aka, a falsy value) . This is identical to a short-circuit or with "last value" semantics. The notation of the Elvis operator was inspired by the ternary conditional operator, ? :
, since the Elvis operator expression A ?: B
is approximately equivalent to the ternary conditional expression A ? A : B
.
The name "Elvis operator" refers to the fact that when its common notation, ?:
, is viewed sideways, it resembles an emoticon of Elvis Presley with his signature hairstyle.[1]
A similar operator is the null coalescing operator, where the boolean truth(yness) check is replaced with a check for non-null instead. This is usually written ??
, and can be seen in languages like C#[2] or Dart[3]
Alternative syntaxes
In several languages, such as Common Lisp, Clojure, Lua, Object Pascal, Perl, Python, Ruby, and JavaScript, the OR operator (typically ||
or or
) has the same behavior as the above: returning its first operand if it would evaluate to a truthy value, and otherwise evaluating and returning its second operand, which may be a truthy or falsy value. When the left-hand side is truthy, the right-hand side is not even evaluated; it is "short-circuited". This is different than the behavior in other languages such as C/C++, where the result of ||
will always be a (proper) boolean.
Example
Boolean variant
In a language that supports the Elvis operator, something like this:
x = f() ?: g()
will set x
equal to the result of f()
if that result is truthy, and to the result of g()
otherwise.
It is equivalent to this example, using the conditional ternary operator:
x = f() ? f() : g()
except that it does not evaluate f()
twice if it yields truthy. Note the possibility of arbitrary behaviour if f()
is not a state-independent function that always returns the same result.
Object reference variant
Шаблон:Main
This code will result in a reference to an object that is guaranteed to not be null. Function f()
returns an object reference instead of a boolean, and may return null, which is universally regarded as falsy:
x = f() ?: "default value"
Languages supporting the Elvis operator
- Perl since version v5.10 provides the Logical Defined Or operator:
//
, equivalent todefined $a ? $a : $b
[4] - In GNU C and C++ (that is: in C and C++ with GCC extensions), the second operand of the ternary operator is optional.[5] This has been the case since at least GCC 2.95.3 (March 2001), and seems to be the original Elvis operator.[6]
- In Apache Groovy, the "Elvis operator"
?:
is documented as a distinct operator;[7] this feature was added in Groovy 1.5[8] (December 2007). Groovy, unlike GNU C and PHP, does not simply allow the second operand of ternary?:
to be omitted; rather, binary?:
must be written as a single operator, with no whitespace in between. - In PHP, it is possible to leave out the middle part of the ternary operator since PHP 5.3.[9] (June 2009).
- The Fantom programming language has the
?:
binary operator that compares its first operand withnull
. - In Kotlin, the Elvis operator returns its left-hand side if it is not null, and its right-hand side otherwise.[10] A common pattern is to use it with
return
, like this: Шаблон:Code - In Gosu, the
?:
operator returns the right operand if the left is null as well. - In C#, the null-conditional operator,
?.
is referred to as the "Elvis operator",[11] but it does not perform the same function. Instead, the null-coalescing operator??
does. - In ColdFusion and CFML, the Elvis operator was introduced using the
?:
syntax. - The Xtend programming language has an Elvis operator.[12]
- In Google's Closure Templates, the Elvis operator is a null coalescing operator, equivalent to
isNonnull($a) ? $a : $b
.[13] - In Ballerina, the Elvis operator
L ?: R
returns the value ofL
if it's not nil. Otherwise, return the value ofR
.[14] - In JavaScript, the nullish coalescing (
??
) operator is a logical operator that returns its right-hand side operand when its left-hand side operand isnull
orundefined
, and otherwise returns its left-hand side operand.[15]
See also
?:
or conditional operator, when used as a ternary operator- Safe navigation operator, often
?.
- Spaceship operator
<=>
- Option type
References