Elvis operator
In certain computer programming languages, the Elvis operator, often written ?:
, or
or ||
, is a binary operator that returns its first operand if that operand evaluates to a true value, and otherwise evaluates and returns its second operand. 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 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 quiff.[1]
A similar operator is the null coalescing operator, where the check for boolean truthiness is replaced with a check for non-null instead. This is usually written ??
, and can be seen in languages like C#.[2]
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 a true value, 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 the f()
twice if it is true.
Object reference variant
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:
x = f() ?: "default value"
Languages supporting the Elvis operator
- In GNU C and C++ (that is: in C and C++ with GCC extensions), the second operand of the ternary operator is optional.[3] This has been the case since at least GCC 2.95.3 (March 2001), and seems to be the original elvis operator.[4]
- In Apache Groovy, the "Elvis operator"
?:
is documented as a distinct operator;[5] this feature was added in Groovy 1.5[6] (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.[7] (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.[8] A common pattern is to use it with
return
, like this:val foo = bar() ?: return
- 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",[9] 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.[10]
- In Google's Closure Templates, the Elvis operator is a null coalescing operator, equivalent to
isNonnull($a) ? $a : $b
.[11] - Swift supports this concept with its Nil-coalescing operator
??
,[12] e.g.(a ?? b)
. - SQL supports this concept with its COALESCE function, e.g.
COALESCE(a, b)
. - In Ballerina, the Elvis operator
L ?: R
returns the value ofL
if it's not nil. Otherwise, return the value ofR
.[13] - Clojure supports this concept with the
or
[14] macro, e.g.(or a b)
. In the case of Clojure, it is var-arg, and not binary, e.g.(or a b c d e)
will return the first non false value. - Dart language provides ?? operator which returns right side value if left side value is null
- TypeScript supports this concept with its nullish-coalescing operator
??
, e.g.(a ?? b)
, since v3.7.[15] - JavaScript supports this concept, same as TypeScript[16]
- Lua supports this concept with the
or
[17] logical operator, e.g.(a or b)
.
Analogous use of the short-circuiting OR operator
In several languages, such as Common Lisp, Clojure, Lua, 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 true in a boolean environment, and otherwise evaluating and returning its second operand. When the left hand side is true, 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 boolean.
See also
?:
or conditional operator, when used as a ternary operator- Safe navigation operator, often
?.
- Spaceship operator
<=>
- Option type
References
- Joyce Farrell. Java Programming. p. 276. ISBN 978-1285081953.
The new operator is called Elvis operator because it uses a question mark and a colon together (?:); if you view it sideways, it reminds you of Elvis Presley.
- "?? Operator". C# Reference. Microsoft. Retrieved 5 December 2018.
- "Using the GNU Compiler Collection (GCC): Conditionals". gcc.gnu.org.
- "Using and Porting the GNU Compiler Collection (GCC): C Extensions". gcc.gnu.org.
- "Elvis Operator (?: )".
- "The Apache Groovy programming language - Groovy 1.5 release notes". groovy-lang.org.
- "PHP: Comparison Operators - Manual". PHP website. Retrieved 2014-02-17.
- "Null Safety - Kotlin Programming Language". Kotlin.
- Albahari, Joseph; Albahari, Ben (2015). C# 6.0 in a Nutshell (6 ed.). O'Reilly Media. p. 59. ISBN 978-1491927069.
- Efftinge, Sven. "Xtend - Expressions". eclipse.org.
- "Closure Templates - Expressions". GitHub.
- "The Swift Programming Language (Swift 4.1): Basic Operators". developer.apple.com.
- "Elvis Operator - Ballerina Programming Language". Ballerina.
- "clojure.core or macro API reference".
- "nullish coalescing commit by Kingwl · Pull Request #32883 · microsoft/TypeScript". GitHub. Retrieved 2019-10-08.
- "Nullish coalescing operator (??)". mozilla. Retrieved 2021-01-28.
- "Lua or operator Reference".