Elvis operator
In certain computer programming languages, the Elvis operator, often written ?:, 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 signature hairstyle.[1]
A similar operator is the null coalescing operator, where the boolean truth check is replaced with a check for non-null instead. This is usually written ??, and can be seen in languages like C#.[2]
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 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.
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
- Perl since version v5.10 provides the Logical Defined Or operator:
//, equivalent todefined $a ? $a : $b[3] - In GNU C and C++ (that is: in C and C++ with GCC extensions), the second operand of the ternary operator is optional.[4] This has been the case since at least GCC 2.95.3 (March 2001), and seems to be the original Elvis operator.[5]
- In Apache Groovy, the "Elvis operator"
?:is documented as a distinct operator;[6] this feature was added in Groovy 1.5[7] (December 2007). Groovy, unlike GNU C and PHP, does not simply allow the second operand of ternary?:to be omitted; rather, binary?:must be written as a single operator, with no whitespace in between. - In PHP, it is possible to leave out the middle part of the ternary operator since PHP 5.3.[8] (June 2009).
- The Fantom programming language has the
?:binary operator that compares its first operand withnull. - In Kotlin, the Elvis operator returns its left-hand side if it is not null, and its right-hand side otherwise.[9] A common pattern is to use it with
return, like this:val foo = bar() ?: return - In Gosu, the
?:operator returns the right operand if the left is null as well. - In C#, the null-conditional operator,
?.is referred to as the "Elvis operator",[10] but it does not perform the same function. Instead, the null-coalescing operator??does. - In ColdFusion and CFML, the Elvis operator was introduced using the
?:syntax. - The Xtend programming language has an Elvis operator.[11]
- In Google's Closure Templates, the Elvis operator is a null coalescing operator, equivalent to
isNonnull($a) ? $a : $b.[12] - In Ballerina, the Elvis operator
L ?: Rreturns the value ofLif it's not nil. Otherwise, return the value ofR.[13] - In JavaScript, the nullish coalescing (
??) operator is a logical operator that returns its right-hand side operand when its left-hand side operand isnullorundefined, and otherwise returns its left-hand side operand.[14]
See also
?:or conditional operator, when used as a ternary operator- Safe navigation operator, often
?. - Spaceship operator
<=> - Option type
References
- ↑ Joyce Farrell (7 February 2013). 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". Microsoft. https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-coalescing-operator. Retrieved 5 December 2018.
- ↑ "perlop - Perl operators and precedence -Perldoc Browser". Perl.org. https://perldoc.perl.org/5.10.0/perlop#C-style-Logical-Defined-Or. Retrieved 2023-01-19.
- ↑ "Using the GNU Compiler Collection (GCC): Conditionals with omitted operands". https://gcc.gnu.org/onlinedocs/gcc/extensions-to-the-c-language-family/conditionals-with-omitted-operands.html.
- ↑ "Using and Porting the GNU Compiler Collection (GCC): C Extensions". https://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC70.
- ↑ "Elvis Operator (?: )". http://docs.groovy-lang.org/latest/html/documentation/index.html#_elvis_operator.
- ↑ "The Apache Groovy programming language - Groovy 1.5 release notes". http://groovy-lang.org/releasenotes/groovy-1.5.html.
- ↑ "PHP: Comparison Operators - Manual". PHP website. http://php.net/manual/en/language.operators.comparison.php#language.operators.comparison.ternary. Retrieved 2014-02-17.
- ↑ "Null Safety - Kotlin Programming Language". https://kotlinlang.org/docs/reference/null-safety.html#elvis-operator.
- ↑ 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". https://eclipse.org/xtend/documentation/203_xtend_expressions.html#elvis-operator.
- ↑ "Closure Templates - Expressions". 29 October 2021. https://github.com/google/closure-templates/blob/master/documentation/reference/expressions.md#null-coalescing-operator--null-coalescing-operator.
- ↑ "Elvis Operator - Ballerina Programming Language". https://ballerina.io/learn/by-example/elvis-operator.html.
- ↑ "Nullish coalescing operator (??) - JavaScript | MDN" (in en-US). https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing.
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