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 to defined $a ? $a : $b [3]
  • In GNU C and C++ (that is: in C and C++ with GCC extensions), the second operand of the ternary operator is optional.[4] This has been the case since at least GCC 2.95.3 (March 2001), and seems to be the original Elvis operator.[5]
  • In Apache Groovy, the "Elvis operator" ?: is documented as a distinct operator;[6] this feature was added in Groovy 1.5[7] (December 2007). Groovy, unlike GNU C and PHP, does not simply allow the second operand of ternary ?: to be omitted; rather, binary ?: must be written as a single operator, with no whitespace in between.
  • In PHP, it is possible to leave out the middle part of the ternary operator since PHP 5.3.[8] (June 2009).
  • The Fantom programming language has the ?: binary operator that compares its first operand with null.
  • In Kotlin, the Elvis operator returns its left-hand side if it is not null, and its right-hand side otherwise.[9] A common pattern is to use it with return, like this: val foo = bar() ?: return
  • In Gosu, the ?: operator returns the right operand if the left is null as well.
  • In C#, the null-conditional operator, ?. is referred to as the "Elvis operator",[10] but it does not perform the same function. Instead, the null-coalescing operator ?? does.
  • In ColdFusion and CFML, the Elvis operator was introduced using the ?: syntax.
  • The Xtend programming language has an Elvis operator.[11]
  • In Google's Closure Templates, the Elvis operator is a null coalescing operator, equivalent to isNonnull($a) ? $a : $b.[12]
  • In Ballerina, the Elvis operator L ?: R returns the value of L if it's not nil. Otherwise, return the value of R.[13]
  • In JavaScript, the nullish coalescing (??) operator is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined, and otherwise returns its left-hand side operand.[14]

See also

References

  1. ↑ 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.
  2. ↑ "?? Operator". C# Reference. Microsoft. Retrieved 5 December 2018.
  3. ↑ "perlop - Perl operators and precedence -Perldoc Browser". Perl.org. Retrieved 2023-01-19.
  4. ↑ "Using the GNU Compiler Collection (GCC): Conditionals with omitted operands". gcc.gnu.org.
  5. ↑ "Using and Porting the GNU Compiler Collection (GCC): C Extensions". gcc.gnu.org.
  6. ↑ "Elvis Operator (?: )".
  7. ↑ "The Apache Groovy programming language - Groovy 1.5 release notes". groovy-lang.org.
  8. ↑ "PHP: Comparison Operators - Manual". PHP website. Retrieved 2014-02-17.
  9. ↑ "Null Safety - Kotlin Programming Language". Kotlin.
  10. ↑ Albahari, Joseph; Albahari, Ben (2015). C# 6.0 in a Nutshell (6 ed.). O'Reilly Media. p. 59. ISBN 978-1491927069.
  11. ↑ Efftinge, Sven. "Xtend - Expressions". eclipse.org.
  12. ↑ "Closure Templates - Expressions". GitHub. 29 October 2021.
  13. ↑ "Elvis Operator - Ballerina Programming Language". Ballerina. Archived from the original on 2018-12-20. Retrieved 2018-12-19.
  14. ↑ "Nullish coalescing operator (??) - JavaScript | MDN". developer.mozilla.org. Retrieved 2023-01-05.
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