Elvis operator

In certain

, 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(iness) 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

:

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

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 to defined $a ? $a : $b ^[4]
• In GNU C and C++ (that is: in C and C++ with GCC extensions), the second operand of the ternary operator is optional.^[5] This has been the case since at least GCC 2.95.3 (March 2001), and seems to be the original Elvis operator.^[6]
• In
  Apache Groovy, the "Elvis operator" ?: is documented as a distinct operator;^[7] this feature was added in Groovy 1.5^[8]
  (December 2007). Groovy, unlike GNU C and PHP, does not simply allow the second operand of ternary ?: to be omitted; rather, binary ?: must be written as a single operator, with no whitespace in between.
• In PHP, it is possible to leave out the middle part of the ternary operator since PHP 5.3.^[9] (June 2009).
• The Fantom programming language has the ?: binary operator that compares its first operand with null.
• In Kotlin, the Elvis operator returns its left-hand side if it is not null, and its right-hand side otherwise.^[10] A common pattern is to use it with return, like this: val foo = bar() ?: return
• In Gosu, the ?: operator returns the right operand if the left is null as well.
• In
  null-coalescing operator
  ?? does.
• In
  CFML
  , the Elvis operator was introduced using the ?: syntax.
• The Xtend programming language has an Elvis operator.^[12]
• In Google's Closure Templates, the Elvis operator is a null coalescing operator, equivalent to isNonnull($a) ? $a : $b.^[13]
• In Ballerina, the Elvis operator L ?: R returns the value of L if it's not nil. Otherwise, return the value of R.^[14]
• 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.^[15]

See also

• ternary operator
• Safe navigation operator, often ?.
• Spaceship operator
  <=>
• Option type

References

1. 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. ^ "Conditional expressions". Dart Language. Google.
4. ^ "perlop - Perl operators and precedence -Perldoc Browser". Perl.org. Retrieved 2023-01-19.
5. ^ "Using the GNU Compiler Collection (GCC): Conditionals with omitted operands". gcc.gnu.org.
6. ^ "Using and Porting the GNU Compiler Collection (GCC): C Extensions". gcc.gnu.org.
7. ^ "Elvis Operator (?: )".
8. ^ "The Apache Groovy programming language - Groovy 1.5 release notes". groovy-lang.org.
9. ^ "PHP: Comparison Operators - Manual". PHP website. Retrieved 2014-02-17.
10. ^ "Null Safety - Kotlin Programming Language". Kotlin.
11. ISBN 978-1491927069
    .
12. ^ Efftinge, Sven. "Xtend - Expressions". eclipse.org.
13. ^ "Closure Templates - Expressions". GitHub. 29 October 2021.
14. ^ "Elvis Operator - Ballerina Programming Language". Ballerina. Archived from the original on 2018-12-20. Retrieved 2018-12-19.
15. ^ "Nullish coalescing operator (??) - JavaScript | MDN". developer.mozilla.org. Retrieved 2023-01-05.