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. The Elvis operator is a variant of the ternary conditional operator, ? : in the sense that the expression with the Elvis operator A ?: B is approximately equivalent to the expression with the ternary operator A ? A : B.

This article is about the use of a ?: operator as a binary operator. For use as a ternary operator, see ?:.

Some computer programming languages (e.g. C#) have different semantics for the ?? operator: instead of the first operand having to result in a boolean, it must result in an object reference.[1] If the resulting object reference is not null, it is returned. Otherwise the value of the second operand (which may also be null) is returned. This distinction is necessary because in C#, references are not implicitly convertible to a boolean.[2]

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.[3]

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.[4] This has been the case since at least GCC 2.95.3[5] (March 2001).
  • 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]
  • Swift supports this concept with its Nil-coalescing operator ??,[13] 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 of L if it's not nil. Otherwise, return the value of R.[14]
  • Clojure supports this concept with the or[15] 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.[16]

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."

See also

References
  1. "?? Operator". C# Reference. Microsoft. Retrieved 5 December 2018.
  2. "The bool type". C# 6.0 draft specification. Microsoft. Retrieved 5 December 2018.
  3. 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.
  4. "Using the GNU Compiler Collection (GCC): Conditionals". 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.
  13. "The Swift Programming Language (Swift 4.1): Basic Operators". developer.apple.com.
  14. "Elvis Operator - Ballerina Programming Language". Ballerina.
  15. "clojure.core or macro API reference".
  16. "nullish coalescing commit by Kingwl · Pull Request #32883 · microsoft/TypeScript". GitHub. Retrieved 2019-10-08.
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