Elvis operator

It has been suggested that this article be merged into Short-circuit evaluation. (Discuss) Proposed since November 2020.

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, or in the other direction, his smirk.^[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 with null.
• 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 of L if it's not nil. Otherwise, return the value of R.^[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, 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.

See also[]

• ?: or conditional operator, when used as a ternary operator
• Safe navigation operator, often ?.
• Spaceship operator <=>
• Option type

References[]