In certain computer programming languages, the Elvis operator, often written ?:, is a binary operator that returns the evaluated first operand if that operand evaluates to a value likened to logically true (according to a language-dependent convention, in other words, a truthy value), and otherwise returns the evaluated second operand (in which case the first operand evaluated to a value likened to logically false, in other words, a falsy value). 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 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]
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.
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 conditional ternary operator:
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.
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Main article: null coalescing operator |
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" //, equivalent to defined $a ? $a : $b [4]?: is documented as a distinct operator;[7] this feature was added in Groovy 1.5[8] (December 2007). Groovy, unlike GNU C and PHP, does not simply allow the second operand of ternary ?: to be omitted; rather, binary ?: must be written as a single operator, with no whitespace in between.?: binary operator that compares its first operand with null.return, like this: val foo = bar() ?: return?: operator returns the right operand if the left is null as well.?. is referred to as the "Elvis operator",[11] but it does not perform the same function. Instead, the null-coalescing operator ?? does.?: syntax.isNonnull($a) ? $a : $b.[13]L ?: R returns the value of L if it's not nil. Otherwise, return the value of R.[14]??) 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]