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Programming language construct

Short circuit evaluation, minimal evaluation, or McCarthy evaluation (after John McCarthy) is the semantics of some Boolean operators in some programming languages in which the second argument is executed or evaluated only if the first argument does not suffice to determine the value of the expression: when the first argument of the AND function evaluates to false, the overall value must be false; and when the first argument of the OR function evaluates to true, the overall value must be true. In programming languages with lazy evaluation (Lisp, Perl, Haskell), the usual Boolean operators short circuit. In others (Ada, Java, Delphi), both short circuit and standard Boolean operators are available. For some Boolean operations, like exclusive or (XOR), it is impossible to short circuit, because both operands are always needed to determine a result. Short circuit operators are, in effect, control structures rather than simple arithmetic operators, as they are not strict. In imperative language terms (notably C and C++), where side effects are important, short circuit operators introduce a sequence point: they completely evaluate the first argument, including any side effects, before (optionally) processing the second argument. ALGOL 68 used proceduring to achieve user defined short circuit operators and procedures. The use of short circuit operators has been criticized as problematic:

In any programming language that implements short circuit evaluation, the expression x and y is equivalent to the conditional expression if x then y else x, and the expression x or y is equivalent to if x then x else y. In either case, x is only evaluated once. The generalized definition above accommodates loosely typed languages that have more than the two truth values True and False, where short circuit operators may return the last evaluated subexpression. This is called "last value" in the table below. For a strictly typed language, the expression is simplified to if x then y else false and if x then true else y respectively for the boolean case.

Short circuit logic, with or without side effects, have been formalized based on Hoare's conditional. A result is that non short circuiting operators can be defined out of short circuit logic to have the same sequence of evaluation.

Short circuiting can lead to errors in branch prediction on modern central processing units (CPUs), and dramatically reduce performance. A notable example is highly optimized ray with axis aligned box intersection code in ray tracing. Some compilers can detect such cases and emit faster code, but programming language semantics may constrain such optimizations. An example of a compiler unable to optimize for such a case is Java's Hotspot virtual machine (VM) as of 2012.