Logical OR

The || (Logical OR) operator is a binary operator in C++ that performs a logical disjunction on two expressions. It yields a prvalue of type bool, returning true if at least one of its operands evaluates to true, and false strictly when both operands evaluate to false.

expression1 || expression2

Operand Conversion

Before the operation is performed, both expression1 and expression2 are contextually converted to bool. For arithmetic and pointer types, any non-zero value or non-null pointer evaluates to true, while zero or null evaluates to false.

Short-Circuit Evaluation

The || operator enforces strict left-to-right evaluation and utilizes short-circuiting.

1. expression1 (the left operand) is evaluated first.
2. If expression1 evaluates to true, the overall expression is guaranteed to be true. Consequently, expression2 (the right operand) is not evaluated at all.
3. If expression1 evaluates to false, expression2 is evaluated to determine the final result.

There is a strict sequence point (or sequenced-before relationship in modern C++) after the evaluation of the left operand and before the evaluation of the right operand. All side effects of the left expression are guaranteed to be committed before the right expression is evaluated.

Precedence and Associativity

• Associativity: Left-to-right. An expression like a || b || c is parsed as (a || b) || c.
• Precedence: The || operator has lower precedence than the Logical AND operator (&&), equality operators (==, !=), and relational operators (<, >), but higher precedence than assignment operators (=, +=) and the ternary conditional operator (?:).

Alternative Token

C++ provides the alternative spelling or defined in the standard. It is a primary token and behaves identically to || at the compiler level.

expression1 or expression2

Operator Overloading

The || operator can be overloaded for user-defined types by defining operator||. However, doing so fundamentally alters the operator’s evaluation semantics:

• Loss of Short-Circuiting: When overloaded, the operator behaves as a standard function call. Both operands (arguments) are evaluated before the function is invoked, destroying the short-circuit property.
• Sequencing Changes: Prior to C++17, the evaluation order of the operands in an overloaded || was unsequenced. Since C++17, the left operand is sequenced before the right operand, but both are still unconditionally evaluated.

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