fallthrough

Fallthrough refers to the transfer of control from one case in a multi-way branch to another case within a switch-like construct, typically occurring when a terminating statement such as a break, return, or throw is not encountered. In languages that use implicit fallthrough, once a case matches, execution continues into subsequent cases until a terminating statement or the end of the construct is reached. This behavior can be intentional for sharing logic, but it can also cause bugs if it happens unintentionally.

In languages such as C, C++, Java, JavaScript, and PHP, fallthrough is the default behavior. If a case lacks a break (or another terminating statement), execution spills into the next case. Programmers often rely on breaks to end a case, and the absence of a break can lead to subtle errors, especially when cases are added or reordered. Some compilers offer warnings for unintended fallthrough.

Other languages handle fallthrough differently. Go requires an explicit fallthrough statement to continue to the next case, making the control flow more deliberate. Swift uses a fallthrough keyword to indicate intentional continuation to the next case; without it, match cases do not fall through. Rust does not permit implicit fallthrough in its match expressions, encouraging exhaustive and non-overlapping patterns.

Fallthrough design affects readability, bug risk, and maintainability. Common practice includes commenting intentional fallthrough, minimizing opportunities for accidental continuation, and using language-specific mechanisms (such as breaks, explicit fallthrough, or pattern matching) to make control flow clear. Understanding a language’s rules helps prevent errors and guides the structuring of switch-like constructs.