C++ attribute: fallthrough (since C++17) (original) (raw)

Indicates that the fall through from the previous case label is intentional and should not be diagnosed by a compiler that warns on fallthrough.

[edit] Syntax

| | | | | ----------------------- | | | | [[fallthrough]] | | | | | | |

[edit] Explanation

May only be applied to a null statement to create a fallthrough statement ([[fallthrough]];).

A fallthrough statement may only be used in a switch statement, where the next statement to be executed is a statement with a case or default label for that switch statement. If the fallthrough statement is inside a loop, the next (labeled) statement must be part of the same iteration of that loop.

[edit] Example

void f(int n) { void g(), h(), i(); switch (n) { case 1: case 2: g(); [[fallthrough]]; case 3: // no warning on fallthrough h(); case 4: // compiler may warn on fallthrough if (n < 3) { i(); [[fallthrough]]; // OK } else { return; } case 5: while (false) { [[fallthrough]]; // ill-formed: next statement is not // part of the same iteration } case 6: [[fallthrough]]; // ill-formed, no subsequent case or default label } }

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 2406	C++17	[[fallthrough]] could appear in a loopnested inside the target switch statement	prohibited

[edit] References