C++ attribute: fallthrough (since C++17)
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fallthrough
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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
Run this code
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 loop nested inside the target switch statement |
prohibited |
[edit] References
- C++23 standard (ISO/IEC 14882:2024):
- 9.12.6 Fallthrough attribute [dcl.attr.fallthrough]
- C++20 standard (ISO/IEC 14882:2020):
- 9.12.5 Fallthrough attribute [dcl.attr.fallthrough]
- C++17 standard (ISO/IEC 14882:2017):
- 10.6.5 Fallthrough attribute [dcl.attr.fallthrough]
[edit] See also
C documentation for fallthrough