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

 

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Attributes

assume (C++23)

carries_dependency (C++11)(until C++26)

deprecated (C++14)

fallthrough (C++17)

indeterminate (C++26)

likely (C++20)

maybe_unused (C++17)

nodiscard (C++17)

noreturn (C++11)

no_unique_address (C++20)

optimize_for_synchronized (TM TS)

unlikely (C++20)

 

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

Syntax

[[fallthrough]]

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.

Example

Run this code

void f(int n){void g(), h(), i();   switch(n){case1:case2: g();[[fallthrough]];case3:// no warning on fallthrough h();case4:// compiler may warn on fallthroughif(n <3){ i();[[fallthrough]];// OK}else{return;}case5:while(false){[[fallthrough]];// ill-formed: next statement is not// part of the same iteration}case6:[[fallthrough]];// ill-formed, no subsequent case or default label}}

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

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]

See also

C documentation for fallthrough