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

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

C++ language

Declarations

Overview declaration syntax decl-specifier-seq declarator Specifiers typedef inline function specifier virtual function specifier explicit function specifier friend constexpr (C++11) storage class specifiers class union enum decltype (C++11) auto (C++11) alignas (C++11) const/volatile elaborated type specifier attributes (C++11) Declarators reference pointer array

Block declarations simple-declaration structured binding declaration (C++17) alias declaration(C++11) namespace alias definition using-declaration using-directive static_assert declaration (C++11) asm-definition opaque enum declaration(C++11) Other declarations namespace definition function declaration class template declaration function template declaration explicit template instantiation(C++11) explicit template specialization linkage specification attribute declaration (C++11) empty declaration

Attributes

noreturnlrm; (C++11)

carries_dependency (C++11)

deprecated (C++14)

fallthrough (C++17)

nodiscard (C++17)

maybe_unused (C++17)

likelyunlikely (C++20)(C++20)

no_unique_address (C++20)

expectsensuresassert (C++20)(C++20)(C++20)

optimize_for_synchronized (TM TS)

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

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:
      [[fallthrough]]; // ill-formed, no subsequent case or default label
  }
}