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

 

C++

Language

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Freestanding and hosted implementations

Named requirements

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Concepts library (C++20)

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Technical Specifications

 

C++ language

 

Declarations

Overview declaration syntax decl-specifier-seq declarator Specifiers typedef inline virtual function specifier explicit function specifier friend constexpr (C++11) consteval (C++20) storage class specifiers classstruct union enum decltype (C++11) auto (C++11) alignas (C++11) constvolatile 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

noreturn‎ (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
  }
}