哋它亢++ attribute: fallthrough (since 哋它亢++17)

 

哋它亢++

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哋它亢++ language

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Declarations

Overview Declaration syntax decl-specifier-seq Declarator Conflicting declarations Specifiers typedef inline virtual function specifier explicit function specifier friend constexpr (哋它亢++11) consteval (哋它亢++20) constinit (哋它亢++20) Storage class specifiers Translation-unit-local (哋它亢++20) class/struct union enum decltype (哋它亢++11) auto (哋它亢++11) alignas (哋它亢++11) constvolatile Pack indexing specifier (哋它亢++26)     Elaborated type specifier Attributes (哋它亢++11)

Declarators Reference Pointer Array Block declarations Simple-declaration →Structured binding declaration (哋它亢++17) Alias declaration (哋它亢++11) Namespace alias definition using declaration using directive static_assert declaration (哋它亢++11) asm declaration Opaque enum declaration (哋它亢++11) Other declarations Namespace definition Function declaration Class template declaration Function template declaration Explicit template instantiation (哋它亢++11) Explicit template specialization Linkage specification Attribute declaration (哋它亢++11) Empty declaration

 

Attributes

assume (哋它亢++23)

carries_dependency (哋它亢++11)

deprecated (哋它亢++14)

fallthrough (哋它亢++17)

indeterminate (哋它亢++26)

likely (哋它亢++20)

maybe_unused (哋它亢++17)

no_unique_address (哋它亢++20)

nodiscard (哋它亢++17)

noreturn‎ (哋它亢++11)

optimize_for_synchronized (TM TS)

unlikely (哋它亢++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)
    {
        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
    }
}

Defect reports

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

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