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{{cpp/attribute/title|fallthrough}}
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{{cpp/language/attributes/title|fallthrough|notes={{mark since c++17}}}}
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{{cpp/attribute/navbar}}
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{{cpp/language/attributes/navbar}}
  
 
Indicates that the fall through from the previous case label is intentional and should not be diagnosed by a compiler that warns on fallthrough.
 
Indicates that the fall through from the previous case label is intentional and should not be diagnosed by a compiler that warns on fallthrough.
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{{sdsc begin}}
 
{{sdsc begin}}
 
{{sdsc|1=
 
{{sdsc|1=
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{{ttb|<nowiki>[[fallthrough]]</nowiki>;}}
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{{ttb|{{c/core|[[fallthrough]]}}}}
 
}}
 
}}
 
{{sdsc end}}
 
{{sdsc end}}
  
 
===Explanation===
 
===Explanation===
−
Appears in a [[cpp/language/switch|switch]] statement on a line of its own (technically as an attribute of a [[cpp/language/statements#Expression_statements|null statement]]), immediately before a case label. Indicates that the fall through from the previous case label is intentional and should not be diagnosed by a compiler that warns on fallthrough.
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May only be applied to a [[cpp/language/statements#Expression statements|null statement]] to create a ''fallthrough statement'' ({{c|[[fallthrough]];}}).
 +
 
 +
A fallthrough statement may only be used in a [[cpp/language/switch|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===
 
===Example===
 
{{example
 
{{example
 
|code=
 
|code=
−
void f(int n) {
+
void f(int n)
−
  void g(), h(), i();
+
{
−
  switch (n) {
+
    void g(), h(), i();
−
     case 1:
+
   
−
    case 2:
+
    switch (n)
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      g();
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     {
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    [[fallthrough]];
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        case 1:
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    case 3: // no warning on fallthrough
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        case 2:
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      h();
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            g();
−
    case 4: // compiler may warn on fallthrough
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            [[fallthrough]];
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      i();
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        case 3: // no warning on fallthrough
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      [[fallthrough]]; // ill­formed, not before a case label
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            h();
−
  }
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        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
 +
    }
 
}
 
}
 
}}
 
}}
  
−
{{langlinks|ja|zh}}
+
===Defect reports===
 +
{{dr list begin}}
 +
{{dr list item|wg=cwg|dr=2406|std=C++17|before={{c|[[fallthrough]]}} could appear in a loop<br>nested inside the target switch statement|after=prohibited}}
 +
{{dr list end}}
 +
 
 +
===References===
 +
{{ref std c++23}}
 +
{{ref std|section=9.12.6|title=Fallthrough attribute|id=dcl.attr.fallthrough}}
 +
{{ref std end}}
 +
{{ref std c++20}}
 +
{{ref std|section=9.12.5|title=Fallthrough attribute|id=dcl.attr.fallthrough}}
 +
{{ref std end}}
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{{ref std c++17}}
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{{ref std|section=10.6.5|title=Fallthrough attribute|id=dcl.attr.fallthrough}}
 +
{{ref std end}}
 +
 
 +
===See also===
 +
{{dsc begin}}
 +
{{dsc see c|c/language/attributes/fallthrough}}
 +
{{dsc end}}
 +
 
 +
{{langlinks|es|ja|zh}}

Latest revision as of 11:43, 12 August 2024

 
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(C++23)
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fallthrough
(C++17)
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(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.

Contents

[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
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