50 lines
2.8 KiB
Markdown
50 lines
2.8 KiB
Markdown
[dcl.attr.fallthrough]
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# 9 Declarations [[dcl]](./#dcl)
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## 9.13 Attributes [[dcl.attr]](dcl.attr#fallthrough)
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### 9.13.5 Fallthrough attribute [dcl.attr.fallthrough]
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[1](#1)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L9773)
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The [*attribute-token*](dcl.attr.grammar#nt:attribute-token "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") fallthrough may be applied to a [null statement](stmt.expr#def:statement,null "8.3 Expression statement [stmt.expr]"); such a statement is a fallthrough statement[.](#1.sentence-1)
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No [*attribute-argument-clause*](dcl.attr.grammar#nt:attribute-argument-clause "9.13.1 Attribute syntax and semantics [dcl.attr.grammar]") shall be present[.](#1.sentence-2)
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A fallthrough statement may only appear within
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an enclosing switch statement ([[stmt.switch]](stmt.switch "8.5.3 The switch statement"))[.](#1.sentence-3)
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The next statement that would be executed after a fallthrough statement
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shall be a labeled statement whose label is a case label or
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default label for the same switch statement and,
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if the fallthrough statement is contained in an iteration statement,
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the next statement shall be part of the same execution of
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the substatement of the innermost enclosing iteration statement[.](#1.sentence-4)
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The program is ill-formed if there is no such statement[.](#1.sentence-5)
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[2](#2)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L9789)
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*Recommended practice*: The use of a fallthrough statement should suppress
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a warning that an implementation might otherwise issue
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for a case or default label that is reachable
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from another case or default label along some path of execution[.](#2.sentence-1)
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The value of
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a [*has-attribute-expression*](cpp.cond#nt:has-attribute-expression "15.2 Conditional inclusion [cpp.cond]") for the fallthrough attribute
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should be 0 if the attribute does not cause suppression of such warnings[.](#2.sentence-2)
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Implementations should issue a warning
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if a fallthrough statement is not dynamically reachable[.](#2.sentence-3)
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[3](#3)
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[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/declarations.tex#L9802)
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[*Example [1](#example-1)*: void f(int n) {void g(), h(), i(); switch (n) {case 1:case 2: g(); [[fallthrough]]; case 3: // warning on fallthrough discourageddo {[[fallthrough]]; // error: next statement is not part of the same substatement execution} while (false); case 6:do {[[fallthrough]]; // error: next statement is not part of the same substatement execution} while (n--); case 7:while (false) {[[fallthrough]]; // error: next statement is not part of the same substatement execution}case 5: h(); case 4: // implementation may warn on fallthrough i(); [[fallthrough]]; // error}} â *end example*]
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