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Difference between revisions of "cpp/language/attributes/maybe unused"

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< cpp‎ | language‎ | attributes
(Applied LWG 2733 (Applying maybe_unused to a label) as a C++26 feature, this is the preference of CWG.)
m (→‎References: Consistent order: newest to oldest)
 
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This attribute can appear in the declaration of the following entities:
 
This attribute can appear in the declaration of the following entities:
 
* [[cpp/language/classes|class/struct/union]]: {{c|struct [[maybe_unused]] S;}}
 
* [[cpp/language/classes|class/struct/union]]: {{c|struct [[maybe_unused]] S;}}
−
* {{lt|cpp/language/typedef|typedef}}, including those declared by [[cpp/language/type alias|alias declaration]]: {{c|[[maybe_unused]] typedef S* PS;}}, {{c|1=using PS [[maybe_unused]] = S*;}}
+
* {{lt|cpp/language/typedef}}, including those declared by [[cpp/language/type alias|alias declaration]]: {{c|[[maybe_unused]] typedef S* PS;}}, {{c|1=using PS [[maybe_unused]] = S*;}}
 
* variable, including [[cpp/language/static|static data member]]<!--clang used to get this wrong-->: {{c|[[maybe_unused]] int x;}}
 
* variable, including [[cpp/language/static|static data member]]<!--clang used to get this wrong-->: {{c|[[maybe_unused]] int x;}}
 
* [[cpp/language/data members|non-static data member]]: {{c|union U { [[maybe_unused]] int n; };}},
 
* [[cpp/language/data members|non-static data member]]: {{c|union U { [[maybe_unused]] int n; };}},
−
* {{lt|cpp/language/function|function}}: {{c|[[maybe_unused]] void f();}}
+
* {{lt|cpp/language/function}}: {{c|[[maybe_unused]] void f();}}
 
* [[cpp/language/enum|enumeration]]: {{c|enum [[maybe_unused]] E {};}}
 
* [[cpp/language/enum|enumeration]]: {{c|enum [[maybe_unused]] E {};}}
 
* enumerator: {{c|1=enum { A [[maybe_unused]], B [[maybe_unused]] = 42 };}}
 
* enumerator: {{c|1=enum { A [[maybe_unused]], B [[maybe_unused]] = 42 };}}
−
* {{lt|cpp/language/structured binding|structured binding}}: {{c|1=[[maybe_unused]] auto [a, b] = std::make_pair(42, 0.23);}}
+
* {{lt|cpp/language/structured binding}}: {{c|1=[[maybe_unused]] auto [a, b] = std::make_pair(42, 0.23);}}
  
 
For entities declared {{c|[[maybe_unused]]}}, if the entities or their structured bindings are unused, the warning on unused entities issued by the compiler is suppressed.
 
For entities declared {{c|[[maybe_unused]]}}, if the entities or their structured bindings are unused, the warning on unused entities issued by the compiler is suppressed.
Line 49: Line 49:
 
{{dr list item|wg=cwg|dr=2360|std=C++17|before=could not apply {{c|[[maybe_unused]]}} to structured bindings|after=allowed}}
 
{{dr list item|wg=cwg|dr=2360|std=C++17|before=could not apply {{c|[[maybe_unused]]}} to structured bindings|after=allowed}}
 
{{dr list end}}
 
{{dr list end}}
 +
 +
===References===
 +
{{ref std c++23}}
 +
{{ref std|section=9.12.8|title=Maybe unused attribute|id=dcl.attr.unused}}
 +
{{ref std end}}
 +
{{ref std c++20}}
 +
{{ref std|section=9.12.7|title=Maybe unused attribute|id=dcl.attr.unused}}
 +
{{ref std end}}
 +
{{ref std c++17}}
 +
{{ref std|section=10.6.6|title=Maybe unused attribute|id=dcl.attr.unused}}
 +
{{ref std end}}
  
 
===See also===
 
===See also===

Latest revision as of 11:44, 12 August 2024

 
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maybe_unused
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Suppresses warnings on unused entities.

Contents

[edit] Syntax

[[maybe_unused]]

[edit] Explanation

This attribute can appear in the declaration of the following entities:

For entities declared [[maybe_unused]], if the entities or their structured bindings are unused, the warning on unused entities issued by the compiler is suppressed.

For labels declared [[maybe_unused]], if they are unused, the warning on unused labels issued by the compiler is suppressed.

(since C++26)

[edit] Example

Run this code
#include <cassert>
 
[[maybe_unused]] void f([[maybe_unused]] bool thing1,
                        [[maybe_unused]] bool thing2)
{
    [[maybe_unused]] lb: // the label “lb” is not used, no warning
    [[maybe_unused]] bool b = thing1 && thing2;
    assert(b); // in release mode, assert is compiled out, and “b” is unused
               // no warning because it is declared [[maybe_unused]]
} // parameters “thing1” and “thing2” are not used, no warning
 
int main() {}

[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 2360 C++17 could not apply [[maybe_unused]] to structured bindings allowed

[edit] References

  • C++23 standard (ISO/IEC 14882:2024):
  • 9.12.8 Maybe unused attribute [dcl.attr.unused]
  • C++20 standard (ISO/IEC 14882:2020):
  • 9.12.7 Maybe unused attribute [dcl.attr.unused]
  • C++17 standard (ISO/IEC 14882:2017):
  • 10.6.6 Maybe unused attribute [dcl.attr.unused]

[edit] See also

C documentation for maybe_unused
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