Difference between revisions of "cpp/language/attributes/maybe unused"
From cppreference.com
< cpp | language | attributes
(Applied LWG 2733 (Applying maybe_unused to a label) as a C++26 feature, this is the preference of CWG.) |
m (1) {{lt}} uses last part of the path as the default title. 2) I like asymmetric quotes like in “thing1”. Shouldn't they eventually become a part of C++ syntax?.) |
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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/ | + | * {{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/ | + | * {{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/ | + | * {{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. | ||
Revision as of 04:21, 12 January 2024
Suppresses warnings on unused entities.
Contents |
Syntax
[[maybe_unused]]
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Explanation
This attribute can appear in the declaration of the following entities:
- class/struct/union: struct [[maybe_unused]] S;
- typedef, including those declared by alias declaration: [[maybe_unused]] typedef S* PS;, using PS [[maybe_unused]] = S*;
- variable, including static data member: [[maybe_unused]] int x;
- non-static data member: union U { [[maybe_unused]] int n; };,
- function: [[maybe_unused]] void f();
- enumeration: enum [[maybe_unused]] E {};
- enumerator: enum { A [[maybe_unused]], B [[maybe_unused]] = 42 };
- structured binding: [[maybe_unused]] auto [a, b] = std::make_pair(42, 0.23);
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.
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For labels declared [[maybe_unused]], if they are unused, the warning on unused labels issued by the compiler is suppressed. |
(since C++26) |
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() {}
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 |
See also
| C documentation for maybe_unused
|
