Standard library header <cstdalign> (C++11) (until C++20) , <stdalign.h> (C++11)

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Standard library headers

Language support
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Strings
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Text processing
`<clocale>`
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Numerics
`<cfenv>` (C++11)
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Time
`<chrono>` (C++11)
`<ctime>`
C compatibility
`<ccomplex>` ( C++11/17/20* )
`<ciso646>` (until C++20)
`<cstdalign>` ( C++11/17/20* )
`<cstdbool>` ( C++11/17/20* )
`<ctgmath>` ( C++11/17/20* )

Input/output
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`<ios>`
`<istream>`
`<ostream>`
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`<sstream>`
`<streambuf>`
`<strstream>` ( C++98/26* )
`<syncstream>` (C++20)
Concurrency support
`<atomic>` (C++11)
`<barrier>` (C++20)
`<condition_variable>` (C++11)
`<future>` (C++11)
`<hazard_pointer>` (C++26)
`<latch>` (C++20)
`<mutex>` (C++11)
`<rcu>` (C++26)
`<semaphore>` (C++20)
`<shared_mutex>` (C++14)
`<stdatomic.h>` (C++23)
`<stop_token>` (C++20)
`<thread>` (C++11)
Execution support
`<execution>` (C++17)

This header was originally in the C standard library as <stdalign.h> .

C compatibility header. alignas and alignof are keywords in C++, but they were not keywords in C until C23.

Macros
__alignas_is_defined (C++11) (deprecated)	C compatibility macro constant, expands to integer literal 1 (macro constant)
__alignof_is_defined (C++11) (deprecated)	C compatibility macro constant, expands to integer literal 1 (macro constant)

Notes

<cstdalign> is deprecated in C++17 and removed in C++20. Corresponding <stdalign.h> is still available in C++20.

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
LWG 3827	C++11	C programs no longer need the compatibility macro `__alignas_is_defined` since C23	deprecated this macro in C++

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