alignas specifier (since C++11)
Specifies the alignment requirement of a type or an object.
Syntax
alignas( expression )
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alignas( type-id )
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alignas( pack ... )
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Explanation
The alignas specifier may be applied to the declaration of a variable or a non-bitfield class data member, or it can be applied to the declaration or definition of a class/struct/union or enumeration. It cannot be applied to a function parameter or to the exception parameter of a catch clause.
The object or the type declared by such a declaration will have its alignment requirement equal to the strictest (largest) non-zero expression of all alignas
specifiers used in the declaration, unless it would weaken the natural alignment of the type.
If the strictest (largest) alignas
on a declaration is weaker than the alignment it would have without any alignas
specifiers (that is, weaker than its natural alignment or weaker than alignas
on another declaration of the same object or type), the program is ill-formed:
struct alignas(8) S {}; struct alignas(1) U { S s; }; // error: alignment of U would have been 8 without alignas(1)
Invalid non-zero alignments, such as alignas(3) are ill-formed.
Valid non-zero alignments that are weaker than another alignas on the same declaration are ignored.
alignas(0) is always ignored.
Notes
As of the ISO C11 standard, the C language has the _Alignas keyword and defines alignas as a preprocessor macro expanding to the keyword in the header <stdalign.h>, but in C++ this is a keyword, and the headers <stdalign.h> and <cstdalign> do not define such macro. They do, however, define the macro constant __alignas_is_defined.
Keywords
Example
// every object of type sse_t will be aligned to 16-byte boundary struct alignas(16) sse_t { float sse_data[4]; }; // the array "cacheline" will be aligned to 128-byte boundary alignas(128) char cacheline[128]; int main() { }
See also
alignof operator | queries alignment requirements of a type (since C++11) |
(C++11) |
obtains the type's alignment requirements (class template) |