alignas-specifier.md

description	title	ms.date	f1_keywords	helpviewer_keywords
Learn more about: alignas specifier	alignas (C++)	11/01/2023	alignas	alignas [C++] __alignof keyword [C++] alignof [C++] types [C++], alignment requirements

alignas (C++)

The alignas specifier changes the alignment of a type or object in memory.

Syntax

alignas(expression) alignas(type-id) alignas(pack...)

Remarks

You can use alignas specifier on a struct, class, union, or variable declaration.

For alignas(expression), the expression must be an integral constant expression that is 0 or a power of 2 (1, 2, 4, 8, 16, ...). All other expressions are ill-formed.

Use alignas instead of __declspec(align(#)) for code portability.

A common use of alignas is to control the alignment of a user-defined type, as shown in the following example:

structalignas(8) S1 { int x; }; static_assert(alignof(S1) == 8, "alignof(S1) should be 8");

When multiple alignas are applied to the same declaration, the one with the largest value is used. An alignas value of 0 is ignored.

The following example shows how to use alignas with a user-defined type:

classalignas(4) alignas(16) C1 {}; // `alignas(0)` ignoredunionalignas(0) U1 { int i; float f; }; union U2 { int i; float f; }; static_assert(alignof(C1) == 16, "alignof(C1) should be 16"); static_assert(alignof(U1) == alignof(U2), "alignof(U1) should be equivalent to alignof(U2)");

You can supply a type as the alignment value. The type's default alignment is used as the alignment value, as shown in the following example:

structalignas(double) S2 { int x; }; static_assert(alignof(S2) == alignof(double), "alignof(S2) should be equivalent to alignof(double)");

A template parameter pack (alignas (pack...)) can be used for the alignment value. The largest alignment value of all the elements in the pack is used.

template <typename... Ts> classalignas(Ts...) C2 { char c; }; static_assert(alignof(C2<>) == 1, "alignof(C2<>) should be 1"); static_assert(alignof(C2<short, int>) == 4, "alignof(C2<short, int>) should be 4"); static_assert(alignof(C2<int, float, double>) == 8, "alignof(C2<int, float, double>) should be 8");

If multiple alignas are applied, the resulting alignment is the largest of all the alignas values, and can't be less than the natural alignment of the type it's applied to.

The declaration and definition of user-defined types must have the same alignment value.

// Declaration of `C3`classalignas(16) C3; // Definition of `C3` with differing alignment valueclassalignas(32) C3 {}; // Error: C2023 'C3': Alignment (32) different from prior declaration (16)intmain() { alignas(2) int x; // ill-formed because the natural alignment of int is 4 }

See also

#pragma pack
Alignment
alignof
Compiler error C2023
Compiler warning C4359