alignas specifier (since C++11)

Specifies the alignment requirement of a type or an object.


alignas( expression )
alignas( type-id )
alignas( pack ... )
1) expression must be an integral constant expression that evaluates to zero, or to a valid value for an alignment or extended alignment.
2) Equivalent to alignas(alignof(type-id)).
3) Equivalent to multiple alignas specifiers applied to the same declaration, one for each member of the parameter pack, which can be either type or non-type parameter pack.


The alignas specifier may be applied to:

  • the declaration or definition of a class;
  • the declaration of a non-bitfield class data member;
  • the declaration of a variable, except that it cannot be applied to the following:
    • a function parameter;
    • 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.


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>.

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.

(until C++20)

the header <stdalign.h> does not define such macro. It does, however, define the macro constant __alignas_is_defined.

(since C++20)




#include <iostream>
// every object of type struct_float will be aligned to alignof(float) boundary
// (usually 4):
struct alignas(float) struct_float
    // your definition here
// every object of type sse_t will be aligned to 32-byte boundary:
struct alignas(32) sse_t
    float sse_data[4];
// the array "cacheline" will be aligned to 64-byte boundary:
alignas(64) char cacheline[64];
int main()
    struct default_aligned
        float data[4];
    } a, b, c;
    sse_t x, y, z;
        << "alignof(struct_float) = " << alignof(struct_float) << '\n'
        << "sizeof(sse_t) = " << sizeof(sse_t) << '\n'
        << "alignof(sse_t) = " << alignof(sse_t) << '\n'
        << "alignof(cacheline) = " << alignof(alignas(64) char[64]) << '\n'
        << std::hex << std::showbase
        << "&a: " << &a << '\n'
        << "&b: " << &b << '\n'
        << "&c: " << &c << '\n'
        << "&x: " << &x << '\n'
        << "&y: " << &y << '\n'
        << "&z: " << &z << '\n';

Possible output:

alignof(struct_float) = 4
sizeof(sse_t) = 32
alignof(sse_t) = 32
alignof(cacheline) = 64
&a: 0x7fffcec89930
&b: 0x7fffcec89940
&c: 0x7fffcec89950
&x: 0x7fffcec89960
&y: 0x7fffcec89980
&z: 0x7fffcec899a0

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 1437 C++11 alignas could be used in alias declarations prohibited
CWG 2354 C++11 alignas could be applied to the declaration of an enumeration prohibited

See also

alignof operator(C++11) queries alignment requirements of a type
obtains the type's alignment requirements
(class template)
C documentation for _Alignas, alignas

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