alignas specifier (since C++11)

 

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Specifies the alignment requirement of a type or an object.

Syntax

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

1) alignas(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))

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.

Explanation

The alignas specifier may be applied to:

• the declaration or definition of a class/struct/union or enumeration;
• 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.

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

alignas

Example

Run this code

// every object of type sse_t will be aligned to 256-byte boundary
struct alignas(256) sse_t
{
    float sse_data[4];
};
 
// the array "cacheline" will be aligned to 128-byte boundary
alignas(128) char cacheline[128];
 
 
#include <iostream>
int main()
{
    sse_t x, y, z;
    struct default_aligned { float data[4]; } a, b, c;
 
    std::cout << std::hex << std::showbase
              << "&x: " << &x << '\n'
              << "&y: " << &y << '\n'
              << "&z: " << &z << '\n'
              << "&a: " << &a << '\n'
              << "&b: " << &b << '\n'
              << "&c: " << &c << '\n';
}

Possible output:

&x: 0x7fffd901bb00
&y: 0x7fffd901bc00
&z: 0x7fffd901bd00
&a: 0x7fffd901bad0
&b: 0x7fffd901bae0
&c: 0x7fffd901baf0

See also

alignof operator(C++11)	queries alignment requirements of a type
alignment_of (C++11)	obtains the type's alignment requirements (class template)