alignas specifier (since C++11)

 

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Overview declaration syntax decl-specifier-seq declarator Specifiers typedef inline virtual function specifier explicit function specifier    friend constexpr (C++11) consteval (C++20) constinit (C++20) storage class specifiers translation-unit-local (C++20) class/struct union enum decltype (C++11) auto (C++11) alignas (C++11) constvolatile elaborated type specifier attributes (C++11)

Declarators reference pointer array Block declarations simple-declaration → structured binding declaration (C++17) alias declaration(C++11) namespace alias definition using-declaration using-directive static_assert declaration (C++11) asm-declaration opaque enum declaration(C++11) Other declarations namespace definition function declaration class template declaration function template declaration explicit template instantiation(C++11) explicit template specialization linkage specification attribute declaration (C++11) empty declaration

 

Specifies the alignment requirement of a type or an object.

Syntax

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.

Explanation

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.

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>.

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)

Keywords

alignas

Example

Run this code

// 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];
 
 
#include <iostream>
int main()
{
    struct default_aligned { float data[4]; } a, b, c;
    sse_t x, y, z;
 
    std::cout
        << "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
alignment_of (C++11)	obtains the type's alignment requirements (class template)
C documentation for _Alignas