alignas specifier (since C++11)

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

C++ language

General topics
Preprocessor Comments	Keywords Escape sequences
Flow control
Conditional execution statements
if	switch
Iteration statements (loops)
for range-for (C++11)	while do-while
Jump statements
continue - break	goto - return
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++17*)
noexcept specifier (C++11)
Exceptions
throw-expression try block	handler
Namespaces
Namespace declaration	Namespace aliases
Types
Fundamental types Enumeration types Function types	Class/struct types Union types
Specifiers
const/volatile decltype (C++11) auto (C++11)	constexpr (C++11) consteval (C++20) constinit (C++20)
Storage duration specifiers
Initialization
Default-initialization Value-initialization Zero-initialization Copy-initialization Direct-initialization	Aggregate initialization List-initialization (C++11)   Constant initialization Reference initialization

Expressions
Value categories Order of evaluation	Operators Operator precedence
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions static_cast const_cast	Explicit conversions dynamic_cast reinterpret_cast
Memory allocation
new expression	delete expression
Classes
Class declaration Constructors this pointer	Access specifiers friend specifier
Class-specific function properties
Virtual function override specifier (C++11)   final specifier (C++11)	explicit (C++11) static
Special member functions
Default constructor Copy constructor Move constructor (C++11)	Copy assignment Move assignment (C++11) Destructor
Templates
Class template Function template	Template specialization Parameter packs (C++11)
Miscellaneous
Inline assembly	History of C++

 

Declarations

Overview Declaration syntax decl-specifier-seq Declarator Conflicting declarations 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 Pack indexing specifier (C++26)     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

#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];
};
 
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'
        << 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
&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

References

• C++23 standard (ISO/IEC 14882:2024):

• 9.12.4 Carries dependency attribute [dcl.attr.depend]

• C++20 standard (ISO/IEC 14882:2020):

• 9.12.3 Carries dependency attribute [dcl.attr.depend]

• C++17 standard (ISO/IEC 14882:2017):

• 10.6.3 Carries dependency attribute [dcl.attr.depend]

• C++14 standard (ISO/IEC 14882:2014):

• 7.6.4 Carries dependency attribute [dcl.attr.depend]

• C++11 standard (ISO/IEC 14882:2011):

• 7.6.4 Carries dependency attribute [dcl.attr.depend]

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, alignas