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Showing content from https://en.cppreference.com/w/cpp/language/../string/../utility/../language/alignas.html below:

alignas specifier (since C++11) - cppreference.com

Specifies the alignment requirement of a type or an object.

2) Equivalent to alignas(alignof( type-id )).

Equivalent to multiple alignas specifiers applied to the same declaration, one for each member of the

, which can be either type or constant 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.

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

alignas

#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

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• 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]

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