alignas specifier (since C++11)

Specifies the alignment requirement of a type or an object.

Syntax

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

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

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

Keywords

alignas

Example

#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';
}

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.

References

See also