Specifies thealignment requirement of a type or an object.

[edit]Syntax

[edit]Explanation

Thealignas specifier may be applied to:

• the declaration or definition of aclass;
• 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 itsalignment requirement equal to the strictest (largest) non-zeroexpression of allalignas 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 anyalignas specifiers (that is, weaker than its natural alignment or weaker thanalignas 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 asalignas(3) are ill-formed.

Valid non-zero alignments that are weaker than anotheralignas on the same declaration are ignored.

alignas(0) is always ignored.

[edit]Notes

As of the ISO C11 standard, the C language has the_Alignas keyword and definesalignas as a preprocessor macro expanding to the keyword in the header<stdalign.h>.

In C++, this is a keyword, and

[edit]Keywords

alignas

[edit]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) = 4sizeof(sse_t) = 32alignof(sse_t) = 32&a: 0x7fffcec89930&b: 0x7fffcec89940&c: 0x7fffcec89950&x: 0x7fffcec89960&y: 0x7fffcec89980&z: 0x7fffcec899a0

[edit]Defect reports

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

[edit]References

[edit]See also

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