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alignof operator (since C++11)

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Queries alignment requirements of a type.

Contents

[edit] Syntax

alignof( type-id )

Returns a value of type std::size_t.

[edit] Explanation

Returns the alignment, in bytes, required for any instance of the type indicated by type-id, which is either complete object type, an array type whose element type is complete, or a reference type to one of those types.

If the type is reference type, the operator returns the alignment of referenced type; if the type is array type, alignment requirement of the element type is returned.

[edit] Notes

See alignment for the meaning and properties of the value returned by alignof.

[edit] Keywords

alignof

[edit] Example

#include <iostream>
 
struct Foo
{
    int   i;
    float f;
    char  c;
};
 
// Note: `alignas(alignof(long double))` below can be simplified to simply 
// `alignas(long double)` if desired.
struct alignas(alignof(long double)) Foo2
{
    // put your definition here
}; 
 
struct Empty {};
 
struct alignas(64) Empty64 {};
 
int main()
{
    std::cout << "Alignment of"  "\n"
        "- char            : " << alignof(char)    << "\n"
        "- pointer         : " << alignof(int*)    << "\n"
        "- class Foo       : " << alignof(Foo)     << "\n"
        "- class Foo2      : " << alignof(Foo2)    << "\n"
        "- empty class     : " << alignof(Empty)   << "\n"
        "- empty class\n"
        "  with alignas(64): " << alignof(Empty64) << "\n";
}

Possible output:

Alignment of
- char            : 1
- pointer         : 8
- class Foo       : 4
- class Foo2      : 16
- empty class     : 1
- empty class
  with alignas(64): 64

[edit] References

  • C++23 standard (ISO/IEC 14882:2023):
  • 7.6.2.6 Alignof [expr.alignof]
  • C++20 standard (ISO/IEC 14882:2020):
  • 7.6.2.5 Alignof [expr.alignof]
  • C++17 standard (ISO/IEC 14882:2017):
  • 8.3.6 Alignof [expr.alignof]
  • C++14 standard (ISO/IEC 14882:2014):
  • 5.3.6 Alignof [expr.alignof]
  • C++11 standard (ISO/IEC 14882:2011):
  • 5.3.6 Alignof [expr.alignof]

[edit] 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 1305 C++11 type-id could not represent a reference to an array
with an unknown bound but a complete element type
allowed

[edit] See also

alignment requirement restricts the addresses at which an object may be allocated
alignas specifier(C++11) specifies that the storage for the variable should be aligned by specific amount[edit]
obtains the type's alignment requirements
(class template) [edit]
C documentation for _Alignof