alignof operator (since C++11)

 

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Queries alignment requirements of a type.

Syntax

alignof( type-id )

Returns a value of type std::size_t.

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.

Notes

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

Keywords

alignof

Example

Run this code

#include <iostream>
 
struct Foo
{
    int   i;
    float f;
    char  c;
};
 
/ Note: alignas(alignof(long double)) below can be
/ simplified to alignas(long double) if desired.
struct alignas(alignof(long double) Foo2
{
    / put your definition here
}; 
 
struct Empty {};
 
struct alignas(64) Empty64 {};
 
#define SHOW(expr) std::cout << #expr << " = " << (expr) << '\n'
 
int main()
{
    SHOW(alignof(char);
    SHOW(alignof(int*);
    SHOW(alignof(Foo));
    SHOW(alignof(Foo2));
    SHOW(alignof(Empty));
    SHOW(alignof(Empty64));
}

Possible output:

alignof(char) = 1
alignof(int*) = 8
alignof(Foo) = 4
alignof(Foo2) = 16
alignof(Empty) = 1
alignof(Empty64) = 64

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

References

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

• 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]

See also

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

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