alignof operator (since C++11)

Queries alignment requirements of a type.

Syntax

Returns a value of type std::size_t.

Explanation

Returns the alignment, in bytes, required for 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

#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));
}

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.

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