# `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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