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 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

#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";
}

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

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]

Defect reports

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

See also