aligned_union Class

Article
08/03/2021

Provides a POD type large enough and suitably aligned to store a union type, and the size required.

Syntax

template <std::size_t Len, class... Types>
struct aligned_union;

template <std::size_t Len, class... Types>
using aligned_union_t = typename aligned_union<Len, Types...>::type;

Parameters

Len
The alignment value for the largest type in the union.

Types
The distinct types in the underlying union.

Remarks

Use the class template to get the alignment and size needed to store a union in uninitialized storage. The member typedef type names a POD type suitable for storage of any type listed in Types; the minimum size is Len. The static member alignment_value of type std::size_t contains the strictest alignment required of all the types listed in Types.

Example

The following example shows how to use aligned_union to allocate an aligned stack buffer to place a union.

// std__type_traits__aligned_union.cpp
#include <iostream>
#include <type_traits>

union U_type
{
    int i;
    float f;
    double d;
    U_type(float e) : f(e) {}
};

typedef std::aligned_union<16, int, float, double>::type aligned_U_type;

int main()
{
    // allocate memory for a U_type aligned on a 16-byte boundary
    aligned_U_type au;
    // do placement new in the aligned memory on the stack
    U_type* u = new (&au) U_type(1.0f);
    if (nullptr != u)
    {
        std::cout << "value of u->i is " << u->i << std::endl;
        // must clean up placement objects manually!
        u->~U_type();
    }
}

value of u->i is 1065353216

Requirements

Header: <type_traits>

Namespace: std