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std::aligned_storage

Defined in header `<type_traits>`
template< std::size_t Len, std::size_t Align = /default-alignment/ > struct aligned_storage;		(since C++11)

Provides the member typedef type, which is a PODType suitable for use as uninitialized storage for any object whose size is at most Len and whose alignment requirement is a divisor of Align.

The default value of Align is the most stringent (the largest) alignment requirement for any object whose size is at most Len. If the default value is not used, Align must be the value of alignof(T) for some type T, or the behavior is undefined.

The behavior is undefined if Len == 0.

Member types

Name	Definition
`type`	the POD type of at least size `Len` with alignment requirement `Align`

Helper types

template< std::size_t Len, std::size_t Align = /*default-alignment*/ >
using aligned_storage_t = typename aligned_storage<Len, Align>::type;

(since C++14)

Notes

The type defined by std::aligned_storage<>::type can be used to create uninitialized memory blocks suitable to hold the objects of given type, optionally aligned stricter than their natural alignment requirement, for example on a cache or page boundary.

As with any other uninitialized storage, the objects are created using placement new and destroyed with explicit destructor calls.

Possible implementation

Except for default argument, aligned_storage is expressible in terms of alignas:

template<std::size_t Len, std::size_t Align /* default alignment not implemented */>
struct aligned_storage {
    typedef struct {
        alignas(Align) unsigned char data[Len];
    } type;
};

Example

A primitive static vector class, demonstrating creation, access, and destruction of objects in aligned storage.

#include <iostream>
#include <type_traits>
#include <string>
 
template<class T, std::size_t N>
class static_vector
{
    // properly aligned uninitialized storage for N T's
    typename std::aligned_storage<sizeof(T), alignof(T)>::type data[N];
    std::size_t m_size = 0;
 
public:
    // Create an object in aligned storage
    template<typename ...Args> void emplace_back(Args&&... args) 
    {
        if( m_size >= N ) // possible error handling
            throw std::bad_alloc{};
        new(data+m_size) T(std::forward<Args>(args)...);
        ++m_size;
    }
 
    // Access an object in aligned storage
    const T& operator[](std::size_t pos) const 
    {
        return *reinterpret_cast<const T*>(data+pos);
    }
 
    // Delete objects from aligned storage
    ~static_vector() 
    {
        for(std::size_t pos = 0; pos < m_size; ++pos) {
            reinterpret_cast<const T*>(data+pos)->~T();
        }
    }
};
 
int main()
{
    static_vector<std::string, 10> v1;
    v1.emplace_back(5, '*');
    v1.emplace_back(10, '*');
    std::cout << v1[0] << '\n' << v1[1] << '\n';
}

Output:

*****
**********