NAME

std::aligned_storage - std::aligned_storage

Synopsis

Defined in header <type_traits>
template< std::size_t Len, std::size_t Align = (since C++11)
/*default-alignment*/ > (deprecated in C++23)
struct aligned_storage;

Provides the nested type type, which is a trivial standard-layout type 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.

It is implementation-defined whether any extended alignment is supported.

The behavior of a program that adds specializations for aligned_storage is
undefined.

Member types

Name Definition
type a trivial and standard-layout type of at least size Len with alignment
requirement Align

Helper types

template< std::size_t Len, std::size_t Align =
/*default-alignment*/ > (since C++14)
using aligned_storage_t = typename aligned_storage<Len, (deprecated in C++23)
Align>::type;

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 {
struct type {
alignas(Align) unsigned char data[Len];
};
};

Example

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

// Run this code

#include <new>
#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
std::aligned_storage_t<sizeof(T), alignof(T)> 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{};

// construct value in memory of aligned storage
// using inplace operator new
::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
{
// Note: std::launder is needed after the change of object model in P0137R1
return *std::launder(reinterpret_cast<const T*>(&data[pos]));
}

// Destroy objects from aligned storage
~static_vector()
{
for(std::size_t pos = 0; pos < m_size; ++pos) {
// Note: std::launder is needed after the change of object model in P0137R1
std::destroy_at(std::launder(reinterpret_cast<T*>(&data[pos])));
}
}
};

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:

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