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 satisfies TrivialType and StandardLayoutType
and 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.

If the program adds specializations for std::aligned_storage, the behavior 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 <cstddef>
#include <iostream>
#include <new>
#include <string>
#include <type_traits>

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:¶

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

See also¶

alignas specifier(C++11) specifies that the storage for the variable should be
aligned by specific amount
alignment_of obtains the type's alignment requirements
(C++11) (class template)
aligned_alloc allocates aligned memory
(C++17) (function)
aligned_union defines the type suitable for use as uninitialized
(C++11)(deprecated in C++23) storage for all given types
(class template)
max_align_t trivial type with alignment requirement as great as any
(C++11) other scalar type
(typedef)
launder pointer optimization barrier
(C++17) (function template)