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) (deprecated in C++23) | |
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.
Contents |
[edit]Member types
| Name | Definition |
type | a trivial and standard-layout type of at least size Len with alignment requirement Align |
[edit]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) (deprecated in C++23) | |
[edit]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.
[edit]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)unsignedchar data[Len];};}; |
[edit]Example
A primitive static vector class, demonstrating creation, access, and destruction of objects in aligned storage.
#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 storagetemplate<typename ...Args>void emplace_back(Args&&... args){if(m_size >= N)// Possible error handlingthrowstd::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 storageconst T& operator[](std::size_t pos)const{// Note: std::launder is needed after the change of object model in P0137R1return*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 P0137R1std::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:
***** **********
[edit]See also
alignas(C++11) | specifies that the storage for the variable should be aligned by specific amount (specifier) |
(C++11) | obtains the type's alignment requirements (class template) |
(C++17) | allocates aligned memory (function) |
(since C++11)(deprecated in C++23) | defines the type suitable for use as uninitialized storage for all given types (class template) |
(C++11) | trivial type with alignment requirement as great as any other scalar type (typedef) |
(C++17) | pointer optimization barrier (function template) |
