std::aligned_storage
From Cppreference
| Defined in header <type_traits>
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| template< std::size_t Len, std::size_t Align = default-alignment >
struct aligned_storage; |
(C++11 feature) | |
Provides the member typedef type, which is a POD 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.
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[edit] Member types
| Name | Definition |
| type | the POD type of size Len with alignment requirement Align |
[edit] Notes
The type defined by std::aligned_storage can be used to create uninitialized memory blocks suitable to hold the objects of given type, optionally aligned stricter than necessary, for example on a cache or page boundary.
[edit] Equivalent definition
Except for default argument, aligned_storage is expressible in terms of alignas:
template<std::size_t Len, std::size_t Align> struct aligned_storage { typedef struct { alignas(Align) unsigned char data[Len]; } type; }; |
[edit] Example
A primitive static vector class, demonstrating creation, access, and destruction of objects in aligned storage
#include <iostream> #include <type_traits> #include <string> template<typename T, std::size_t N> class static_vector { // propertly aligned uninitialized storage for N T's typename std::aligned_storage <sizeof(T), std::alignment_of<T>::value>::type data[N]; std::size_t m_size; public: static_vector() : m_size(0) {}; // Create an object in aligned storage template<typename ...Args> void emplace_back(Args&&... args) { new(data+m_size) T(std::forward<Args>(args)...); m_size++; // bounds check omitted } // Access an object in aligned storage const T& operator[](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(std::string(5, '*')); v1.emplace_back(std::string(10, '*')); std::cout << v1[0] << '\n' << v1[1] << '\n'; }
Output:
***** **********
[edit] See also
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(keyword) | ||
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obtains the type's alignment requirements (class template) |
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defines the type suitable for use as uninitialized storage for all given types (class template) |
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POD type with alignment requirement as great as any other scalar type (typedef) |
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