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std::aligned_storage

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Defined in header <type_traits>
template< std::size_t Len, std::size_t Align = /*default-alignment*/ >
struct aligned_storage;
(a partir do C++ 11)
Fornece a type membro typedef, que é um tipo POD adequado para utilização como armazenamento não inicializada para qualquer objecto cujo tamanho é no máximo Len e cujo alinhamento requisito é um divisor de Align. O valor padrão de Align é a exigência de alinhamento mais rigoroso (maior) para qualquer objeto cujo tamanho é no máximo Len.
Original:
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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Índice

[editar] Tipos de membro

Nome
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Name
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Definition
type
o tipo de POD Len tamanho com Align exigência de alinhamento
Original:
the POD type of size Len with alignment requirement Align
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[editar] Notas

O tipo definido por std::aligned_storage pode ser usado para criar blocos de memória não inicializadas adequados para armazenar os objetos de determinado tipo, opcionalmente alinhados mais rigorosa do que o necessário, por exemplo, em um cache ou limite de página.
Original:
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.
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[editar] Possível implementação

Com exceção de argumento padrão, aligned_storage é expresso em termos de alignas:
Original:
Except for default argument, aligned_storage is expressible in terms of alignas:
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template<std::size_t Len, std::size_t Align>
struct aligned_storage {
    typedef struct {
        alignas(Align) unsigned char data[Len];
    } type;
};

[editar] Exemplo

Uma classe vetor primitivo estática, demonstrando criação, acesso e destruição de objetos no armazenamento alinhados
Original:
A primitive static vector class, demonstrating creation, access, and destruction of objects in aligned storage
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#include <iostream>
#include <type_traits>
#include <string>
 
template<class 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:

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

[editar] Veja também

alignas especificador
especifica que o armazenamento para a variável devem ser alinhados por (C++11) quantidade específica
Original:
specifies that the storage for the variable should be aligned by specific amount (C++11)
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[edit]
obtém requisitos do tipo de alinhamento
Original:
obtains the type's alignment requirements
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(modelo de classe) [edit]
define o tipo adequado para utilização como armazenamento não inicializados para todos os tipos de dados
Original:
defines the type suitable for use as uninitialized storage for all given types
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(modelo de classe) [edit]
Tipo POD com exigência de alinhamento tão grande quanto qualquer outro tipo escalar
Original:
POD type with alignment requirement as great as any other scalar type
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(typedef) [edit]