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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;
(depuis C++11)
Fournit la type membre typedef, qui est un type POD peut être utilisé comme stockage non initialisée pour tout objet dont la taille est au plus Len et dont l'alignement exigence est un diviseur de Align. La valeur par défaut est le Align exigence d'alignement les plus strictes (le plus grand) pour tout objet dont la taille est au plus 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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Sommaire

[modifier] Types de membres

Nom
Original:
Name
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Definition
type
le type de POD de Len taille avec Align exigence d'alignement
Original:
the POD type of size Len with alignment requirement Align
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[modifier] Notes

Le type défini par std::aligned_storage peut être utilisé pour créer des blocs de mémoire non initialisée appropriés pour contenir les objets de type donné, éventuellement alignées plus strictes que nécessaire, par exemple sur un cache ou limite de page .
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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[modifier] Mise en œuvre possible

Sauf pour argument par défaut, aligned_storage est exprimable en termes 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;
};

[modifier] Exemple

Une classe vecteur primitif statique, démontrant la création, l'accès et la destruction d'objets dans le stockage alignés
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';
}

Résultat :

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

[modifier] Voir aussi

alignas spécificateur
spécifie que le stockage de la variable doit être aligné par (C++11) quantité spécifique
Original:
specifies that the storage for the variable should be aligned by specific amount (C++11)
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[edit]
obtient exigences d'alignement du type
Original:
obtains the type's alignment requirements
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(classe générique) [edit]
définit le type approprié pour être utilisé comme stockage non initialisée pour tous les types de données
Original:
defines the type suitable for use as uninitialized storage for all given types
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(classe générique) [edit]
(C++11)
Type de POD avec exigence d'alignement aussi grand que n'importe quel autre type scalaire
Original:
POD type with alignment requirement as great as any other scalar type
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(typedef) [edit]