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std::aligned_storage

Da cppreference.com.
< cpp‎ | types

 
 
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initializer_list(C++11)
bitset
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Tipo di supporto
Tipi base
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aligned_storage(C++11)
aligned_union(C++11)
decay(C++11)
enable_if(C++11)
conditional(C++11)
common_type(C++11)
underlying_type(C++11)
result_of(C++11)
Digitare costanti tratto
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Type trait constants
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integral_constant(C++11)
 
Defined in header <type_traits>
template< std::size_t Len, std::size_t Align = /*default-alignment*/ >
struct aligned_storage;
(dal C++11)
Fornisce il type membro typedef, che è un tipo POD adatto all'uso come memoria non inizializzata per qualsiasi oggetto la cui dimensione è al massimo e Len cui allineamento requisito è un divisore di Align. Il valore predefinito di Align è il requisito più rigoroso allineamento (il più grande) per qualsiasi oggetto la cui dimensione è al massimo 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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Indice

[modifica] Membri tipi

Nome
Original:
Name
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Definition
type
il tipo di POD Len dimensioni con Align allineamento requisito
Original:
the POD type of size Len with alignment requirement Align
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[modifica] Note

Tipo definito dal std::aligned_storage può essere usato per creare blocchi di memoria non inizializzate adatti a contenere gli oggetti di tipo determinato, eventualmente allineati rigorose del necessario, per esempio in una cache o limite di pagina.
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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[modifica] Possibile implementazione

Fatta eccezione per argomento di default, aligned_storage è esprimibile in termini di 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;
};

[modifica] Esempio

Una classe primitiva vettore statico, dimostrando la creazione, l'accesso, e la distruzione di oggetti in memoria allineati
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:

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

[modifica] Vedi anche

alignas specificatore
specifica che il deposito per la variabile deve essere allineato (C++11) quantità specifica
Original:
specifies that the storage for the variable should be aligned by specific amount (C++11)
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[edit]
ottiene requisiti di allineamento del tipo
Original:
obtains the type's alignment requirements
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(classe template) [edit]
definisce il tipo adatto per l'uso come memoria non inizializzata per tutti i tipi di dati
Original:
defines the type suitable for use as uninitialized storage for all given types
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(classe template) [edit]
Tipo di POD con il requisito di allineamento così grande come qualsiasi altro scalare
Original:
POD type with alignment requirement as great as any other scalar type
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(typedef) [edit]