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value initialization

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C++
 
Linguaggio C + +
Temi generali
Original:
General topics
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Controllo del flusso
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Flow control
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Dichiarazioni esecuzione condizionale
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Conditional execution statements
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Iterazione dichiarazioni
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Iteration statements
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Vai dichiarazioni
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Jump statements
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Funzioni
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Functions
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dichiarazione di funzione
lambda funzione dichiarazione
funzione di modello
specificatore inline
eccezioni specifiche (deprecato)
noexcept specificatore (C++11)
Eccezioni
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Exceptions
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Spazi dei nomi
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Namespaces
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Tipi
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Types
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decltype specifier (C++11)
Specifiers
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Specifiers
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cv specificatori
Durata di stoccaggio specificatori
constexpr specificatore (C++11)
specificatore auto (C++11)
alignas specificatore (C++11)
Inizializzazione
Original:
Initialization
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Letterali
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Literals
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Espressioni
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Expressions
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rappresentazioni alternative
Utilities
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Utilities
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Tipi
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Types
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typedef declaration
Tipo alias dichiarazione (C++11)
attributi (C++11)
Lancia
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Casts
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conversioni implicite
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
Fusione C-stile e funzionale
Occupazione della memoria
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Memory allocation
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Classi
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Classes
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Specifiche per una classe di funzioni proprietà
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Class-specific function properties
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esplicito (C++11)
statico
Funzioni membro speciali
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Special member functions
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Modelli
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Templates
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classe template
funzione di modello
modello di specializzazione
parametri confezioni (C++11)
Varie
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Miscellaneous
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Montaggio in linea
 
Fornisce il valore iniziale di default per un nuovo oggetto.
Original:
Provides the default initial value to a new object.
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Indice

[modifica] Sintassi

T object {}; (1) (dal C++11)
T();

T{};

(2)
(dal C++11)
new T ();

new T {};

(3)
(dal C++11)

[modifica] Spiegazione

Valore di inizializzazione viene eseguita in tre situazioni:
Original:
Value initialization is performed in three situations:
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1)
quando una variabile di nome (automatico, statico, o filo-locale) è dichiarata con l'inizializzatore costituito da una coppia di parentesi graffe. (dal C++11)
Original:
when a named variable (automatic, static, or thread-local) is declared with the initializer consisting of a pair of braces. (dal C++11)
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2)
quando un oggetto senza nome temporaneo viene creato con l'inizializzatore costituito da una coppia vuota di parentesi o graffe.
Original:
when a nameless temporary object is created with the initializer consisting of an empty pair of parentheses or braces.
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3)
quando un oggetto con durata di stoccaggio dinamico viene creato da una nuova espressione con l'inizializzatore costituito da una coppia di parentesi vuote o graffe.
Original:
when an object with dynamic storage duration is created by a new-expression with the initializer consisting of an empty pair of parentheses or braces.
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Gli effetti del valore di inizializzazione sono:
Original:
The effects of value initialization are:
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  • Se T è un tipo di classe con almeno un fornito dall'utente costruttore di qualsiasi tipo, il costruttore di default è chiamato.
    Original:
    If T is a class type with at least one user-provided constructor of any kind, the costruttore di default is called.
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  • Se T è un non-union tipo di classe, senza alcun costruttore forniti dall'utente, l'oggetto è inizializzati a zero e poi il costruttore predefinito implicito dichiarato è chiamato (a meno che non è banale)
    Original:
    If T is an non-union class type without any user-provided constructors, then the object is inizializzati a zero and then the implicitly-declared default constructor is called (unless it's trivial)
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  • Se T è un tipo di matrice, ogni elemento della matrice è valore-inizializzato
    Original:
    If T is an array type, each element of the array is value-initialized
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  • Altrimenti, l'oggetto è inizializzati a zero.
    Original:
    Otherwise, the object is zero-initialized.
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[modifica] Note

Il T object(); sintassi non inizializzare un oggetto, ma dichiara una funzione che non prende argomenti e T rendimenti. Il modo per valore inizializzare una variabile denominata prima di C + 11 era T object = T();, che inizializza un valore temporaneo e quindi copia-inizializza l'oggetto: la maggior parte dei compilatori ottimizzare la copia in questo caso.
Original:
The syntax T object(); does not initialize an object; it declares a function that takes no arguments and returns T. The way to value-initialize a named variable before C++11 was T object = T();, which value-initializes a temporary and then copy-initializes the object: most compilers optimize out the copy in this case.
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I riferimenti non può essere valore inizializzato.
Original:
References cannot be value-initialized.
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Tutti i contenitori standard (std::vector, std::list, ecc) valore-inizializzare i loro elementi quando costruito con un argomento size_type singolo o quando viene coltivata da una chiamata a resize().
Original:
All standard containers (std::vector, std::list, etc) value-initialize their elements when constructed with a single size_type argument or when grown by a call to resize().
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[modifica] Esempio

#include <string>
#include <vector>
#include <iostream>
 
struct T1 {
    int mem1;
    std::string mem2;
}; // no constructors
struct T2 { 
    int mem1;
    std::string mem2;
    T2(const T2&) {} // a constructor, but no default
};
struct T3 { 
    int mem1;
    std::string mem2;
    T3() {} // user-provided default ctor
};
 
std::string s{}; // calls default ctor, the value is "" (empty string)
int main()
{
    int n{};     // non-class value-initialization, value is 0
    double f = double(); // non-class value-init, value is 0.0
    int* a = new int[10](); // array of 10 zeroes
 
    T1 t1{}; // no ctors: zero-initialized
             // t1.mem1 is zero-initialized
             // t1.mem2 is default-initialized
//    T2 t2{}; // error: has a ctor, but no default ctor
    T3 t3{}; // user-defined default ctor:
             // t3.mem1 is default-initialized (the value is indeterminate)
             // t3.mem2 is default-initialized
 
    std::vector<int> v(3); // value-initializes three ints
 
    std::cout << s.size() << ' ' << n << ' ' << f << ' ' << a[9] << ' ' << v[2] << '\n';
    std::cout << t1.mem1 << ' ' << t3.mem1 << '\n';
    delete[] a;
}

Output: 

0 0 0 0 0
0 4199376

[modifica] Vedi anche

Estratto da "https://it.cppreference.com/mwiki/index.php?title=cpp/language/value_initialization&oldid=25997"