Namespace
Varianti

Default constructors

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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
Original:
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
Original:
Special member functions
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costruttore di default
costruttore di copia
spostare costruttore (C++11)
Modelli
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Templates
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classe template
funzione di modello
modello di specializzazione
parametri confezioni (C++11)
Varie
Original:
Miscellaneous
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Montaggio in linea
 
Un costruttore predefinito è un costruttore che può essere chiamato senza argomenti (sia definito con un elenco di parametri vuoto, o con argomenti di default previsti per ogni parametro). Un tipo con un costruttore pubblico predefinito è DefaultConstructible.
Original:
A default constructor is a constructor which can be called with no arguments (either defined with an empty parameter list, or with default arguments provided for every parameter). A type with a public default constructor is DefaultConstructible.
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Indice

[modifica] Sintassi

ClassName ( ) ; (1)
ClassName :: ClassName ( ) body (2)
ClassName() = delete ; (3) (dal C++11)
ClassName() = default ; (4) (dal C++11)

[modifica] Spiegazione

1)
Dichiarazione di un costruttore predefinito
Original:
Declaration of a default constructor
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2)
Definizione della funzione di costruzione al di fuori del corpo della classe
Original:
Definition of the constructor outside the class body
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3)
Inibire la generazione automatica di un costruttore predefinito
Original:
Inhibiting the automatic generation of a default constructor
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4)
Esplicitamente costringendo la generazione automatica di un costruttore predefinito
Original:
Explicitly forcing the automatic generation of a default constructor
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ClassName è l'identificatore della classe contenitrice
Original:
ClassName is the identifier of the enclosing class
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I costruttori di default vengono chiamati quando:
Original:
The default constructors are called when:
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  • creazione di oggetti o matrici di statica, la durata di archiviazione locale del thread, e automatico che vengono dichiarati senza un inizializzatore, (T obj; o T arr[10];)
    Original:
    creating objects or arrays of static, thread-local, and automatic storage duration that are declared without an initializer, (T obj; or T arr[10];)
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  • la creazione di oggetti di durata di conservazione dinamica quando la nuova espressione viene scritto senza un inizializzatore (new T;)
    Original:
    creating objects of dynamic storage duration when the new-expression is written without an initializer (new T;)
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  • la creazione di array di durata di stoccaggio dinamico con il new T[n] espressione
    Original:
    creating arrays of dynamic storage duration with the expression new T[n]
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  • creazione di valore-inizializzati gli oggetti temporanei con l'espressione cast T().
    Original:
    creating value-initialized temporary objects with the cast expression T().
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[modifica] Implicitamente-ha dichiarato costruttore predefinito

Se non definiti dall'utente costruttori di qualsiasi tipo sono previsti per un tipo di classe (struct, class o union), il compilatore sempre dichiarare un costruttore predefinito come membro inline public della sua classe. Se alcuni definiti dall'utente costruttori sono presenti, l'utente può comunque forzare la generazione del costruttore implicitamente dichiarata con la parola chiave default (dal C++11).
Original:
If no user-defined constructors of any kind are provided for a class type (struct, class, or union), the compiler will always declare a default constructor as an inline public member of its class. If some user-defined constructors are present, the user may still force the generation of the implicitly declared constructor with the keyword default (dal C++11).
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[modifica] Soppresso costruttore predefinito implicito dichiarato

Il costruttore di default implicitamente dichiarate o inadempiente per T classe è (C fino + 11) undefined / definito come cancellati (dal C++11) se una delle seguenti condizioni:
Original:
The implicitly-declared or defaulted default constructor for class T is undefined (C fino + 11) / defined as deleted (dal C++11) if any of the following is true:
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  • T ha un membro del tipo di riferimento (senza una coppia-o-uguale initializer(dal C++11)).
    Original:
    T has a member of reference type (without a brace-or-equal initializer(dal C++11)).
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  • T ha un elemento const (senza initializer(dal C++11) tutore-o-uguale) o una definita dall'utente costruttore di default.
    Original:
    T has a const member (without a brace-or-equal initializer(dal C++11)) or a user-defined default constructor.
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  • T ha un membro (senza initializer(dal C++11) tutore-o-uguale), che ha un costruttore predefinito cancellato, o il suo costruttore di default è ambigua o inaccessibili da questo costruttore.
    Original:
    T has a member (without a brace-or-equal initializer(dal C++11)), which has a deleted default constructor, or its default constructor is ambiguous or inaccessible from this constructor.
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  • T ha una base diretta o virtuale che ha un costruttore predefinito cancellato, o è ambiguo o inaccessibili da questo costruttore.
    Original:
    T has a direct or virtual base which has a deleted default constructor, or it is ambiguous or inaccessible from this constructor.
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  • T ha una base diretta o virtuale che ha un distruttore cancellato, o un distruttore che è inaccessibile da questo costruttore.
    Original:
    T has a direct or virtual base which has a deleted destructor, or a destructor that is inaccessible from this constructor.
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  • T è un union con almeno un membro di variante con non banale constructor(dal C++11) predefinito.
    Original:
    T is a union with at least one variant member with non-trivial default constructor(dal C++11).
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  • T è un union e tutti i suoi membri sono varianti const.
    Original:
    T is a union and all of its variant members are const.
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[modifica] Costruttore di default Trivial

Il costruttore predefinito implicito dichiarato per T classe è banale se tutte le seguenti condizioni:
Original:
The implicitly-declared default constructor for class T is trivial if all of the following is true:
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  • T non ha funzioni membro virtuali
    Original:
    T has no virtual member functions
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  • T non ha classi base virtuali
    Original:
    T has no virtual base classes
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  • T non ha membri non statici con inizializzatori di coppia-o-uguale (dal C++11)
    Original:
    T has no non-static members with brace-or-equal initializers (dal C++11)
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  • Ogni base diretta di T dispone di un costruttore predefinito semplice
    Original:
    Every direct base of T has a trivial default constructor
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  • Ogni membro non statico di tipo classe dispone di un costruttore predefinito semplice
    Original:
    Every non-static member of class type has a trivial default constructor
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Un costruttore predefinito banale è un costruttore che non esegue alcuna azione. Gli oggetti con costruttori di default banali possono essere creati utilizzando reinterpret_cast su qualsiasi deposito, ad esempio, sulla memoria allocata con std::malloc. Tutti i tipi di dati compatibili con il linguaggio C (tipo POD) sono banalmente default-costruibile.
Original:
A trivial default constructor is a constructor that performs no action. Objects with trivial default constructors can be created by using reinterpret_cast on any storage, e.g. on memory allocated with std::malloc. All data types compatible with the C language (POD types) are trivially default-constructible.
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[modifica] Implicitamente definito costruttore predefinito

Se il costruttore predefinito implicito dichiarati non viene eliminato o banale, è definito (cioè, un corpo di funzione viene generato e compilato) dal compilatore, ed ha esattamente lo stesso effetto come definito dall'utente costruttore con corpo vuoto e vuoto inizializzatore lista. Cioè, chiama i costruttori predefiniti delle basi e dei membri non statici di questa classe.
Original:
If the implicitly-declared default constructor is not deleted or trivial, it is defined (that is, a function body is generated and compiled) by the compiler, and it has exactly the same effect as a user-defined constructor with empty body and empty initializer list. That is, it calls the default constructors of the bases and of the non-static members of this class.
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[modifica] Esempio

struct A {
    int x;
    A(int x = 1) : x(x) {} // user-defined default ctor
};
struct B : A {
    // B::B() is implicitly-defined, calls A::A()
};
struct C {
    A obj;
    // C::C() is implicitly-defined, calls A::A()
};
struct D : A {
    D(int y) : A(y) {}
    // D::D() is not declared because another constructor exists
};
struct E : A
{
    E(int y) : A(y) {}
    E() = default; // explicitly defaulted, calls A::A()
};
 
struct F {
    int& ref; // reference member
    const int c; // const member
    // Bad::Bad() is deleted
};
 
int main()
{
    A a;
    B b;
//  D d; // compile error
    E e;
//  F f; // compile error
}