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Default constructors

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C++
 
Linguagem C++
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Funções membro especiais
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construtor padrão
copiar construtor
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Assembly embutido
 
Um construtor padrão é um construtor que pode ser chamado sem argumentos (ou definidos com uma lista de parâmetros vazia, ou com argumentos padrão fornecidos para cada parâmetro). Um tipo com um construtor público padrão é 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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Índice

[editar] Sintaxe

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

[editar] Explicação

1)
Declaração de um construtor padrão
Original:
Declaration of a default constructor
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2)
Definição do construtor fora do corpo da classe
Original:
Definition of the constructor outside the class body
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3)
Inibindo a geração automática de um construtor padrão
Original:
Inhibiting the automatic generation of a default constructor
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4)
Explicitamente forçar a geração automática de um construtor padrão
Original:
Explicitly forcing the automatic generation of a default constructor
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ClassName é o identificador da classe de inclusão
Original:
ClassName is the identifier of the enclosing class
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Os construtores padrão são chamados quando:
Original:
The default constructors are called when:
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  • criação de objetos ou matrizes de duração estática de armazenamento, de segmento local, e automático que são declarados sem um inicializador, (T obj; ou 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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  • criação de objetos do tempo de armazenamento dinâmico quando a nova expressão é escrito sem um inicializador (new T;)
    Original:
    creating objects of dynamic storage duration when the new-expression is written without an initializer (new T;)
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  • criação de matrizes de duração de armazenamento dinâmico com o new T[n] expressão
    Original:
    creating arrays of dynamic storage duration with the expression new T[n]
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  • criação de valor-inicializados objetos temporários com a expressão de conversão T().
    Original:
    creating value-initialized temporary objects with the cast expression T().
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[editar] Implicitamente declarou construtor padrão

Se nenhum construtor definidas pelo usuário de qualquer espécie são fornecidos para um tipo de classe (struct, class, ou union), o compilador sempre declarar um construtor padrão como membro inline public de sua classe. Se alguns construtores definidos pelo usuário estão presentes, o usuário pode ainda forçar a geração do construtor implicitamente declarado com a palavra-chave default (desde 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 (desde C++11).
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[editar] Excluídos construtor padrão implicitamente declarado

O construtor padrão implicitamente declarado ou inadimplente para T classe é (até C++11) indefinido / definido como excluídos (desde C++11) se qualquer uma das seguintes situações:
Original:
The implicitly-declared or defaulted default constructor for class T is undefined (até C++11) / defined as deleted (desde C++11) if any of the following is true:
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  • T tem um membro do tipo de referência (sem uma cinta-ou-igual initializer(desde C++11)).
    Original:
    T has a member of reference type (without a brace-or-equal initializer(desde C++11)).
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  • T tem um membro const (sem initializer(desde C++11) cinta-ou-igual) ou definido pelo usuário construtor padrão.
    Original:
    T has a const member (without a brace-or-equal initializer(desde C++11)) or a user-defined default constructor.
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  • T tem um membro (sem initializer(desde C++11) cinta-ou-igual), que tem um construtor padrão excluído, ou seu construtor padrão é ambíguo ou inacessível a partir deste construtor.
    Original:
    T has a member (without a brace-or-equal initializer(desde C++11)), which has a deleted default constructor, or its default constructor is ambiguous or inaccessible from this constructor.
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  • T tem uma base direta ou virtual que tem um construtor padrão excluído, ou é ambíguo ou inacessível a partir deste construtor.
    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 tem uma base direta ou virtual que tem um destrutor excluído, ou um destruidor que é inacessível a partir deste construtor.
    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 é um union com pelo menos um membro com a variante não-trivial constructor(desde C++11) padrão.
    Original:
    T is a union with at least one variant member with non-trivial default constructor(desde C++11).
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  • T é um union e todas as suas variantes são membros const.
    Original:
    T is a union and all of its variant members are const.
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[editar] Construtor padrão trivial

O construtor padrão implicitamente declarado para T classe é trivial se todo o seguinte é verdadeiro:
Original:
The implicitly-declared default constructor for class T is trivial if all of the following is true:
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  • T não tem funções de membro virtual
    Original:
    T has no virtual member functions
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  • T não tem classes base virtuais
    Original:
    T has no virtual base classes
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  • T não possui membros não-estáticos com inicializadores de cinta-ou-iguais (desde C++11)
    Original:
    T has no non-static members with brace-or-equal initializers (desde C++11)
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  • Cada base direta de T tem um construtor padrão trivial
    Original:
    Every direct base of T has a trivial default constructor
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  • Cada membro não-estático do tipo classe tem um construtor padrão trivial
    Original:
    Every non-static member of class type has a trivial default constructor
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Um construtor padrão trivial é um construtor que não executa nenhuma ação. Objetos com construtores padrão triviais podem ser criados usando reinterpret_cast em qualquer armazenamento, por exemplo, em memória alocada com std::malloc. Todos os tipos de dados compatíveis com a linguagem C (tipos POD) são trivialmente padrão-constructible.
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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[editar] Implicitamente definido construtor padrão

Se o construtor padrão implicitamente declarado não é excluído ou trivial, é definido (isto é, um corpo de função é gerado e compilado) pelo compilador, e ele tem exatamente o mesmo efeito que um construtor definido pelo usuário com o corpo vazio e vazio lista de inicializador. Ou seja, ele chama os construtores padrão das bases e dos membros não-estáticos desta 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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[editar] Exemplo

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
}
Obtida de "https://pt.cppreference.com/mwiki/index.php?title=cpp/language/default_constructor&oldid=20909"