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list initialization

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Linguagem C + +
Tópicos gerais
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General topics
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Controle de fluxo
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Flow control
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Declarações execução condicional
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Conditional execution statements
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Jump statements
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Funções
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Functions
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declaração da função
lambda declaração da função
modelo de função
linha especificador
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Exceções
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Exceptions
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Types
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decltype specifier (C++11)
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cv especificadores
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constexpr especificador (C++11)
auto especificador (C++11)
alignas especificador (C++11)
Inicialização
Original:
Initialization
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Literais
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Literals
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Expressões
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Expressions
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representações alternativas
Utilitários
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Utilities
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Tipos
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Types
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typedef declaration
tipo de alias declaração (C++11)
atributos (C++11)
Lança
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Casts
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conversões implícitas
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
Elenco C-estilo e funcional
Alocação de memória
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Memory allocation
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Classes
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Classes
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Classe propriedades específicas de função
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Class-specific function properties
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Funções membro especiais
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Special member functions
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Modelos
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Templates
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modelo de classe
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Miscellaneous
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Assembly embutido
 
Inicializa um objeto de braced-init-lista
Original:
Initializes an object from braced-init-list
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Índice

[editar] Sintaxe

T object { arg1, arg2, ... }; (1)
T { arg1, arg2, ... }; (2)
new T { arg1, arg2, ... }; (3)
return { arg1, arg2, ... } ; (4)
function( { arg1, arg2, ... } ) ; (5)
object[ { arg1, arg2, ... } ] ; (6)
T( { arg1, arg2, ... } ) (7)
Class { T member = { arg1, arg2, ... }; }; (8)
Class::Class() : member{arg1, arg2, ...} {... (9)
T object = {arg1, arg2, ...}; (10)

[editar] Explicação

Lista de inicialização é realizada nas seguintes situações:
Original:
List initialization is performed in the following situations:
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1)
inicialização de uma variável chamada com uma lista cinta-fechado de expressões ou listas aninhadas (preparou-init-list)
Original:
initialization of a named variable with a brace-enclosed list of expressions or nested lists (braced-init-list)
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2)
inicialização de um temporário sem nome com uma lista apoiou-init
Original:
initialization of an unnamed temporary with a braced-init-list
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3)
inicialização de um objeto com duração de armazenamento dinâmico, com uma expressão nova, onde o inicializador é uma cinta-init-lista
Original:
initialization of an object with dynamic storage duration with a new-expression, where the initializer is a brace-init-list
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4)
em comunicado return com braced-init-lista usada como a expressão de retorno
Original:
in a return statement with braced-init-list used as the return expression
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5)
em uma expressão chamada de função, com preparei-init-list usado como um argumento
Original:
in a function call expression, with braced-init-list used as an argument
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6)
em uma expressão subscrito com um operador definido pelo usuário []
Original:
in a subscript expression with a user-defined operator[]
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7)
em uma expressão de conversão funcional ou outro inicialização-direta, com preparei-init-lista usado como o argumento do construtor
Original:
in a functional cast expression or other direct-initialization, with braced-init-list used as the constructor argument
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8)
em um inicializador de membro não-estático de dados
Original:
in a non-static data member initializer
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9)
em uma lista de inicializador construtor
Original:
in a constructor initializer list
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10)
no lado direito do sinal de igual (semelhante ao cópia de inicialização)
Original:
on the right-hand-side of the equals sign (similar to cópia de inicialização)
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Os efeitos da inicialização lista de um objeto de T tipo são:
Original:
The effects of list initialization of an object of type T are:
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  • Se a lista braced-init-se vazio e T é um tipo de classe com um construtor padrão, valor de inicialização é realizada.
    Original:
    If the braced-init-list is empty and T is a class type with a default constructor, valor de inicialização is performed.
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  • Caso contrário, se T é uma especialização de std::initializer_list, um objeto std::initializer_list novo do mesmo tipo é construído e usado para dirigir-inicializar ou copiar-inicializar o objeto de T tipo, dependendo do contexto.
    Original:
    Otherwise, if T is a specialization of std::initializer_list, a new std::initializer_list object of the same type is constructed and used to direct-initialize or copy-initialize the object of type T, depending on context.
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  • Caso contrário, os constuctors de T são considerados, em duas fases:
    Original:
    Otherwise, the constuctors of T are considered, in two phases:
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  • Todos os construtores que levam std::initializer_list como o único argumento, ou como o primeiro argumento, se os argumentos restantes têm valores padrão, são examinados, e acompanhado por resolução de sobrecarga contra um único argumento do tipo std::initializer_list
    Original:
    All constructors that take std::initializer_list as the only argument, or as the first argument if the remaining arguments have default values, are examined, and matched by overload resolution against a single argument of type std::initializer_list
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  • Se a etapa anterior não produz uma partida, todos os construtores de T participar de resolução de sobrecarga em relação ao conjunto de argumentos que consiste nos elementos da braced-lista-init, com a restrição de que apenas conversões de estreitamento são permitidos. Se nesta fase produz um construtor explícito como o melhor para uma cópia de lista de inicialização, compilação falhar (nota, em simples cópia de inicialização, construtores explícitos não são consideradas em todos)
    Original:
    If the previous stage does not produce a match, all constructors of T participate in overload resolution against the set of arguments that consists of the elements of the braced-init-list, with the restriction that only narrowing conversions are allowed. If this stage produces an explicit constructor as the best match for a copy-list-initialization, compilation fails (note, in simple copy-initialization, explicit constructors are not considered at all)
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  • Caso contrário, se T é tipo de referência, um prvalue temporária do tipo referenciado é a lista inicializada, ea referência é vinculado a essa temporário.
    Original:
    Otherwise, if T is reference type, a prvalue temporary of the referenced type is list-initialized, and the reference is bound to that temporary.
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  • Caso contrário, se a lista preparou-init-tem apenas um elemento, T é direct-inicializado ou cópia inicializado, dependendo do contexto, exceto que as conversões de estreitamento não são permitidos.
    Original:
    Otherwise, if the braced-init-list has only one element, T is direct-inicializado or cópia inicializado, depending on context, except that narrowing conversions are not allowed.
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  • Caso contrário, se a lista preparou-init-não tem elementos, T é valor inicializado.
    Original:
    Otherwise, if the braced-init-list has no elements, T is valor inicializado.
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[editar] Estreitando conversões

lista de inicialização-limita o conversões implícitas permitido ao proibir o seguinte:
Original:
list-initialization limits the allowed conversões implícitas by prohibiting the following:
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  • conversão de um tipo de ponto flutuante para um tipo inteiro
    Original:
    conversion from a floating-point type to an integer type
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  • conversão de um long double para double ou float e conversão de double para float, salvo se a fonte é uma expressão constante cujo valor pode ser armazenado exatamente no tipo de destino
    Original:
    conversion from a long double to double or to float and conversion from double to float, except where the source is a constant expression whose value can be stored exactly in the target type
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  • conversão do tipo de enumeração inteiro ou sem escopo de tipo inteiro que não pode representar todos os valores do original, salvo fonte é uma expressão constante cujo valor pode ser armazenado exatamente no tipo de destino
    Original:
    conversion from integer or unscoped enumeration type to integer type that cannot represent all values of the original, except where source is a constant expression whose value can be stored exactly in the target type
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[editar] Notas

Preparou-init-lista não é uma expressão e não tem nenhum tipo em seu próprio: por exemplo, ao chamar um modelo de função, preparou-init-lista argumento não pode ser utilizado para dedução do tipo de modelo. A exceção especial é feita para o auto palavra-chave, o que deduz qualquer preparou-init-lista como std::initializer_list.
Original:
Braced-init-list is not an expression and has no type on its own: for example, when calling a function template, braced-init-list argument cannot be used for template type deduction. A special exception is made for the keyword auto, which deduces any braced-init-list as std::initializer_list.
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[editar] Exemplo

#include <iostream>
#include <vector>
#include <map>
#include <string>
 
struct Foo {
    std::vector<int> mem = {1,2,3}; // list-initialization of a non-static member
    std::vector<int> mem2;
    Foo() : mem2{-1, -2, -3} {} // list-initialization of a member in constructor
};
 
std::pair<std::string, std::string> f(std::pair<std::string, std::string> p)
{
    return {p.second, p.first}; // list-initialization in return statement
}
 
int main()
{
    int n0{};     // value-initialization (to zero)
    int n1{1};    // direct-list-initialization
    std::string s1{'a', 'b', 'c', 'd'}; // initializer-list constructor call
    std::string s2{s1, 2, 2};           // regular constructor call
    std::string s3{0x61, 'a'}; // initializer-list ctor is preferred to (int, char)
 
    int n2 = {1}; // copy-list-initialization
    double d = double{1.2}; // list-initialization of a temporary, then copy-init
 
    std::map<int, std::string> m = { // nested list-initialization
           {1, "a"},
           {2, {'a', 'b', 'c'} },
           {3, s1}
    };
 
    std::cout << f({"hello", "world"}).first // list-initialization in function call
              << '\n';
 
    const int (&ar)[2] = {1,2}; // binds a lvalue reference to a temporary array
    int&& r1 = {1}; // binds a rvalue reference to a temporary int
//  int& r2 = {2}; // error: cannot bind rvalue to a non-const lvalue ref
 
//  int bad{1.0}; // error: narrowing conversion
    unsigned char uc1{10}; // okay
//  unsigned char uc2{-1}; // error: narrowing conversion
 
    Foo f;
 
    std::cout << n0 << ' ' << n1 << ' ' << n2 << '\n'
              << s1 << ' ' << s2 << ' ' << s3 << '\n';
    for(auto p: m)
        std::cout << p.first << ' ' << p.second << '\n';
    for(auto n: f.mem)
        std::cout << n << ' ';
    for(auto n: f.mem2)
        std::cout << n << ' ';
}

Output:

world
0 1 1
abcd cd aa
1 a
2 abc
3 abcd
1 2 3 -1 -2 -3


[editar] Veja também