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auto specifier (a partir do C++ 11)

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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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Instruções de iteração
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Iteration statements
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Ir declarações
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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
especificações de exceção (obsoleta)
noexcept especificador (C++11)
Exceções
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Exceptions
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Namespaces
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Namespaces
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Tipos
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Types
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decltype specifier (C++11)
Especificadores
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Specifiers
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cv especificadores
armazenamento duração especificadores
constexpr especificador (C++11)
auto especificador (C++11)
alignas especificador (C++11)
Inicialização
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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
modelo de função
especialização de modelo
pacotes de parâmetros (C++11)
Diversos
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Miscellaneous
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Assembly embutido
 
Especifica que o tipo da variável que está a ser declarado será automaticamente deduzida da sua inicializador. Para funções, especifica que o tipo de retorno é um tipo de retorno de fuga.
Original:
Specifies that the type of the variable that is being declared will be automatically deduced from its initializer. For functions, specifies that the return type is a trailing return type.
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Índice

[editar] Sintaxe

auto variable initializer (a partir do C++ 11)
auto function -> return type (a partir do C++ 11)

[editar] Explicação

1)
Ao declarar variáveis ​​no escopo de bloco, em âmbito namespace, em declarações de inicialização de loops, etc, o tipo da variável pode ser omitido ea palavra-chave auto pode ser usado em vez.
Original:
When declaring variables in block scope, in namespace scope, in init statements of for loops, etc, the type of the variable may be omitted and the keyword auto may be used instead.
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Uma vez que o tipo do inicializador foi determinada, o compilador determina o tipo que irá substituir a auto palavra-chave como se estivesse usando as regras para a dedução modelo argumento de uma chamada de função. O auto palavra-chave pode ser acompanhado por modifica, como const ou &, que irão participar na dedução tipo. Por exemplo, dado const auto& i = expr;, o tipo de i é exactamente o tipo de u argumento numa template<class U> void f(const U& u) modelo imaginário se a chamada de função f(expr) foi compilado.
Original:
Once the type of the initializer has been determined, the compiler determines the type that will replace the keyword auto as if using the rules for template argument deduction from a function call. The keyword auto may be accompanied by modifies, such as const or &, which will participate in the type deduction. For example, given const auto& i = expr;, the type of i is exactly the type of the argument u in an imaginary template template<class U> void f(const U& u) if the function call f(expr) was compiled.
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2)
Em um declaração da função, o auto palavra-chave não realizar a detecção automática de tipo. Serve apenas como uma parte da sintaxe de fuga retorno tipo.
Original:
In a declaração da função, the keyword auto does not perform automatic type detection. It only serves as a part of the trailing return type syntax.
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[editar] Notas

Até C + +11, auto teve a semântica de um armazenamento duração especificador.
Original:
Until C++11, auto had the semantic of a armazenamento duração especificador.
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[editar] Exemplo

#include <iostream>
#include <cmath>
#include <typeinfo>
 
template<class T, class U>
auto add(T t, U u) -> decltype(t + u) // the return type of add is the type of operator+(T,U)
{
    return t + u;
}
 
auto get_fun(int arg)->double(*)(double) // same as double (*get_fun(int))(double)
{
    switch (arg) {
        case 1: return std::fabs;
        case 2: return std::sin;
        default: return std::cos;
    }
}
 
int main()
{
    auto a = 1 + 2;
    std::cout << "type of a: " << typeid(a).name() << '\n';
    auto b = add(1, 1.2);
    std::cout << "type of b: " << typeid(b).name() << '\n';
    //auto int c; //compile-time error
    auto d = {1, 2};
    std::cout << "type of d: " << typeid(d).name() << '\n';
 
    auto my_lambda = [](int x) { return x + 3; };
    std::cout << "my_lambda: " << my_lambda(5) << '\n';
 
    auto my_fun = get_fun(2);
    std::cout << "type of my_fun: " << typeid(my_fun).name() << '\n';
    std::cout << "my_fun: " << my_fun(3) << '\n';
}

Output:

type of a: int
type of b: double
type of d: std::initializer_list<int>
my_lambda: 8
type of my_fun: double (*)(double)
my_fun: 0.14112