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auto specifier (seit C++11)

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General topics
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Flusskontrolle
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Flow control
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Bedingte Ausführung Aussagen
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Conditional execution statements
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Iterationsanweisungen
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Iteration statements
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Gehe Aussagen
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Jump statements
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Funktionen
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Functions
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Funktion Erklärung
Lambda-Funktion Erklärung
Funktions-Template
inline-Spezifizierer
Exception-Spezifikationen (veraltet)
noexcept Spezifizierer (C++11)
Ausnahmen
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Exceptions
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Namespaces
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Namespaces
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Types
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Types
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decltype specifier (C++11)
Specifiers
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Specifiers
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cv Planer
Lagerdauer Planer
constexpr Spezifizierer (C++11)
auto Spezifizierer (C++11)
alignas Spezifizierer (C++11)
Initialisierung
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Initialization
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Literale
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Literals
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Expressions
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Expressions
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alternative Darstellungen
Utilities
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Utilities
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Types
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Types
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typedef declaration
Typ Aliasdeklaration (C++11)
Attribute (C++11)
Wirft
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Casts
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impliziten Konvertierungen
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-Stil und funktionale Besetzung
Speicherzuweisung
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Memory allocation
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Classes
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Classes
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Class-spezifische Funktion Eigenschaften
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Class-specific function properties
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explizit (C++11)
statisch
Besondere Member-Funktionen
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Special member functions
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Templates
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Templates
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Klassen-Template
Funktions-Template
Template-Spezialisierung
Parameter Packs (C++11)
Verschiedenes
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Miscellaneous
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Inline Montage
 
Gibt an, dass der Typ der Variable, die deklariert wird automatisch von seiner Initialisierung abgeleitet werden. Für Funktionen, gibt an, dass der Rückgabetyp ein hinteres Rückgabetyp ist .
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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Inhaltsverzeichnis

[Bearbeiten] Syntax

auto variable initializer (seit C++11)
auto function -> return type (seit C++11)

[Bearbeiten] Erklärung

1)
Bei der Deklaration von Variablen in Block Umfang, in Namespacebereich, in init Aussagen for-Schleifen, etc, kann der Typ der Variablen weggelassen werden, und das Schlüsselwort auto dürfen stattdessen verwendet werden .
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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Sobald die Art der Initialisierung ermittelt wurde, bestimmt der Compiler den Typ, die das Schlüsselwort auto ersetzen wird, als ob mit den Regeln für die Template-Argument Abzug von einem Funktionsaufruf. Das Schlüsselwort auto kann durch modifiziert begleitet werden, wie const oder &, die in der Art Abzug teilnehmen. Zum Beispiel, da const auto& i = expr;, ist die Art der i genau die Art des Arguments u in einer imaginären Vorlage template<class U> void f(const U& u) wenn der Funktionsaufruf f(expr) kompiliert wurde .
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)
In einer Funktion Erklärung, ist das Schlüsselwort auto keine automatische Erkennung des. Sie dient nur als ein Teil des nachlaufenden Rückgabetyp Syntax .
Original:
In a Funktion Erklärung, 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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[Bearbeiten] Notes

Bis C + +11, auto hatte die semantische eines Lagerdauer Spezifizierer .
Original:
Until C++11, auto had the semantic of a Lagerdauer Spezifizierer.
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[Bearbeiten] Beispiel

#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