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    decltype specifier

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    Types
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    decltype specifier(C++11)
    Specifiers
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    Specifiers
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    Überprüft den deklarierten Typ eines Unternehmens oder fragt den Rückgabetyp eines Ausdrucks .
    Original:
    Inspects the declared type of an entity or queries the return type of an expression.
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    Inhaltsverzeichnis

    [Bearbeiten]Syntax

    decltype (entity) (1) (seit C++11)
    decltype (expression) (2) (seit C++11)

    [Bearbeiten]Erklärung

    1)
    Wenn das Argument ist entweder die unparenthesised Namen eines Objekts / Funktion, oder ein Mitglied Zugang Ausdruck (object.member oder pointer->member), dann die decltype gibt den deklarierten Typ des entity durch diesen Ausdruck angegeben .
    Original:
    If the argument is either the unparenthesised name of an object/function, or is a member access expression (object.member or pointer->member), then the decltype specifies the declared type of the entity specified by this expression.
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    2)
    Wenn das Argument eine andere Ausdruck des Typs T, dann
    Original:
    If the argument is any other expression of type T, then
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    a)
    wenn die Value-Kategorie der expression ist xWert, dann die decltype gibt T&&
    Original:
    if the Value-Kategorie of expression is xvalue, then the decltype specifies T&&
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    b)
    wenn der Wert Kategorie expression ist lvalue, dann die decltype gibt T&
    Original:
    if the value category of expression is lvalue, then the decltype specifies T&
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    c)
    Andernfalls gibt decltype T
    Original:
    otherwise, decltype specifies T
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    Beachten Sie, dass, wenn der Name eines Objekts parenthesised es ein L-Wert Ausdruck wird damit decltype(arg) und decltype((arg)) sind oft verschiedene Arten .
    Original:
    Note that if the name of an object is parenthesised, it becomes an lvalue expression, thus decltype(arg) and decltype((arg)) are often different types.
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    decltype ist nützlich bei der Deklaration von Typen, die nur schwer oder gar nicht zu erklären, unter Verwendung von Standard-Notation, wie Lambda-verwandte Typen oder Arten, die auf Template-Parameter abhängig sind .
    Original:
    decltype is useful when declaring types that are difficult or impossible to declare using standard notation, like lambda-related types or types that depend on template parameters.
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    [Bearbeiten]Keywords

    decltype

    [Bearbeiten]Beispiel

    #include <iostream>   struct A {double x;};const A* a = new A();   decltype( a->x ) x3;// type of x3 is double (declared type) decltype((a->x)) x4 = x3;// type of x4 is const double& (lvalue expression)   template<class T, class U>auto add(T t, U u)-> decltype(t + u);// return type depends on template parameters   int main(){int i =33; decltype(i) j = i*2;   std::cout<<"i = "<< i <<", "<<"j = "<< j <<'\n';   auto f =[](int a, int b)->int{return a*b;};   decltype(f) f2{f};// the type of a lambda function is unique and unnamed i = f(2, 2); j = f2(3, 3);   std::cout<<"i = "<< i <<", "<<"j = "<< j <<'\n';}

    Output: 

    i = 33, j = 66 i = 4, j = 9

    [Bearbeiten]Siehe auch

    (C++11)
    erhält die Art des Ausdrucks in unevaluierten Kontext
    Original:
    obtains the type of expression in unevaluated context
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