constexpr specifier (seit C++11) – cppreference.com

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constexpr - bedeutet, der Wert eines Ausdrucks kann zur Übersetzungszeit berechnet werden.
Original:
constexpr - specifies that the value of a variable or function can be computed at compile time
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Erklärung

constexpr Spezifizierer legt fest, dass es möglich sein muss, den Wert des Ausdrucks zur Übersetzungszeit zu berechnen. Der Ausdruck kann dann dort verwendet werden, wo nur zur Kompilierungszeit konstante Ausdrücke erlaubt sind. constexpr bedeutet const .

Original:

constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time, they then can be used where only compile time konstante Ausdrücke are allowed. constexpr implies const.

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'constexpr Variablen' muss die folgenden Anforderungen erfüllen:

Original:

constexpr variables must satisfy the following requirements:

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Es muss sofort gebaut werden oder ein Wert zugewiesen .
Original:
it must be immediately constructed or assigned a value.
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der Konstruktor Parameter oder der Wert zugewiesen werden darf nur literalen Werten, constexpr Variablen und Funktionen .
Original:
the constructor parameters or the value to be assigned must contain only literal values, constexpr variables and functions.
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der Konstruktor verwendet, um das Objekt (entweder implizit oder explizit) konstruieren müssen den Anforderungen der constexpr Konstruktor. Im Falle der expliziten Konstruktor, muss es constexpr angegeben haben .
Original:
the constructor used to construct the object (either implicit or explicit) must satisfy the requirements of constexpr constructor. In the case of explicit constructor, it must have constexpr specified.
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'constexpr Funktionen' muss die folgenden Anforderungen erfüllen:

Original:

constexpr functions must satisfy the following requirements:

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es darf nicht virtuell
Original:
it must not be virtual
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seine Rückkehr Typ muss LiteralType sein
Original:
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jeder seiner Parameter müssen wörtliche Typ sein
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each of its parameters must be literal type
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die Funktion Körper muss entweder gelöscht oder ausgefallen oder enthalten nur die folgenden:
Original:
the function body must be either deleted or defaulted or contain only the following:
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null Aussagen
Original:
null statements
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static_assert Erklärungen
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static_assert declarations
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typedef Erklärungen und Alias Erklärungen, die nicht definieren Klassen oder Aufzählungen
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typedef declarations and Alias declarations that do not define classes or enumerations
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mit Erklärungen
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using declarations
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using-Direktiven
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using directives
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genau ein return Anweisung, die nur literalen Werten, constexpr Variablen und Funktionen enthält .
Original:
exactly one return statement that contains only literal values, constexpr variables and functions.
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'constexpr Konstruktor' muss folgenden Anforderungen genügen:

Original:

constexpr constructor must satisfy the following requirements:

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jeder seiner Parameter müssen wörtliche Typ sein
Original:
each of its parameters must be literal type
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Die Klasse muss keine virtuellen Basisklassen
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the class must have no virtual base classes
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der Konstruktor Körper muss entweder gelöscht oder ausgefallen oder enthalten nur die folgenden:
Original:
the constructor body must be either deleted or defaulted or contain only the following:
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null Aussagen
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null statements
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static_assert Erklärungen
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static_assert declarations
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typedef Erklärungen und Alias Erklärungen, die nicht definieren Klassen oder Aufzählungen
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typedef declarations and Alias declarations that do not define classes or enumerations
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mit Erklärungen
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using declarations
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using-Direktiven
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using directives
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Der Konstruktor muss keine Funktion-try-Block
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the constructor must not have a function-try block
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jede Basisklasse und jeder Nicht-statisches Element muss initialisiert werden, entweder in der initializer_list oder durch Klammer-oder-gleich-Initialisierung. Ende jeder Bauherr beteiligt sind, müssen eine constexpr Konstruktor und jede Klausel jeder Klammer-oder-gleich initializer sein muss ein konstanter Ausdruck sein
Original:
every base class and every non-static member must be initialized, either in the initializer_list or by brace-or-equal initializer. End every constructor involved must be a constexpr constructor and every clause of every brace-or-equal initializer must be a constant expression
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jede implizite Konvertierung einbezogen muss ein konstanter Ausdruck sein
Original:
every implicit conversion involved must be a constant expression
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Keywords

constexpr

Beispiel

Definition eines constexpr Funktion, die Fakultäten und eine buchstäbliche Art, die Zeichenfolgenliterale erstreckt berechnet

Original:

Definition of a constexpr function which computes factorials and a literal type that extends string literals

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#include <iostream>
#include <stdexcept>

// constexpr functions use recursion rather than iteration
constexpr int factorial(int n)
{
    return n <= 1 ? 1 : (n * factorial(n-1));
}

// literal class
class conststr {
    const char * p;
    std::size_t sz;
 public:
    template<std::size_t N>
    constexpr conststr(const char(&a)[N]) : p(a), sz(N-1) {}
    // constexpr functions signal errors by throwing exceptions from operator ?:
    constexpr char operator[](std::size_t n) {
        return n < sz ? p[n] : throw std::out_of_range("");
    }
    constexpr std::size_t size() { return sz; }
};
constexpr std::size_t countlower(conststr s, std::size_t n = 0,
                                             std::size_t c = 0) {
    return n == s.size() ? c :
           s[n] >= 'a' && s[n] <= 'z' ? countlower(s, n+1, c+1) :
           countlower(s, n+1, c);
}

// output function that requires a compile-time constant, for testing
template<int n> struct constN {
    constN() { std::cout << n << '\n'; }
};

int main()
{
    std::cout << "4! = " ;
    constN<factorial(4)> out1; // computed at compile time

    volatile int k = 8;
    std::cout << k << "! = " << factorial(k) << '\n'; // computed at run time

    std::cout << "Number of lowercase letters in \"Hello, world!\" is ";
    constN<countlower("Hello, world!")> out2; // implicitly converted to conststr
}

Output:

4! = 24
8! = 40320
Number of lowercase letters in "Hello, world!" is 9