constexpr specifier (depuis C++11) — cppreference.com

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constexpr - indique que la valeur d'une variable ou une fonction peut être calculée à la compilation
Original:
constexpr - specifies that the value of a variable or function can be computed at compile time
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Explication

constexpr spécificateur déclare qu'il est possible d'évaluer la valeur de la fonction ou variable au moment de la compilation, ils peuvent ensuite être utilisés lorsque que le temps de compiler expressions constantes sont autorisés. constexpr implique 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 expressions constantes are allowed. constexpr implies const.

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'constexpr les variables' doit satisfaire aux exigences suivantes:

Original:

constexpr variables must satisfy the following requirements:

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il doit être immédiatement construit ou assigné une valeur .
Original:
it must be immediately constructed or assigned a value.
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le constructeur paramètres ou la valeur à attribuer doit contenir uniquement des valeurs littérales, des variables et des fonctions constexpr .
Original:
the constructor parameters or the value to be assigned must contain only literal values, constexpr variables and functions.
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le constructeur utilisé pour construire l'objet (implicite ou explicite) doit satisfaire aux exigences de constructeur constexpr. Dans le cas de constructeur explicite, il doit avoir constexpr spécifié .
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 fonctions' doit satisfaire aux exigences suivantes:

Original:

constexpr functions must satisfy the following requirements:

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il ne doit pas être virtuelle
Original:
it must not be virtual
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son type de retour doit être LiteralType
Original:
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chacun de ses paramètres doivent être de type littéral
Original:
each of its parameters must be literal type
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le corps de la fonction doit être soit supprimée, soit par défaut ou contiennent seulement ce qui suit:
Original:
the function body must be either deleted or defaulted or contain only the following:
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déclarations nulles
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null statements
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static_assert déclarations
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static_assert declarations
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typedef déclarations et déclarations alias qui ne définissent pas des classes ou des énumérations
Original:
typedef declarations and alias declarations that do not define classes or enumerations
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l'utilisation de déclarations
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using declarations
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en utilisant les directives
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using directives
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exactement une déclaration return qui contient des valeurs littérales, des variables que constexpr et fonctions .
Original:
exactly one return statement that contains only literal values, constexpr variables and functions.
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'constexpr constructeur' doit satisfaire aux exigences suivantes:

Original:

constexpr constructor must satisfy the following requirements:

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chacun de ses paramètres doivent être de type littéral
Original:
each of its parameters must be literal type
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la classe ne doit pas avoir de classes de base virtuelles
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the class must have no virtual base classes
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le corps du constructeur doit être soit supprimée, soit par défaut ou contiennent seulement ce qui suit:
Original:
the constructor body must be either deleted or defaulted or contain only the following:
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déclarations nulles
Original:
null statements
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static_assert déclarations
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static_assert declarations
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typedef déclarations et déclarations alias qui ne définissent pas des classes ou des énumérations
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typedef declarations and alias declarations that do not define classes or enumerations
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l'utilisation de déclarations
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using declarations
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en utilisant les directives
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using directives
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le constructeur ne doit pas avoir un bloc de fonction-try
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the constructor must not have a function-try block
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chaque classe de base et chaque élément non-statique doit être initialisé, soit dans le initializer_list ou par initialisation entretoise-ou-égal. Terminer chaque constructeur concerné doit être un constructeur constexpr et chacune des clauses de chaque initialiseur accolades ou égal doit être une expression constante
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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chaque conversion implicite en question doit être une expression constante
Original:
every implicit conversion involved must be a constant expression
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Mots-clés

constexpr

Exemple

Définition d'une fonction qui calcule constexpr factorielles et un type littéral qui s'étend des chaînes littérales

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
}

Résultat :

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