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    constexpr specifier (depuis C++11)

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    <metanoindex/>

     
    C++
     
    Langage C++
    Sujets gnraux
    Contrle de flux
    Instructions conditionnelles
    Instructions d'itration
    Instructions de saut
    Fonctions
    dclaration de fonction
    expression lambda
    fonction gnrique
    spcificateur inline
    spcification d'exception (obsolte)
    spcificateur noexcept (C++11)
    Exceptions
    Espaces de noms
    Types
    spcificateur decltype (C++11)
    Qualificatifs
    qualificatifs const et volatile
    qualificatifs de stockage
    qualificatif constexpr (C++11)
    qualificatif auto (C++11)
    qualificatif alignas (C++11)
    Initialisation
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    oprateurs alternatifs
    Utilitaires
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    dclaration typedef
    dclaration d'alias de type (C++11)
    attributs (C++11)
    Jette
    Original:
    Casts
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    conversions implicites
    conversion const_cast
    conversion static_cast
    conversion dynamic_cast
    conversion reinterpret_cast
    conversions style C et style fonction
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    Divers
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    • constexpr - indique que la valeur d'une variable ou une fonction peut tre calcule 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 spcificateur dclare qu'il est possible d'valuer la valeur de la fonction ou variable au moment de la compilation, ils peuvent ensuite tre utiliss lorsque que le temps de compiler expressions constantes sont autoriss. 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 tre immdiatement construit ou assign une valeur .
      Original:
      it must be immediately constructed or assigned a value.
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    • le constructeur paramtres ou la valeur attribuer doit contenir uniquement des valeurs littrales, 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 utilis pour construire l'objet (implicite ou explicite) doit satisfaire aux exigences de constructeur constexpr. Dans le cas de constructeur explicite, il doit avoir constexpr spcifi .
      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 tre virtuelle
      Original:
      it must not be virtual
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    • son type de retour doit tre LiteralType
      Original:
      its return type must be LiteralType
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    • chacun de ses paramtres doivent tre de type littral
      Original:
      each of its parameters must be literal type
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    • le corps de la fonction doit tre soit supprime, soit par dfaut 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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    • dclarations nulles
      Original:
      null statements
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    • static_assert dclarations
      Original:
      static_assert declarations
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    • typedef dclarations et dclarations alias qui ne dfinissent pas des classes ou des numrations
      Original:
      typedef declarations and alias declarations that do not define classes or enumerations
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    • l'utilisation de dclarations
      Original:
      using declarations
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    • en utilisant les directives
      Original:
      using directives
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    • exactement une dclaration return qui contient des valeurs littrales, 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 paramtres doivent tre de type littral
      Original:
      each of its parameters must be literal type
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    • la classe ne doit pas avoir de classes de base virtuelles
      Original:
      the class must have no virtual base classes
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    • le corps du constructeur doit tre soit supprime, soit par dfaut 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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    • dclarations nulles
      Original:
      null statements
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    • static_assert dclarations
      Original:
      static_assert declarations
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    • typedef dclarations et dclarations alias qui ne dfinissent pas des classes ou des numrations
      Original:
      typedef declarations and alias declarations that do not define classes or enumerations
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    • l'utilisation de dclarations
      Original:
      using declarations
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    • en utilisant les directives
      Original:
      using directives
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    • le constructeur ne doit pas avoir un bloc de fonction-try
      Original:
      the constructor must not have a function-try block
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    • chaque classe de base et chaque lment non-statique doit tre initialis, soit dans le initializer_list ou par initialisation entretoise-ou-gal. Terminer chaque constructeur concern doit tre un constructeur constexpr et chacune des clauses de chaque initialiseur accolades ou gal doit tre 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 tre une expression constante
      Original:
      every implicit conversion involved must be a constant expression
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    Mots-cls

    constexpr

    Exemple

    Dfinition d'une fonction qui calcule constexpr factorielles et un type littral qui s'tend des chanes littrales

    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
}

    Rsultat : 

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