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      constexpr specifier (depuis C++11)

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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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      [modifier]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:
        its return type must be LiteralType
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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
        Original:
        null statements
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      • static_assert déclarations
        Original:
        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
        Original:
        using declarations
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      • en utilisant les directives
        Original:
        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
        Original:
        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
        Original:
        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
        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 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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      [modifier]Mots-clés

      constexpr

      [modifier]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 iterationconstexprint factorial(int n){return n <=1?1:(n * factorial(n-1));}   // literal classclass conststr {constchar* p;std::size_t sz;public:template<std::size_t N>constexpr conststr(constchar(&a)[N]): p(a), sz(N-1){}// constexpr functions signal errors by throwing exceptions from operator ?:constexprchar operator[](std::size_t n){return n < sz ? p[n]:throwstd::out_of_range("");}constexprstd::size_t size(){return sz;}};constexprstd::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 testingtemplate<int n>struct constN { constN(){std::cout<< n <<'\n';}};   int main(){std::cout<<"4! = "; constN<factorial(4)> out1;// computed at compile time   volatileint 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
      Récupérée de « https://fr.cppreference.com/mwiki/index.php?title=cpp/language/constexpr&oldid=41552 »
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