constexpr specifier (desde C++11)

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• constexpr - especifica que o valor de uma variável ou função pode ser calculado em tempo de compilação

  Original:

  constexpr - specifies that the value of a variable or function can be computed at compile time

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[editar] Explicação

• deve ser imediatamente construídos ou atribuído um valor.

  Original:

  it must be immediately constructed or assigned a value.

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• o construtor parâmetros ou o valor a ser atribuído deve conter apenas valores literais, variáveis ​​constexpr e funções.

  Original:

  the constructor parameters or the value to be assigned must contain only literal values, constexpr variables and functions.

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• o construtor usado para construir o objeto (implícita ou explícita) deve satisfazer os requisitos do construtor constexpr. No caso do construtor explícito, deve ter constexpr especificado.

  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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• não deve ser virtual

  Original:

  it must not be virtual

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• seu tipo de retorno deve ser LiteralType

  Original:

  its return type must be LiteralType

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• cada um de seus parâmetros devem ser do tipo literal

  Original:

  each of its parameters must be literal type

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• o corpo da função deve ser excluído ou inadimplente ou conter apenas o seguinte:

  Original:

  the function body must be either deleted or defaulted or contain only the following:

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• declarações nulos

  Original:

  null statements

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• static_assert declarações

  Original:

  static_assert declarations

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• typedef declarações e declarações alias que não definem classes ou enumerações

  Original:

  typedef declarations and alias declarations that do not define classes or enumerations

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• usando declarações

  Original:

  using declarations

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• usando directivas

  Original:

  using directives

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• exatamente uma declaração return que contém somente os valores literais, variáveis ​​constexpr e funções.

  Original:

  exactly one return statement that contains only literal values, constexpr variables and functions.

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• cada um de seus parâmetros devem ser do tipo literal

  Original:

  each of its parameters must be literal type

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• a classe não deve ter classes base virtuais

  Original:

  the class must have no virtual base classes

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• o corpo do construtor deve ser excluído ou inadimplente ou conter apenas o seguinte:

  Original:

  the constructor body must be either deleted or defaulted or contain only the following:

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• declarações nulos

  Original:

  null statements

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• static_assert declarações

  Original:

  static_assert declarations

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• typedef declarações e declarações alias que não definem classes ou enumerações

  Original:

  typedef declarations and alias declarations that do not define classes or enumerations

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• usando declarações

  Original:

  using declarations

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• usando directivas

  Original:

  using directives

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• o construtor não deve ter um bloco try-função

  Original:

  the constructor must not have a function-try block

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• cada classe de base e todos os membros não-estático deve ser inicializado, seja no initializer_list ou inicializador de cinta-ou-igual. Terminar cada construtor envolvido deve ser um construtor constexpr e cada cláusula de cada inicializador cinta-ou-igual deve ser uma expressão 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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• cada conversão implícita envolvido deve ser uma expressão constante

  Original:

  every implicit conversion involved must be a constant expression

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[editar] Palavras-chave

constexpr

[editar] Exemplo

#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
}

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