constexpr specifier (desde C++11) - cppreference.com

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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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Explicação

constexpr especificador declara que é possível avaliar o valor da função ou variável em tempo de compilação, então eles podem ser usados onde apenas o tempo de compilação expressões constantes são permitidos. constexpr implica 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 expressões constantes are allowed. constexpr implies const.

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'constexpr variáveis' deve satisfazer os seguintes requisitos:

Original:

constexpr variables must satisfy the following requirements:

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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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'constexpr funções' deve satisfazer os seguintes requisitos:

Original:

constexpr functions must satisfy the following requirements:

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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:
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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
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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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'constexpr construtor' deve satisfazer os seguintes requisitos:

Original:

constexpr constructor must satisfy the following requirements:

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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
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null statements
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static_assert declarações
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static_assert declarations
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typedef declarações e declarações alias que não definem classes ou enumerações
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typedef declarations and alias declarations that do not define classes or enumerations
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usando declarações
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using declarations
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usando directivas
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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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Palavras-chave

constexpr

Exemplo

Definição de uma função que calcula constexpr fatoriais e um tipo literal que se estende seqüências literais

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
}

Saída:

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