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constexpr specifier (C++11およびそれ以降)

提供: cppreference.com
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  • constexpr - 変数や関数の値はコンパイル時に計算できることを指定します
    Original:
    constexpr - specifies that the value of a variable or function can be computed at compile time
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[編集] 説明

constexprのみが許可され、コンパイル時の場所定数式指定子は、それがコンパイル時に関数や変数の値を評価することが可能であることを宣言し、それらはその後使用することができます。 constexprconst暗示.
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 定数式 are allowed. constexpr implies const.
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'constexpr変数'は、次の要件を満たす必要があります
Original:
constexpr variables must satisfy the following requirements:
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  • それはすぐに構築するか、値を割り当てなければなりません.
    Original:
    it must be immediately constructed or assigned a value.
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  • コンストラクターのパラメーターまたはリテラル値のみ、constexpr変数と関数が含まれている必要が割り当てられる値.
    Original:
    the constructor parameters or the value to be assigned must contain only literal values, constexpr variables and functions.
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  • オブジェクト(暗黙的または明示的な)を構築するために使用されるコンストラクタはconstexprコンストラクタの要件を満たす必要があります。明示的なコンストラクタの場合には、それはconstexpr指定しておく必要があります.
    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機能'は、次の要件を満たす必要があります
Original:
constexpr functions must satisfy the following requirements:
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  • それは仮想であってはなりません
    Original:
    it must not be virtual
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  • その戻り値の型はLiteralTypeでなければなりません
    Original:
    its return type must be LiteralType
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  • その各パラメータはリテラル型でなければなりません
    Original:
    each of its parameters must be literal type
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  • 関数の本体には、削除されるか、またはデフォルト値または以下を含むだけである必要があります
    Original:
    the function body must be either deleted or defaulted or contain only the following:
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  • 空文
    Original:
    null statements
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  • static_assert宣言
    Original:
    static_assert declarations
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  • typedefクラスや列挙型を定義していない宣言とエイリアス宣言
    Original:
    typedef declarations and エイリアス declarations that do not define classes or enumerations
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  • 宣言を使用
    Original:
    using declarations
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  • ディレクティブを使用しています
    Original:
    using directives
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  • リテラル値のみ、return変数と関数が含まれています正確に1つのconstexpr文.
    Original:
    exactly one return statement that contains only literal values, constexpr variables and functions.
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'constexprコンストラクタ'次の要件を満たす必要があります
Original:
constexpr constructor must satisfy the following requirements:
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  • その各パラメータはリテラル型でなければなりません
    Original:
    each of its parameters must be literal type
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  • クラスには仮想基底クラスを持っていてはいけない
    Original:
    the class must have no virtual base classes
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  • コンストラクタ本体が削除されるか、またはデフォルト値または以下を含むだけである必要があります
    Original:
    the constructor body must be either deleted or defaulted or contain only the following:
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  • 空文
    Original:
    null statements
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  • static_assert宣言
    Original:
    static_assert declarations
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  • typedefクラスや列挙型を定義していない宣言とエイリアス宣言
    Original:
    typedef declarations and エイリアス declarations that do not define classes or enumerations
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  • 宣言を使用
    Original:
    using declarations
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  • ディレクティブを使用しています
    Original:
    using directives
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  • コンストラクタは関数tryブロックを持ってはいけません
    Original:
    the constructor must not have a function-try block
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  • すべての基底クラスとすべての非静的メンバはinitializer_listやブレースか等しい初期化子のいずれかによって、初期化する必要があります。関係するすべてのコンストラクタはconstexprのコンストラクタでなければならず、すべてのブレースか等しい初期化子のすべての句が定数式でなければなりません。終了
    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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  • 関係するすべての暗黙的な変換は定数式でなければなりません
    Original:
    every implicit conversion involved must be a constant expression
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[編集] キーワード

constexpr

[編集]

階乗と文字列リテラルを拡張リテラル型を計算constexprの関数の定義

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
}

出力:

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