如何在编译时创建具有 string_views 序列的 constexpr 数组？

分类：编程问答

如何解决如何在编译时创建具有 string_views 序列的 constexpr 数组？

我想创建一个 constexpr std::array<std::string_view,ConstexprNumber>。例如，它应该包含 constexpr std::strings_view's，如下所示：

"text0","text1","text2",..... "textn"

我想出了以下初始解决方案：

#include <iostream>
#include <array>
#include <utility>
#include <string>
#include <string_view>

// Number of strings that we want to generate
constexpr size_t NumberOfTextsToGenerate = 10u;

// constexpr function to build a string
constexpr std::string_view makeString(unsigned int i) {
    return std::string_view("text");
}

// Helper: constexpr function that will create an array of string_views and initialize it
template <unsigned int... ManyIntegers>
constexpr auto generateTextHelper(std::integer_sequence<unsigned int,ManyIntegers...>) {
    return std::array<std::string_view,sizeof...(ManyIntegers)>{ {makeString(ManyIntegers)...}};
}

// Helper: constexpr function that will return an array of string_views as shown above with a specified number of texts
constexpr auto generateTextArray() {
    return generateTextHelper(std::make_integer_sequence<unsigned int,NumberOfTextsToGenerate>());
}

// This is a definition of a std::array<std::string_view,UpperBound> initialized with some text
constexpr auto text = generateTextArray();

int main() {
    for (size_t i{}; i < NumberOfTextsToGenerate; ++i)
        std::cout << text[i] << '\n';
    return 0;
}

但是，当然，“makeString”函数不会做我想要的。或者说，我不知道如何实施正确的解决方案。

我们怎样才能让这样的数组工作呢？是基于这个初始解决方案还是类似的解决方案？

解决方法

您可以执行以下操作：

constexpr size_t NumberOfTextsToGenerate = 10u;

template <std::size_t I>
struct digit_to_end_impl {
    static constexpr const char value[]{ 't','e','x','t',(I + '0'),0 };
};

template <std::size_t I>
struct digit_to_end {
    static constexpr std::string_view value = digit_to_end_impl<I>::value;
};

template <std::size_t I>
constexpr std::string_view makeString() {
    return digit_to_end<I>::value;
}

template <unsigned int... ManyIntegers>
constexpr auto generateTextHelper(std::integer_sequence<unsigned int,ManyIntegers...>) {
    return std::array<std::string_view,sizeof...(ManyIntegers)>{ { makeString<ManyIntegers>()... } };
}

// ...

查看DEMO。

但是，当 NumberOfTextsToGenerate 大于 10 时，这将不起作用。

,

哇哦，我终于做到了！！！

（链接到 godbold）

#include <iostream>
#include <array>
#include <utility>
#include <string>
#include <string_view>


template <class T>
using const_c_str_char_t = std::remove_const_t<std::remove_pointer_t<T>>;

template <auto str1,auto str2,size_t ...indexes1,size_t ...indexes2>
constexpr decltype(auto) string_append_sequence(std::index_sequence<indexes1...>,std::index_sequence<indexes2...>)
{
    using char_type = const_c_str_char_t<decltype(str1())>;
    static_assert(std::is_same_v<char_type,const_c_str_char_t<decltype(str2())>>);

    return std::integer_sequence<char_type,str1()[indexes1]...,str2()[indexes2]...>{};
}

template <class T,T ...values1,T ...values2>
constexpr decltype(auto) append_sequence(std::integer_sequence<T,values1...>,std::integer_sequence<T,values2...>) {
    return std::integer_sequence<T,values1...,values2...>{};
}


template <class sequence_t>
struct string_sequence_to_view;

template <class char_type,char_type ...chars>
struct string_sequence_to_view<std::integer_sequence<char_type,chars...>>
{
    using string_view_t = std::conditional_t<std::is_same_v<char_type,char>,std::string_view,std::wstring_view>;

    static constexpr decltype(auto) get() {
        return string_view_t{c_str};
    }

    static constexpr const char_type c_str[]{chars...,char_type{}};
};

template <class char_type,size_t value,std::enable_if_t<std::is_same_v<char_type,char> || std::is_same_v<char_type,wchar_t>,int> = 0>
constexpr decltype(auto) integer_to_string_sequence()
{
    constexpr auto digits = []()
    {
        if constexpr (std::is_same_v<char_type,char>) {
            return "0123456789abcdefghijklmnopqrstuvwxyz";
        }
        else if constexpr (std::is_same_v<char_type,wchar_t>) {
            return L"0123456789abcdefghijklmnopqrstuvwxyz";
        }
    };

    constexpr size_t remainder = value % 10;
    constexpr size_t next_value = value / 10;

    if constexpr (next_value != 0) {
        return append_sequence(integer_to_string_sequence<char_type,next_value>(),std::integer_sequence<char_type,digits()[remainder]>{});
    }
    else {
        return std::integer_sequence<char_type,digits()[remainder]>{};
    }
}

#define INT_TO_C_STR(char_type,num)    string_sequence_to_view<decltype(integer_to_string_sequence<char_type,num>())>{}.c_str

#define APPEND_C_STR_AS_VIEW(s1,s2)                                            \
    string_sequence_to_view<                                                    \
        decltype(                                                               \
            string_append_sequence<                                             \
                [] { return s1; },\
                [] { return s2; }                                               \
            >(                                                                  \
                std::make_index_sequence<sizeof(s1) / sizeof(s1[0]) - 1>(),\
                std::make_index_sequence<sizeof(s2) / sizeof(s1[0]) - 1>()      \
            )                                                                   \
        )                                                                       \
    >{}.get()

// Number of strings that we want to generate
constexpr size_t NumberOfTextsToGenerate = 20u;

// constexpr function to build a string
template <size_t i>
constexpr std::string_view makeString() {
    return APPEND_C_STR_AS_VIEW("test",INT_TO_C_STR(char,i));
}

template <size_t i>
constexpr std::wstring_view makeStringW() {
    return APPEND_C_STR_AS_VIEW(L"test",INT_TO_C_STR(wchar_t,i));
}

// Helper: constexpr function that will create an array of string_views and initialize it
template <size_t... ManyIntegers>
constexpr auto generateTextHelper(std::integer_sequence<size_t,sizeof...(ManyIntegers)>{ makeString<ManyIntegers>()...};
}

template <size_t... ManyIntegers>
constexpr auto generateTextHelperW(std::integer_sequence<size_t,ManyIntegers...>) {
    return std::array<std::wstring_view,sizeof...(ManyIntegers)>{ makeStringW<ManyIntegers>()...};
}

// Helper: constexpr function that will return an array of string_views as shown above with a specified number of texts
constexpr auto generateTextArray() {
    return generateTextHelper(std::make_integer_sequence<size_t,NumberOfTextsToGenerate>());
}

constexpr auto generateTextArrayW() {
    return generateTextHelperW(std::make_integer_sequence<size_t,NumberOfTextsToGenerate>());
}

// This is a definition of a std::array<std::string_view,UpperBound> initialized with some text
constexpr auto text = generateTextArray();
constexpr auto textW = generateTextArrayW();

int main()
{
    for (size_t i{}; i < NumberOfTextsToGenerate; ++i) {
        std::cout << text[i] << '\n';
    }

    for (size_t i{}; i < NumberOfTextsToGenerate; ++i) {
        std::wcout << textW[i] << L'\n';
    }

    return 0;
}

输出：

test0
test1
test2
test3
test4
test5
test6
test7
test8
test9
test10
test11
test12
test13
test14
test15
test16
test17
test18
test19

编辑：支持 wchar_t 字符串。

,

我现在找到了一个最终满足我需求的解决方案。

因为 SO 用户提供了许多很好的答案以及对我的问题 here 的其他答案，

我现在将使用以下代码。

#include <iostream>
#include <algorithm>
#include <iterator>
#include <array>
#include <string>

// Specification for the output that we want to generate
constexpr const char BaseString[]{ "text" };    // Some example text
constexpr size_t StartIndex = 1u;               // Start index. So first array element will be "Text1"
constexpr size_t NumberOfElements = 20u;        // Number of elements to create. Last array element will be "Text20"

// These templates are used to generate a std::array
namespace ConstexprGenerator {
    // To create something like "text123" as constexpr
    template <const size_t numberToConvert,const char* Text>
    class Converter {
    public:
        // Some helper variables for calculating sizes
        static constexpr size_t TextLength{ std::char_traits<char>::length(Text) };
        static constexpr size_t NumberOfDigits{ ([]() constexpr noexcept {size_t result = 0; int temp = numberToConvert; for (; temp != 0; temp /= 10) ++result; return result; }()) };
        static constexpr size_t ArrayLength{ (numberToConvert ? 1u : 2u) + NumberOfDigits + TextLength };

        // Here we will store the string
        std::array<char,ArrayLength> internalString{};

        // Constructor: Copy text and Convert number to character digits
        constexpr Converter() noexcept {
            size_t i{ 0 };  for (; i < TextLength; ++i) internalString[i] = Text[i]; // Copy text
            if (numberToConvert == 0) internalString[i] = '0';  // In case that the given number is 0,then simply copy '0' character   
            else {
                i = NumberOfDigits + TextLength - 1;            // Convert number to character digits
                int number = numberToConvert; for (; number; number /= 10)
                    internalString[i--] = number % 10 + '0';
            }
        }
        constexpr std::array<char,ArrayLength> get() const { return *this; };              // getter
        constexpr operator std::array<char,ArrayLength>() const { return internalString; } // type cast
    };

    // Templated variable. Will have always a different type,depending on the template parameters
    template<const size_t numberToConvert,const char* Text>
    constexpr auto Converted = Converter<numberToConvert,Text>{}.get();

    // Generate a std::array with n elements that consist of const char *,pointing to Textx...Texty
    template <int... ManyIntegers>
    constexpr auto generateTextHelper(std::integer_sequence<size_t,ManyIntegers...>) noexcept {
        return std::array<const char*,sizeof...(ManyIntegers)>{ {Converted<ManyIntegers + StartIndex,BaseString>.data()...}};
    }
    // Generate the required number of texts
    constexpr auto generateTextArray()noexcept {
        return generateTextHelper(std::make_integer_sequence<size_t,NumberOfElements>());
    }
}
// This is a constexpr array
constexpr auto text = ConstexprGenerator::generateTextArray();

int main() {
    std::copy(text.begin(),text.end(),std::ostream_iterator<const char*>(std::cout,"\n"));
    return 0;
}

使用 MSVC、Clang 和 gcc 进行测试

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