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编译时文本到数字转换(atoi)

compile-time-constant compile-time template-meta-programming c++11 c++

1

Kirill Frolov · 技术社区 · 6 年前

我知道有两种方法可以解决这个问题:

2) 通过模板元编程。

下面是我的解决方案,我很高兴听到一些建议。

http://coliru.stacked-crooked.com/a/0b8f1fae9d9b714b


#include <stdio.h>

template <typename T> struct Result
{
    T value;
    bool valid;

    constexpr Result(T v) : value(v), valid(true) {}
    constexpr Result() : value(), valid(false) {}
};

template <typename T>
constexpr Result<T> _atoi_oct(const char *s, size_t n, T val, int sign)
{
    return n == 0 ? Result<T>(sign < 0 ? -val : val)
        : *s >= '0' && *s <= '7' 
            ? _atoi_oct(s+1, n-1, val*T(010) + *s - '0', sign)
            : Result<T>();
}

template <typename T>
constexpr Result<T> _atoi_dec(const char *s, size_t n, T val, int sign)
{
    return n == 0 ? Result<T>(sign < 0 ? -val : val)
        : *s >= '0' && *s <= '9'
            ? _atoi_dec(s+1, n-1, val*T(10) + *s - '0', sign)
            : Result<T>();
}

template <typename T>
constexpr Result<T> _atoi_hex(const char *s, size_t n, T val, int sign)
{
    return n == 0 ? Result<T>(sign < 0 ? -val : val)
        : *s >= '0' && *s <= '9'
            ? _atoi_hex(s+1, n-1, val*T(0x10) + *s - '0', sign)
            : *s >= 'a' && *s <= 'f'
                ? _atoi_hex(s+1, n-1, val*T(0x10) + *s - 'a' + 10, sign)
                : *s >= 'A' && *s <= 'F'
                    ? _atoi_hex(s+1, n-1, val*T(0x10) + *s - 'A' + 10, sign)
                    : Result<T>();
}

template <typename T>
constexpr Result<T> _atoi_zero(const char *s, size_t n, int sign = 1)
{
    return n == 0 ? Result<T>()
        : *s >= '0' && *s <= '7'
            ? _atoi_oct(s+1, n-1, T(*s - '0'), sign)
            : *s == 'x' || *s == 'X'
                ? _atoi_hex(s+1, n-1, T(0), sign)
                : Result<T>();
}

template <typename T>
constexpr Result<T> _atoi_sign(const char *s, size_t n, int sign = 1)
{
    return n == 0 ? Result<T>()
        : *s == '0'
            ? _atoi_zero<T>(s+1, n-1, sign)
            : *s > '0' && *s <= '9'
                ? _atoi_dec(s+1, n-1, T(*s - '0'), sign)
                : Result<T>();
}

template <typename T>
constexpr Result<T> _atoi_space(const char *s, size_t n)
{
    return n == 0 ? Result<T>()
        : (*s == ' ' || *s == '\t' || *s == '\n' || *s == '\r' || *s == '\v')
            ? _atoi_space<T>(s+1, n-1)
            : *s == '-'
                ? _atoi_sign<T>(s+1, n-1, -1)
                : *s == '+'
                    ? _atoi_sign<T>(s+1, n-1)
                    : *s == '0'
                        ? _atoi_zero<T>(s+1, n-1)
                        : _atoi_dec(s, n, T(0), 1);
}

template <size_t N> void pstr(const char (&s)[N])
{
    printf("s '%.*s'\n", int(N-1), s);
}

template <typename Str>
__attribute__((always_inline))
void _atoi(Str s)
{
    constexpr auto result = _atoi_space<long>(s.cstr(), sizeof(s.cstr())-1);
    if (result.valid)
        printf("i %ld\n", result.value);
    else
        pstr(reinterpret_cast<const char (&)[sizeof(s.cstr())]>(s.cstr()));
}

#define atoi(STR) _atoi([]() { \
                        struct S { \
                            static constexpr const char (&cstr())[sizeof(STR)] { return STR; } \
                        }; \
                        return S();  \
                    }())

int main()
{
    atoi("42");
    atoi("-1");
    atoi("+1");
    atoi("010");
    atoi("-0x10");
    atoi("--1");
    atoi("x");
    atoi("3x");
    return 0;   
}

基本上我想问的是,我怎样才能转变 在编译时 数字(如“42”)用双引号写在整型值中。我的解决方案看起来太麻烦了。

0 回复 | 直到 6 年前

1

0

Timo 6 年前

here

#include <cstdint>
#include <iostream>

template <typename T>
struct Result
{
    T value{};
    bool valid = false;

    constexpr explicit Result(T v) : value(v), valid(true) {}
    constexpr Result() = default;
};

// converts upper case chars to lower case
constexpr char to_lower(char c)
{
    return c >= 'A' && c <= 'Z'
        ? c - 'A' + 'a'
        : c;
}

// converts a digit char to its numeric value (eg. 'F' -> 15)
constexpr int to_digit(char c)
{
    c = to_lower(c);
    return c >= 'a'
        ? c - 'a' + 10
        : c - '0';
}

// checks whether the given digit fits in the given base (eg. 'A' in 16 (hex) -> true, but '9' in 8 (oct) -> false)
constexpr bool is_digit(char c, int base)
{
    int digit = to_digit(c);
    return 0 <= digit && digit < base;
}

namespace detail
{
    // returns true if c is a sign character (+ or -), sign will hold a valid factor (1 or -1) regardless of the return value
    constexpr bool get_sign(char c, int& sign)
    {
        if (c == '-')
        {
            sign = -1;
            return true;
        }
        else
        {
            sign = 1;
            return c == '+';
        }
    }

    // adds a digit to the right side of the a number
    template <typename T>
    constexpr T append_digit(T value, int base, int digit)
    {
        return value * base + digit;
    }

    // create the actual number from the given string
    template <typename T>
    constexpr T construct_integral(const char* str, std::size_t size, int base)
    {
        T value = 0;
        for (std::size_t i = 0; i < size; i++)        
            value = append_digit(value, base, to_digit(str[i]));

        return value;
    }

    // how many chars are necessary to specify the base (ex. hex -> 0x -> 2) 
    constexpr std::size_t get_base_offset(int base)
    {
        if (base == 8) return 1;
        if (base == 16) return 2;
        return 0;
    }

    // gets the base value according to the number prefix (eg. 0x -> 16 (hex))
    constexpr int get_base(const char* str, std::size_t size)
    {
        return str[0] == '0'
            ? size > 2 && to_lower(str[1]) == 'x'
                ? 16
                : 8
            : 10;
    }

    // checks whether all digits in the string can fit in the given base
    constexpr bool verify_base(const char* str, std::size_t size, int base)
    {
        for (std::size_t i = 0; i < size; i++)
            if (!is_digit(str[i], base))
                return false;

        return true;
    }
}

template <typename T = int>
constexpr Result<T> to_integral(const char* str, std::size_t size)
{
    using namespace detail;

    // remove the sign from the string
    auto sign = 0;    
    if (get_sign(str[0], sign)) 
    {
        ++str;
        --size;
    }

    // get the base and remove its prefix from the string
    auto base = get_base(str, size);
    auto offset = get_base_offset(base);
    str += offset;
    size -= offset;

    // check if the string holds a valid number with respect to its base
    if (!verify_base(str, size, base))
        return {};

    // create the number and apply the sign
    auto unsigned_value = construct_integral<T>(str, size, base);
    return Result<T>(unsigned_value * sign);
}

template <typename T = int, std::size_t N>
constexpr Result<T> to_integral(const char(&str)[N])
{
    static_assert(N > 1, "Empty strings are not allowed");
    return to_integral<T>(str, N - 1);
}

C++ 17可以通过使用代码来减少代码量 std::string_view . 你的 Result<T> 也可以用 std::optional