使用C ++用户定义的文字来初始化数组 [英] Using a C++ user-defined literal to initialise an array

问题描述

我有一堆测试向量，以十六进制字符串形式表示:

I have a bunch of test vectors, presented in the form of hexadecimal strings:

MSG: 6BC1BEE22E409F96E93D7E117393172A
MAC: 070A16B46B4D4144F79BDD9DD04A287C
MSG: 6BC1BEE22E409F96E93D7E117393172AAE2D8A57
MAC: 7D85449EA6EA19C823A7BF78837DFADE

等我需要以某种方式将它们放入C ++程序中，而无需进行过多的编辑.有多种选择:

etc. I need to get these into a C++ program somehow, without too much editing required. There are various options:

• 手动将测试向量编辑为0x6B,0xC1,0xBE,...
形式
• 手动将测试向量编辑为"6BC1BEE22E409F96E93D7E117393172A"的形式，并编写一个在运行时将其转换为字节数组的函数.
• 编写一个程序以解析测试向量并输出C ++代码.

• Edit the test vectors by hand into the form 0x6B,0xC1,0xBE,...
• Edit the test vectors by hand into the form "6BC1BEE22E409F96E93D7E117393172A" and write a function to convert that into a byte array at run time.
• Write a program to parse the test vectors and output C++ code.

但是我最终使用的是:

• 用户定义的文字，

因为很有趣.我定义了一个帮助程序类HexByteArray和一个用户定义的文字运算符HexByteArray operator "" _$ (const char* s)，该运算符解析格式为"0xXX...XX"的字符串，其中XX...XX是十六进制数的偶数. HexByteArray包括对const uint8_t*和std::vector<uint8_t>的转换运算符.所以现在我可以写例如

because fun. I defined a helper class HexByteArray and a user-defined literal operator HexByteArray operator "" _$ (const char* s) that parses a string of the form "0xXX...XX", where XX...XX is an even number of hex digits. HexByteArray includes conversion operators to const uint8_t* and std::vector<uint8_t>. So now I can write e.g.

struct {
  std::vector<uint8_t> MSG ;
  uint8_t* MAC ;
  } Test1 = {
  0x6BC1BEE22E409F96E93D7E117393172A_$,
  0x070A16B46B4D4144F79BDD9DD04A287C_$
  } ;

哪个效果很好.但是现在这是我的问题:我也可以对数组执行此操作吗?例如:

Which works nicely. But now here is my question: Can I do this for arrays as well? For instance:

uint8_t MAC[16] = 0x070A16B46B4D4144F79BDD9DD04A287C_$ ;

甚至

uint8_t MAC[] = 0x070A16B46B4D4144F79BDD9DD04A287C_$ ;

我看不到如何进行这项工作.要初始化数组，我似乎需要std::initializer_list.但据我所知，只有编译器才能实例化此类内容.有什么想法吗?

I can't see how to make this work. To initialise an array, I would seem to need an std::initializer_list. But as far as I can tell, only the compiler can instantiate such a thing. Any ideas?

这是我的代码:

HexByteArray.h

#include <cstdint>
#include <vector>

class HexByteArray
  {
public:
  HexByteArray (const char* s) ;
  ~HexByteArray() { delete[] a ; }

  operator const uint8_t*() && { const uint8_t* t = a ; a = 0 ; return t ; }
  operator std::vector<uint8_t>() &&
    {
    std::vector<uint8_t> v ( a, a + len ) ;
    a = 0 ;
    return v ;
    }

  class ErrorInvalidPrefix { } ;
  class ErrorHexDigit { } ;
  class ErrorOddLength { } ;

private:
  const uint8_t* a = 0 ;
  size_t len ;
  } ;

inline HexByteArray operator "" _$ (const char* s)
  {
  return HexByteArray (s) ;
  }

HexByteArray.cpp

#include "HexByteArray.h"

#include <cctype>
#include <cstring>

HexByteArray::HexByteArray (const char* s)
  {
  if (s[0] != '0' || toupper (s[1]) != 'X') throw ErrorInvalidPrefix() ;
  s += 2 ;

  // Special case: 0x0_$ is an empty array (because 0x_$ is invalid C++ syntax)
  if (!strcmp (s, "0"))
    {
    a = nullptr ; len = 0 ;
    }
  else
    {
    for (len = 0 ; s[len] ; len++) if (!isxdigit (s[len])) throw ErrorHexDigit() ;
    if (len & 1) throw ErrorOddLength() ;
    len /= 2 ;
    uint8_t* t = new uint8_t[len] ;
    for (size_t i = 0 ; i < len ; i++, s += 2)
      sscanf (s, "%2hhx", &t[i]) ;
    a = t ;
    }
  }

推荐答案

使用数字文字运算符模板，其签名为:

template <char...>
result_type operator "" _x();

此外，由于数据是在编译时已知的，因此我们最好将所有内容都设为constexpr.请注意，我们使用 std::array 代替C样式的数组:

Also, since the data is known at compile-time, we might as well make everything constexpr. Note that we use std::array instead of C-style arrays:

#include <cstdint>
#include <array>
#include <vector>

// Constexpr hex parsing algorithm follows:
struct InvalidHexDigit {};
struct InvalidPrefix {};
struct OddLength {};

constexpr std::uint8_t hex_value(char c)
{
    if ('0' <= c && c <= '9') return c - '0';
    // This assumes ASCII:
    if ('A' <= c && c <= 'F') return c - 'A' + 10;
    if ('a' <= c && c <= 'f') return c - 'a' + 10;
    // In constexpr-land, this is a compile-time error if execution reaches it:
    // The weird `if (c == c)` is to work around gcc 8.2 erroring out here even though
    // execution doesn't reach it.
    if (c == c) throw InvalidHexDigit{};
}

constexpr std::uint8_t parse_single(char a, char b)
{
    return (hex_value(a) << 4) | hex_value(b);
}

template <typename Iter, typename Out>
constexpr auto parse_hex(Iter begin, Iter end, Out out)
{
    if (end - begin <= 2) throw InvalidPrefix{};
    if (begin[0] != '0' || begin[1] != 'x') throw InvalidPrefix{};
    if ((end - begin) % 2 != 0) throw OddLength{};

    begin += 2;

    while (begin != end)
    {
        *out = parse_single(*begin, *(begin + 1));
        begin += 2;
        ++out;
    }

    return out;
}

// Make this a template to defer evaluation until later        
template <char... cs>
struct HexByteArray {
    static constexpr auto to_array()
    {
        constexpr std::array<char, sizeof...(cs)> data{cs...};

        std::array<std::uint8_t, (sizeof...(cs) / 2 - 1)> result{};

        parse_hex(data.begin(), data.end(), result.begin());

        return result;
    }

    constexpr operator std::array<std::uint8_t, (sizeof...(cs) / 2)>() const 
    {
        return to_array();
    }

    operator std::vector<std::uint8_t>() const
    {
        constexpr auto tmp = to_array();

        return std::vector<std::uint8_t>{tmp.begin(), tmp.end()};
    }
};

template <char... cs>
constexpr auto operator"" _$()
{
    static_assert(sizeof...(cs) % 2 == 0, "Must be an even number of chars");
    return HexByteArray<cs...>{};
}

演示

示例用法:

auto data_array = 0x6BC1BEE22E409F96E93D7E117393172A_$ .to_array();
std::vector<std::uint8_t> data_vector = 0x6BC1BEE22E409F96E93D7E117393172A_$;

---

请注意， identifier 中的$实际上是gcc扩展，因此它是非标准的C ++.考虑使用_$以外的UDL.

---

As a side note, $ in an identifier is actually a gcc extension, so it's non-standard C++. Consider using a UDL other than _$.

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