在c++ 11中声明字符十六进制常量

Declaring char hexadecimal constants in C++11

本文关键字：字符 十六进制 常量 声明 c++      更新时间：2023-10-16

在代码的各个低级部分，我们需要向设备发送特定的字节以使事情发生。因此，我们有很多代码看起来像:

const char magic_bytes[] = { 0x01, 0xFA, 0x92 };

这会导致错误(在GCC 4.7.2上)

test_char.cpp:6:51: warning: narrowing conversion of ‘250’ from ‘int’ to ‘const char’ inside { } is ill-formed in C++11 [-Wnarrowing]

因为0xFA在-128到127的范围之外。

我能想到两种解决方法:

const char magic_bytes[] = { static_cast<char>(0x01), static_cast<char>(0xFA), static_cast<char>(0x92) };

或:

const unsigned char magic_bytes[] = { 0x01, 0xFA, 0x92 };

这两种方法要么很难看(第一种情况)，要么有其他缺点(在后一种情况下必须强制转换为(const char*))

是否有更好的方式来声明这些字符串?

c++ 11提供了可变模板(GCC支持已经存在一段时间了)来解决这个问题。

template <typename... A>                                                                 
constexpr std::array<char, sizeof...(A)> byte_array(A... v)                              
{ return std::array<char, sizeof...(A)>{{static_cast<char>(v)...}}; }                    
constexpr auto arr = byte_array( 0x01, 0xFA, 0x92 );

或者为了避免在传递给C函数时重复调用.data():

template <std::size_t S>
struct byte_array {
  char data_[S];
  char *data() { return data_; }
  operator char*() { return data_; }
  const char *data() const { return data_; }
  operator const char*() const { return data_; }
  constexpr std::size_t size() const { return S; }
  // one could add support for begin/end and things like that
};
template <typename... A>
constexpr byte_array<sizeof...(A)> make_byte_array(A... v)
{ return byte_array<sizeof...(A)>{{static_cast<char>(v)...}}; }
// beside constexpr, this can be also non-const
auto magic_bytes = make_byte_array( 0x01, 0xFA, 0x92 );
strtok(magic_bytes, "why?");

与普通字符数组相比没有开销

您可以这样做来实现单类型转换:

const unsigned char magic_bytesUC[] = { 0x01, 0xFA, 0x92 };
enum { NBYTES = sizeof(magic_bytesUC) };
const char *magic_bytes = reinterpret_cast<const char*>(magic_bytesUC);

既然您指定了c++ 11，那么我假定您可以使用可变宏。在这种情况下，有一种解决方案在使用时很优雅，但在幕后却很难看。

那么我将从如何使用它开始:

char myBytes1[] = MAKE_BYTES( 0x00, 0x40, 0x80, 0xC0 );
char myBytes2[] = MAKE_BYTES( 0xFF );

现在，后端代码:这可以更短，但我使用了标准的循环方法，因此您可以从中获得一些重用。它可以支持1 - 24字节的列表。通过重复最后几行，它可以变得更大。欢迎来到预处理器元编程的世界。

#define EXPAND(a) a
#define ARGS_COUNT_(
  _96,_95,_94,_93,_92,_91,_90,_89,_88,_87,_86,_85,_84,_83,_82,_81,
  _80,_79,_78,_77,_76,_75,_74,_73,_72,_71,_70,_69,_68,_67,_66,_65,
  _64,_63,_62,_61,_60,_59,_58,_57,_56,_55,_54,_53,_52,_51,_50,_49,
  _48,_47,_46,_45,_44,_43,_42,_41,_40,_39,_38,_37,_36,_35,_34,_33,
  _32,_31,_30,_29,_28,_27,_26,_25,_24,_23,_22,_21,_20,_19,_18,_17,
  _16,_15,_14,_13,_12,_11,_10, _9, _8, _7, _6, _5, _4, _3, _2, _1,
  N,...) N
#define ARGS_COUNT(...) ARGS_COUNT_(__VA_ARGS__,
  96,95,94,93,92,91,90,89,88,87,86,85,84,83,82,81,
  80,79,78,77,76,75,74,73,72,71,70,69,68,67,66,65,
  64,63,62,61,60,59,58,57,56,55,54,53,52,51,50,49,
  48,47,46,45,44,43,42,41,40,39,38,37,36,35,34,33,
  32,31,30,29,28,27,26,25,24,23,22,21,20,19,18,17,
  16,15,14,13,12,11,10, 9, 8, 7, 6, 5, 4, 3, 2, 1)
#define ARGS_HEAD(a,...) (a)
#define ARGS_TAIL(a,...) (__VA_ARGS__)
#define FOREACH(macro,lmacro,list) FOREACH_(ARGS_COUNT list,macro,lmacro,list)
#define FOREACH_(n,macro,lmacro,list) FOREACH__(n,macro,lmacro,list)
#define FOREACH__(n,macro,lmacro,list) FOREACH_##n(macro,lmacro,list)
#define FOREACH_1(macro,lmacro,list) lmacro list
#define FOREACH_2(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_1(macro,lmacro,ARGS_TAIL list)
#define FOREACH_3(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_2(macro,lmacro,ARGS_TAIL list)
#define FOREACH_4(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_3(macro,lmacro,ARGS_TAIL list)
#define FOREACH_5(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_4(macro,lmacro,ARGS_TAIL list)
#define FOREACH_6(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_5(macro,lmacro,ARGS_TAIL list)
#define FOREACH_7(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_6(macro,lmacro,ARGS_TAIL list)
#define FOREACH_8(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_7(macro,lmacro,ARGS_TAIL list)
#define FOREACH_9(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_8(macro,lmacro,ARGS_TAIL list)
#define FOREACH_10(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_9(macro,lmacro,ARGS_TAIL list)
#define FOREACH_11(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_10(macro,lmacro,ARGS_TAIL list)
#define FOREACH_12(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_11(macro,lmacro,ARGS_TAIL list)
#define FOREACH_13(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_12(macro,lmacro,ARGS_TAIL list)
#define FOREACH_14(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_13(macro,lmacro,ARGS_TAIL list)
#define FOREACH_15(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_14(macro,lmacro,ARGS_TAIL list)
#define FOREACH_16(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_15(macro,lmacro,ARGS_TAIL list)
#define FOREACH_17(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_16(macro,lmacro,ARGS_TAIL list)
#define FOREACH_18(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_17(macro,lmacro,ARGS_TAIL list)
#define FOREACH_19(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_18(macro,lmacro,ARGS_TAIL list)
#define FOREACH_20(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_19(macro,lmacro,ARGS_TAIL list)
#define FOREACH_21(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_20(macro,lmacro,ARGS_TAIL list)
#define FOREACH_22(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_21(macro,lmacro,ARGS_TAIL list)
#define FOREACH_23(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_22(macro,lmacro,ARGS_TAIL list)
#define FOREACH_24(macro,lmacro,list) EXPAND(macro ARGS_HEAD list) FOREACH_23(macro,lmacro,ARGS_TAIL list)
#define MAKE_BYTE(x) static_cast<char>(x),
#define MAKE_LAST_BYTE(x) static_cast<char>(x)
//#define MAKE_BYTES(x) { FOREACH(MAKE_BYTE,MAKE_LAST_BYTE,x) } // uncomment this if you would rather use double bracket ((0x00,0x40,...)) syntax
#define MAKE_BYTES(...) { FOREACH(MAKE_BYTE,MAKE_LAST_BYTE,(__VA_ARGS__)) }
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