C convert string to hexadecimal and vice versa

Question

What is the best way to convert a string to hex and vice versa in C     Example    A string like  Hello World  to hex format  48656C6C6F20576F726C64 And from hex 48656C6C6F20576F726C64 to string   Hello World

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This will convert  Hello World  to  48656c6c6f20576f726c64  and will store this hex value in str1 and also will convert  48656c6c6f20576f726c64  to  Hello World     include  lt iostream gt   include lt sstream gt   using namespace std  int hexCharToInt char   string hexToString string   int main         std  string str      std  stringstream str1       str  Hello World       for int i 0 i lt str length   i             str1  lt  lt  std  hex  lt  lt   int str at i             std  cout  lt  lt  str1 str    lt  lt   n       string test    48656c6c6f20576f726c64       std  cout  lt  lt  hexToString test   lt  lt   n       return 0    string hexToString string str       std  stringstream HexString      for int i 0 i lt str length   i             char a   str at i             char b   str at i           int x   hexCharToInt a           int y   hexCharToInt b           HexString  lt  lt   char   16 x  y             return HexString str       int hexCharToInt char a       if a gt   0   amp  amp  a lt   9           return a-48       else if a gt   A   amp  amp  a lt   Z           return a-55       else         return a-87

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string ToHex const string amp  s  bool upper case      true           ostringstream ret       for  string  size type i   0  i  lt  s length      i          ret  lt  lt  std  hex  lt  lt  std  setfill  0    lt  lt  std  setw 2   lt  lt   upper case   std  uppercase   std  nouppercase   lt  lt   int s i        return ret str       int FromHex const string  amp s    return strtoul s c str    NULL  16

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This is a bit faster   static const char  s hexTable 256            00    01    02    03    04    05    06    07    08    09    0a    0b    0c    0d    0e    0f    10    11        12    13    14    15    16    17    18    19    1a    1b    1c    1d    1e    1f    20    21    22    23        24    25    26    27    28    29    2a    2b    2c    2d    2e    2f    30    31    32    33    34    35        36    37    38    39    3a    3b    3c    3d    3e    3f    40    41    42    43    44    45    46    47        48    49    4a    4b    4c    4d    4e    4f    50    51    52    53    54    55    56    57    58    59        5a    5b    5c    5d    5e    5f    60    61    62    63    64    65    66    67    68    69    6a    6b        6c    6d    6e    6f    70    71    72    73    74    75    76    77    78    79    7a    7b    7c    7d        7e    7f    80    81    82    83    84    85    86    87    88    89    8a    8b    8c    8d    8e    8f        90    91    92    93    94    95    96    97    98    99    9a    9b    9c    9d    9e    9f    a0    a1        a2    a3    a4    a5    a6    a7    a8    a9    aa    ab    ac    ad    ae    af    b0    b1    b2    b3        b4    b5    b6    b7    b8    b9    ba    bb    bc    bd    be    bf    c0    c1    c2    c3    c4    c5        c6    c7    c8    c9    ca    cb    cc    cd    ce    cf    d0    d1    d2    d3    d4    d5    d6    d7        d8    d9    da    db    dc    dd    de    df    e0    e1    e2    e3    e4    e5    e6    e7    e8    e9        ea    eb    ec    ed    ee    ef    f0    f1    f2    f3    f4    f5    f6    f7    f8    f9    fa    fb        fc    fd    fe    ff         Convert binary data sequence  beginIt  endIt  to hexadecimal string void dataToHexString const uint8 t const beginIt  const uint8 t const endIt  string amp  str        str clear        str reserve  endIt - beginIt    2       for const uint8 t  it beginIt   it    endIt    it                str    s hexTable  it

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Simplest example using the Standard Library     include  lt iostream gt  using namespace std   int main       char c    n     cout  lt  lt   HEX    lt  lt  hex  lt  lt   int c  lt  lt  endl      output in hexadecimal   cout  lt  lt   ASC   lt  lt  c  lt  lt  endl     output in ascii   return 0      To check the output  codepad returns  6e   and an online ascii-to-hexadecimal conversion tool yields 6e as well  So it works   You can also do this   template lt class T gt  std  string toHexString const T amp  value  int width        std  ostringstream oss      oss  lt  lt  hex      if  width  gt  0            oss  lt  lt  setw width   lt  lt  setfill  0              oss  lt  lt  value      return oss str

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Using lookup tables and the like works  but is just overkill  here are some very simple ways of taking a string to hex and hex back to a string    include  lt stdexcept gt   include  lt sstream gt   include  lt iomanip gt   include  lt string gt   include  lt cstdint gt   std  string string to hex const std  string amp  in        std  stringstream ss       ss  lt  lt  std  hex  lt  lt  std  setfill  0        for  size t i   0  in length    gt  i    i            ss  lt  lt  std  setw 2   lt  lt  static cast lt unsigned int gt  static cast lt unsigned char gt  in i                return ss str        std  string hex to string const std  string amp  in        std  string output       if   in length     2     0            throw std  runtime error  String is not valid length                   size t cnt   in length     2       for  size t i   0  cnt  gt  i    i            uint32 t s   0          std  stringstream ss          ss  lt  lt  std  hex  lt  lt  in substr i   2  2           ss  gt  gt  s           output push back static cast lt unsigned char gt  s               return output

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A string like  quot Hello World quot  to hex format  48656C6C6F20576F726C64   Ah  here you go   include  lt string gt   std  string string to hex const std  string amp  input        static const char hex digits      quot 0123456789ABCDEF quot        std  string output      output reserve input length     2       for  unsigned char c   input                output push back hex digits c  gt  gt  4            output push back hex digits c  amp  15              return output      include  lt stdexcept gt   int hex value unsigned char hex digit        static const signed char hex values 256              -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1           0   1   2   3   4   5   6   7   8   9  -1  -1  -1  -1  -1  -1          -1  10  11  12  13  14  15  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  10  11  12  13  14  15  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1          -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1  -1             int value   hex values hex digit       if  value    -1  throw std  invalid argument  quot invalid hex digit quot        return value     std  string hex to string const std  string amp  input        const auto len   input length        if  len  amp  1  throw std  invalid argument  quot odd length quot         std  string output      output reserve len   2       for  auto it   input begin    it    input end                    int hi   hex value  it             int lo   hex value  it             output push back hi  lt  lt  4   lo             return output      This assumes that a char has 8 bits  so it s not very portable  but you can take it from here

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Here is an other solution  largely inspired by the one by  fredoverflow          Return hexadecimal representation of the input binary sequence     std  string hexitize const std  vector lt char gt  amp  input  const char  const digits    0123456789ABCDEF         std  ostringstream output       for  unsigned char gap   0  beg   input gap   gap  lt  input length    beg   input   gap           output  lt  lt  digits beg  gt  gt  4   lt  lt  digits beg  amp  15        return output str        Length was required parameter in the intended usage

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Why has nobody used sprintf    include  lt string gt   include  lt stdio h gt   static const std  string str    hello world     int main         copy the data from the string to a char array   char  strarr   new char str size   1     strarr str size   1    0    set the null terminator   memcpy strarr  str c str   str size       memory copy to the char array    printf strarr     printf   n nHEX           now print the data   for int i   0  i  lt  str size   1  i            char x   strarr i       sprintf   x    reinterpret cast lt const char  gt  x             DO NOT FORGET TO DELETE   delete strarr      return 0

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You can try this  It s Working      include  lt algorithm gt   include  lt sstream gt   include  lt iostream gt   include  lt iterator gt   include  lt iomanip gt   namespace      const std  string test  hello world      int main        std  ostringstream result     result  lt  lt  std  setw 2   lt  lt  std  setfill  0    lt  lt  std  hex  lt  lt  std  uppercase     std  copy test begin    test end    std  ostream iterator lt unsigned int gt  result            std  cout  lt  lt  test  lt  lt       lt  lt  result str    lt  lt  std  endl

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This will convert Hello World to 48656c6c6f20576f726c64 and print it     include  lt iostream gt   include  lt cstring gt   using namespace std   int main         char hello 20   Hello World        for unsigned int i 0  i lt strlen hello   i            cout  lt  lt  hex  lt  lt   int  hello i       return 0

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I think there is a much simpler and more elegant solution  Some of the above-mentioned methods may even throw unhandled exceptions in some cases  Here is a fool-proof  as in never goes wrong  and very fast code  Just try it and compare the results in terms of speed and compactness    include  lt string gt      Convert string of chars to its representative string of hex numbers void stream2hex const std  string str  std  string amp  hexstr  bool capital   false        hexstr resize str size     2       const size t a   capital    A  - 1    a  - 1       for  size t i   0  c   str 0   amp  0xFF  i  lt  hexstr size    c   str i   2   amp  0xFF                hexstr i      c  gt  0x9F    c   16 - 9    a   c   16    0           hexstr i       c  amp  0xF   gt  9    c   16 - 9    a   c   16    0               Convert string of hex numbers to its equivalent char-stream void hex2stream const std  string hexstr  std  string amp  str        str resize  hexstr size     1    2        for  size t i   0  j   0  i  lt  str size    i    j                  str i     hexstr j   amp        hexstr j    9   hexstr j    lt  lt  4  j            str i      hexstr j   amp        hexstr j    9   hexstr j    amp  0xF            Test the code    include  lt iostream gt  int main         std  string s    Hello World        std  cout  lt  lt   original string     lt  lt  s  lt  lt    n       stream2hex s  s       std  cout  lt  lt   hex format     lt  lt  s  lt  lt    n       hex2stream s  s       std  cout  lt  lt   original one     lt  lt  s  lt  lt    n       and the result is   original string  Hello World  hex format  48656C6C6F20576F726C6421 original one  Hello World

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As of C  17 there s also std  from chars  The following function takes a string of hex characters and returns a vector of T    include  lt charconv gt   template lt typename T gt  std  vector lt T gt  hexstr to vec const std  string amp  str  unsigned char chars per num   2      std  vector lt T gt  out str size     chars per num  0      T value    for  int i   0  i  lt  str size     chars per num  i          std  from chars lt T gt         str data      i   chars per num         str data      i   chars per num    chars per num        value        16            out i    value         return out

[c++] C++ convert string to hexadecimal and vice versa

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