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"

Answer 1

A string like "Hello World" to hex format: 48656C6C6F20576F726C64.

Ah, here you go:

#include <string>  std::string string_to_hex(const std::string& input) {     static const char hex_digits[] = "0123456789ABCDEF";      std::string output;     output.reserve(input.length() * 2);     for (unsigned char c : input)     {         output.push_back(hex_digits[c >> 4]);         output.push_back(hex_digits[c & 15]);     }     return output; }  #include <stdexcept>  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("invalid hex digit");     return value; }  std::string hex_to_string(const std::string& input) {     const auto len = input.length();     if (len & 1) throw std::invalid_argument("odd length");      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 << 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.)