Integer to hex string in C++

To make it lighter and faster I suggest to use direct filling of a string.

template <typename I> std::string n2hexstr(I w, size_t hex_len = sizeof(I)<<1) {
    static const char* digits = "0123456789ABCDEF";
    std::string rc(hex_len,'0');
    for (size_t i=0, j=(hex_len-1)*4 ; i<hex_len; ++i,j-=4)
        rc[i] = digits[(w>>j) & 0x0f];
    return rc;
}

Use std::stringstream to convert integers into strings and its special manipulators to set the base. For example like that:

std::stringstream sstream;
sstream << std::hex << my_integer;
std::string result = sstream.str();

You can do it with C++20 std::format:

std::string s = std::format("{:x}", 42); // s == 2a

Until std::format is widely available you can use the {fmt} library, std::format is based on (godbolt):

std::string s = fmt::format("{:x}", 42); // s == 2a

Disclaimer: I'm the author of {fmt} and C++20 std::format.

Use <iomanip>'s std::hex. If you print, just send it to std::cout, if not, then use std::stringstream

std::stringstream stream;
stream << std::hex << your_int;
std::string result( stream.str() );

You can prepend the first << with << "0x" or whatever you like if you wish.

Other manips of interest are std::oct (octal) and std::dec (back to decimal).

One problem you may encounter is the fact that this produces the exact amount of digits needed to represent it. You may use setfill and setw this to circumvent the problem:

stream << std::setfill ('0') << std::setw(sizeof(your_type)*2) 
       << std::hex << your_int;

So finally, I'd suggest such a function:

template< typename T >
std::string int_to_hex( T i )
{
  std::stringstream stream;
  stream << "0x" 
         << std::setfill ('0') << std::setw(sizeof(T)*2) 
         << std::hex << i;
  return stream.str();
}