Convert a string variable of two chars to hex

Question

I have code where the user inputs two chars into a string variable. I have a function that verifies that the user input is only two chars long, and that it only contains valid hexadecimal digits.

I want to write these digits to a binary file that's 32 bytes long. I tried:

    outFile.write((char*)&string[0], 1);

In a loop that runs 32 times (I want to write one byte at a time) to test, but it just writes the ascii code for the char, not the actual char itself. I expected it to write a nybble and skip a nybble, but it wrote a full byte of ascii information instead. So I tried:

    outFile.write((unsigned char*)&string[0], 1);

But my compiler complains about it being an invalid cast.

I want to solve this problem without converting the string into a c-style string. In other words, I want string to contain two chars and represent one byte of information. Not four (plus null characters).

Answer 1

You have a string that represents an integer. So convert the string to an integer:

unsigned char byte = (unsigned char)std::stoi(string, 0, 16);
outFile:write(static_cast<const char*>(&byte), 1);

Answer 2

As a workaround for your missing stoi you can do this:

#include <iostream>
#include <sstream>
#include <ios>

char hexnum[]{"2F"}; // or whatever, upper or lowercase hex digits allowed.
std::istringstream input(hexnum);
int num=0;
input >> std::hex >> num;
unsigned char byte = num;
outFile.write(static_cast<const char*>(&byte), 1);