Hex to JPG conversion in C++

Question

I am writing a C++ program where I Need to convert some string output which is basically in HEX format. This output string starts with FFD8 and Ends with FFD9 which is basically a JPEG Image.

Now, I want to get the JPEG file from that string output but I don't want to save that string output in a text file and open it in ios::binary mode and then covert it to a JPEG file.

std::string output; //which is FFD8..........FFD9

//******some code*******???

ofstream imageFile;
imageFile.open('Image.jpg');
imageFile<< output;

How I can do that without saving my string output in a file?

Thanks in advance!

Answer 1

My assumption is that you have a string representing hex, and you want to convert that to the byte equivalent.

byte hexCharToByte(const char h){
    if(isdigit(h))
        return h - '0';
    else
        return toupper(h) - 'A' + 10;
}

This is some code I wrote a while back in C that takes a char and converts it to a byte. You can adapt it however you need.

How it works:

• Hex values are represented by 0-15
• If you receive a char that is a number, eg '0', you subtract '0'. The result is 0
• If you receive a letter, say 'a', make sure it's uppercase. 'A' - 'A' is 0. The hex value of 'A' is 10, so we must add 10 to get its hex value.

Answer 2

std::string output; //which is FFD8..........FFD9

int main()
{
    std::ofstream thum("name.jpg", std::ios_base::binary | std::ios_base::out);

    char buf[3];
    buf[2] = 0;

    std::stringstream in(output);
    in.flags(std::ios_base::hex);
    while (in)
    {
        in >> buf[0] >> buf[1];
        long val = strtol(buf, nullptr, 16);
        thum << static_cast<unsigned char>(val & 0xFF);
    }

}