How to retrieve byte values in hex format from a string with c++?

Question

I am working on a project where i use a RFID reader/writer (cr500)

I got a problem on how to convert my string "FFFFFFFFFFFF" to "\xFF\xFF\xFF\xFF\xFF\xFF"

from

string key = "FFFFFFFFFFFF";

to

string raw = "\xFF\xFF\xFF\xFF\xFF\xFF";

How can this be solved using c++ standard library funcions?

All you need is to convert a string representing hex bytes into the represented byte array. Google knows how to do it. — Eugene Sh., May 02 '16 at 17:34
Is this really a string-to-string question, or do you perhaps want the integer value that the string specifies? — Cheers and hth. - Alf, May 02 '16 at 17:34
You can't convert a literal string. You have to make another one. — Weather Vane, May 02 '16 at 17:36
You've had too much exposure to dynamically typed languages. C++ is statically typed, and a `std::string` is no longer your one-size-fits-all data type. I'm somewhat convinced, that this question is the result of starting out with the wrong data type. — IInspectable, May 02 '16 at 17:43
Welcome to SO, where there is no such thing as bad qu.. oh wait, actually there is. But even so, please see [here](http://stackoverflow.com/a/13738023/1460794) for a possible answer to your question. — wally, May 02 '16 at 17:43
but how to send the key to the device. when i am using "\xFF.." it's work perfectly but not with "FFF..." — Mouha Chelali, May 02 '16 at 17:44
[The Definitive C++ Book Guide and List](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — IInspectable, May 02 '16 at 17:47
@πάνταῥεῖ i used it permanently. but i want to read it from a gui and how to convert it — Mouha Chelali, May 02 '16 at 17:54
Well it looks trivial at first hand but actually isn't using standard functions. — πάντα ῥεῖ, May 02 '16 at 18:18
Be careful what you ask for. What do you expect if the key value equals "000000000000" (or any 2 digit hex number with a byte value of `'\x00'`)? — franji1, May 02 '16 at 18:55

πάντα ῥεῖ · Accepted Answer · 2016-05-02T19:10:19.893

Well, I'll take your question literally and concentrate on how to convert from the character literal "FFFFFFFFFFFF" to the target literals value of "\xFF\xFF\xFF\xFF\xFF\xFF" as you're asking for (pun intended).

Here's one way ¹ to achieve what you're looking for:

#include <iostream>
#include <string>
#include <sstream>
#include <iomanip>

int main()
{
    std::string input = "FFFFFFFFFFFF"; // << Let's suppose this is given from the GUI
    std::istringstream iss(input);
    std::string output;
    std::string sbyte;
    while(iss >> std::setw(2) >> sbyte) {
        std::cout << sbyte << std::endl;
        std::istringstream iss2(sbyte);
        unsigned u;
        iss2 >> std::hex >> u;
        output += (unsigned char)u;
    }

    // std::string::data() contains the resulting byte values:
    for(unsigned char x : output) {
        std::cout << std::hex << "\\x" << std::setw(2) << std::setfill('0') 
                  << (unsigned)x; 
    }
    std::cout << std::endl;
}

Output:

FF
FF
FF
FF
FF
FF
\xff\xff\xff\xff\xff\xff

See Live Demo

You can control the input field width using std::setw() as shown above.

Note:

This apparently works with std::string, but not using unsigned u; directly with a former std::setw(2) directive:

#include <iostream>
#include <string>
#include <sstream>
#include <iomanip>

int main()
{
    std::string input = "FFFFFFFFFFFF";
    std::istringstream iss(input);
    std::string output;
    unsigned u;
    while(iss >> std::setw(2) >> std::hex >> u) {
        std::cout << std::hex << std::setw(2) << std::setfill('0') <<  u << std::endl;
        output += (unsigned char)u;
    }

    for(unsigned char x : output) {
        std::cout << std::hex << "\\x" << std::setw(2) << std::setfill('0') 
                  << (unsigned)x; 
    }
    std::cout << std::endl;
}

Live Demo

_{1. You can also split up your input string using std::string::substr() in a loop.}

thank u a lot. and i think a solution: std::string key=""; for(int i=0;i — Mouha Chelali, May 02 '16 at 20:44

How to retrieve byte values in hex format from a string with c++?

1 Answers1