C++ 十六进制格式字符串

Question

The Create method should produce a string of 9 chars composed in this way: som followed by id char plus 4 chars (hex string representing value ) plus 2 chars (hex string representing crc) plus eom char. This's the code:

private: 
     char som = '$';
     char eom = '&';
     bool crc = true;

unsigned char checksum(string data) {
    unsigned char sum = 0x00;

    for (char &c : data)
        sum += c;

    return sum;
}

string epilogue(string data) {
    ostringstream _message;

    if (crc) {
        byte _sum = checksum(data);
        _message << std::hex << (_sum & 0xFF);
    }

    _message << eom;

    return _message.str();
}

string Create(char id, short value) {
    ostringstream _message, _data;

    _data << std::hex << (value & 0xFFFF);

    _message << som << id << _data.str() << epilogue(_data.str());

    return _message.str();
}

The problem is in this line:

_data << std::hex << (value & 0xFFFF);

that correctly outputs an hex string but not of the correct length. Similar problem should be in the line:

_message << std::hex << (_sum & 0xFF);

How can I fix this code to produce exactly the pattern I need? In Java I was using something like this:

String.format("%02X", value & 0xFFFF)

EDIT (runnable code):

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

using namespace std;

typedef unsigned char byte;

class PacketBuilder {
private: 
    char som = '$';
    char eom = '&';
    bool crc = true;

public:
    PacketBuilder(const char som, const char eom, bool crc) {
        this->som = som;
        this->eom = eom;
        this->crc = crc;
    }

    byte checksum(string data) {
        byte sum = 0x00;

        for (char &c : data)
            sum += c;

        return sum;
    }

    string epilogue(string data) {
        ostringstream _message;

        if (crc) {
            byte _sum = checksum(data);
            _message << std::hex << (_sum & 0xFF);
        }

        _message << eom;

        return _message.str();
    }

    string Create(char id, short value) {
        ostringstream _message, _data;

        _data << std::hex << (value & 0xFFFF);

        _message << som << id << _data.str() << epilogue(_data.str());

        return _message.str();
    }
};

int main() {
    PacketBuilder* _pb = new PacketBuilder('$','&',true);

    string _packet = _pb->Create('C',0);

    cout << _packet;

    return 0;
}

Online code

Expected result is:

$C0000c0&

Answer 1

Use std::setw and std::setfill in C++.

Remember to save and restore iomanip flags.

string epilogue(string data) {
    ostringstream _message;

    if (crc) {
        byte _sum = checksum(data);
        const auto save = _message.flags();
        _message << std::hex << std::setw(2) 
            << std::setfill('0') << (_sum & 0xFF);
        _message.flags(save);
    }

    _message << eom;

    return _message.str();
}

string Create(char id, short value) {
    ostringstream _message, _data;

    const auto save = _data.flags();
    _data << std::hex << std::setw(4) 
            << std::setfill('0') << (value & 0xFFFF);
        _message.flags(save);

    _message << som << id << _data.str() << epilogue(_data.str());

    return _message.str();
}

:

format("%02X",
           ^ std::hex << std::uppercase
          ^ std::setw(2)
         ^ std::setfill('0')