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How to convert (const std::vector<unsigned char>& vchIn) to QByteArray?



  • I have a function from a library which requires a const std::vector<unsigned char> as the second argument. The function declaration looks like this:

    bool DecodeBase58(const std::string& str, std::vector<unsigned char>& vchRet);
    

    My first parameter is a QByteArray and I would like to have the result also in a QByteArray. How can I convert const std::vector<unsigned char>& vchIn into a QByteArray? Or even better, how could I change the function below, that it returns a QByteArray?

    static const char* pszBase58 = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
    
    bool DecodeBase58(const char* psz, std::vector<unsigned char>& vch)
    {
        // Skip leading spaces.
        while (*psz && isspace(*psz))
            psz++;
        // Skip and count leading '1's.
        int zeroes = 0;
        int length = 0;
        while (*psz == '1') {
            zeroes++;
            psz++;
        }
        // Allocate enough space in big-endian base256 representation.
        int size = strlen(psz) * 733 /1000 + 1; // log(58) / log(256), rounded up.
        std::vector<unsigned char> b256(size);
        // Process the characters.
        while (*psz && !isspace(*psz)) {
            // Decode base58 character
            const char* ch = strchr(pszBase58, *psz);
            if (ch == nullptr)
                return false;
            // Apply "b256 = b256 * 58 + ch".
            int carry = ch - pszBase58;
            int i = 0;
            for (std::vector<unsigned char>::reverse_iterator it = b256.rbegin(); (carry != 0 || i < length) && (it != b256.rend()); ++it, ++i) {
                carry += 58 * (*it);
                *it = carry % 256;
                carry /= 256;
            }
            assert(carry == 0);
            length = i;
            psz++;
        }
        // Skip trailing spaces.
        while (isspace(*psz))
            psz++;
        if (*psz != 0)
            return false;
        // Skip leading zeroes in b256.
        std::vector<unsigned char>::iterator it = b256.begin() + (size - length);
        while (it != b256.end() && *it == 0)
            it++;
        // Copy result into output vector.
        vch.reserve(zeroes + (b256.end() - it));
        vch.assign(zeroes, 0x00);
        while (it != b256.end())
            vch.push_back(*(it++));
        return true;
    }
    

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  • @Christian-Ehrlicher 's solution is the way to go. I got complicated on mine:

            std::vector<unsigned char> source = {'a'+0,'a'+1,'a'+2,'a'+3,'a'+4,'a'+5,'a'+6,'a'+7,'a'+8,'a'+9};
            QByteArray output; // empty byte array
            for(auto val: source){
                output.push_back(val);
            }
    
            // or use something from algorithm
            QByteArray output2;
            std::copy(source.begin(), source.end(), std::back_inserter(output2));
    
            qInfo() << output;
            qInfo() << output2;
    


  • That is how I ended up doing it:

    QByteArray encodeInput = "Hello world";
    std::vector<unsigned char> encodeResultVector;
    encodeResultVector.reserve(encodeInput.size());
    for(int i = 0; i < encodeInput.size(); ++i)
        encodeResultVector.push_back(static_cast<unsigned char>(encodeInput.at(i)));
    std::string encodeReslutStdString = EncodeBase58(encodeResultVector);
    QByteArray encodedArray(encodeReslutStdString.c_str(), encodeReslutStdString.length());
    
    qDebug() << "encode input  :" << encodeInput;
    qDebug() << "encoded array :" << encodedArray;
    
    QByteArray decodedArray;
    std::vector<unsigned char> vchRet;
    bool ret = DecodeBase58(encodedArray.constData(), vchRet);
    decodedArray.reserve(vchRet.size());
    for(size_t i = 0; i < vchRet.size(); ++i)
        decodedArray.append(vchRet.at(i));
    
    qDebug() << "decoded array :" << decodedArray;
    
    encode input  : "Hello world"
    encoded array : "JxF12TrwXzT5jvT"
    decoded array : "Hello world"
    


  • std::vector<unsinged char> bson = ....
    QByteArray qbson(reinterpret_cast<const char*>(bson.data()), bson.size());
    


  • Can I ask why qt did not insert the static Function fromStdVectorUint8t to QByteArray?
    you can make QString from 4 Basic_string variants char, char16_t, char32_t and wchar_t

    why not bytearry from Std::vector<uint8_t> ?


  • Moderators

    @QtCoder87 said in How to convert (const std::vector<unsigned char>& vchIn) to QByteArray?:

    why not bytearry from Std::vector<uint8_t> ?

    Mainly because there was little demand for it.

    It is very easy to and cheap to reinterpret a uint8_t array as a char array, so QByteArray::fromStdUInt8Vector(const std::vector<uint_8> &vector) wouldn't add much value. Just use QByteArray(const char *data, int size).

    Anyway, future code should not use uint8_t OR char. Instead, we should move towards std::byte.


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