Build QImage from postgresql database via libpqxx

[YuriyRusinov]

Hello. colleagues !

I have to build QImage object from bytea field of postgresql database, previously I do it using pqxx::binarystring as described in theme in here, but when pqxx::binarystring became deprecated and I have to use std::basic_string<std::byte> type instead, I have got invalid image with this, debug shows that binary string became different. Is it problem with libpqxx or Qt ?

[jsulm]

@YuriyRusinov Please show your code.

[YuriyRusinov]

Old code

pqxx::binarystring CVDbPgResult::getCellAsBinaryString0( int row, int column ) const {
    if( m_res == nullptr || row >= m_res->size() || column >= m_res->at(row).size() )
        return pqxx::binarystring(nullptr, 0);

    pqxx::field fCell( m_res->at(row).at(column) );
    pqxx::binarystring fCellB( fCell );
    return fCellB;
}

    int nn = getCellLength( row, column );
    pqxx::binarystring imBytesStr0 = getCellAsBinaryString0( row, column );
    const char* imBytes0 = imBytesStr0.get();
    QByteArray imABytes = QByteArray::fromRawData( imBytes0, nn );
    QImage resImage;
    bool isLoaded = resImage.loadFromData( imABytes );

New code:

    basic_string<std::byte> imBytesStr = getCellAsBinaryString( row, column );
    const void* imV = static_cast<const void*>(imBytesStr.data());
    const char* imBytes = static_cast<const char *>(imV);

std::basic_string<std::byte> CVDbPgResult::getCellAsBinaryString( int row, int column) const {
    if( m_res == nullptr || row >= m_res->size() || column >= m_res->at(row).size() )
        return std::basic_string<std::byte>(nullptr, 0);

    pqxx::field fCell( m_res->at(row).at(column) );
    pqxx::binarystring fCellB( fCell );
    int nn = fCell.size();
    const char* data = fCell.c_str();
    qDebug() << __PRETTY_FUNCTION__ << nn << strlen(data) << fCellB.size();
    const void* vData = static_cast<const void *>(data);

    const std::byte* vres(static_cast<const std::byte*>(vData));
    // (data, nn);//get< std::string >();
    std::basic_string<std::byte> res(vres);
    return res;
}

[Christian Ehrlicher]

I mean - it's clearly stated and obvious: So don't assume that it can be treated as a C-style string unless you've made sure of this yourself.

Use a proper container like e.g. std::vector<>

[YuriyRusinov]

Therefore I need to use something like this vector<std::byte> or vector<char> ?

[YuriyRusinov]

Thanks a lot, problem was solved, using libpqxx method pqxx::field::as< std::basic_string <std::byte> >();

std::basic_string<std::byte> CVDbPgResult::getCellAsBinaryString( int row, int column) const {
    if( m_res == nullptr || row >= m_res->size() || column >= m_res->at(row).size() )
        return std::basic_string<std::byte>(nullptr, 0);

    pqxx::field fCell( m_res->at(row).at(column) );
    return fCell.as< std::basic_string<std::byte> >();
}