<![CDATA[Build QImage from postgresql database via libpqxx]]>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 ?

]]>https://forum.qt.io/topic/142594/build-qimage-from-postgresql-database-via-libpqxxRSS for NodeMon, 06 Jul 2026 14:24:11 GMTWed, 01 Feb 2023 09:25:22 GMT60<![CDATA[Reply to Build QImage from postgresql database via libpqxx on Wed, 01 Feb 2023 17:40:52 GMT]]>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> >();
}
]]>https://forum.qt.io/post/745894https://forum.qt.io/post/745894Wed, 01 Feb 2023 17:40:52 GMT<![CDATA[Reply to Build QImage from postgresql database via libpqxx on Wed, 01 Feb 2023 11:52:48 GMT]]>Therefore I need to use something like this vector<std::byte> or vector<char> ?

]]>https://forum.qt.io/post/745830https://forum.qt.io/post/745830Wed, 01 Feb 2023 11:52:48 GMT<![CDATA[Reply to Build QImage from postgresql database via libpqxx on Wed, 01 Feb 2023 09:44:16 GMT]]>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<>

]]>https://forum.qt.io/post/745815https://forum.qt.io/post/745815Wed, 01 Feb 2023 09:44:16 GMT<![CDATA[Reply to Build QImage from postgresql database via libpqxx on Wed, 01 Feb 2023 09:40:45 GMT]]>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;
}
]]>https://forum.qt.io/post/745814https://forum.qt.io/post/745814Wed, 01 Feb 2023 09:40:45 GMT<![CDATA[Reply to Build QImage from postgresql database via libpqxx on Wed, 01 Feb 2023 09:28:43 GMT]]>@YuriyRusinov Please show your code.

]]>https://forum.qt.io/post/745810https://forum.qt.io/post/745810Wed, 01 Feb 2023 09:28:43 GMT