Using QDataStream with STL containers

Eukaryote

Hi,
I'm trying to use QDataStream for serialization with code that is mostly STL based but compiler never seems to find matching method.
I tried to do the following:

template<typename First, typename Second>
inline QDataStream& operator<< (QDataStream& out, const std::pair<First,Second>& args)
{
    out << args.first;
    out << args.second;
    return out;
}
template<typename First, typename Second>
inline QDataStream& operator>> (QDataStream& in, std::pair<First,Second>& args)
{
    in >> args.first;
    in >> args.second;
    return in;
}
template<typename Container,typename Val>
inline QDataStream& operator<< (QDataStream& out, const Container& args)
{
    out << (int)args.size();
    for (const Val& val : args )
    {
        out << val;
    }
    return out;
}
template<typename Container,typename Val>
inline QDataStream& operator>> (QDataStream& in, Container& args)
{
    int length=0;
    in >> length;
    for (int i = 0 ; i < length ; ++i)
    {
        Val obj;
        in >> obj;
        args.insert(args.end(),obj);
    }
    return in;
}
inline QDataStream& operator<< (QDataStream& out, const std::string& args)
{
    QByteArray temp(args.c_str(),args.size());
    out << temp;
    return out;
}
inline QDataStream& operator>> (QDataStream& in, std::string& args)
{
    QByteArray temp;
    in >> temp;
    args= std::string(temp.constData());
    return in;
}

But it fails . For example for "in >> StatesSet "
I'm getting:

no match for 'operator>>' (operand types are 'QDataStream' and 'StatesSet {aka std::set<long int>}')
     in >> Values;
note: candidate: template<class Container, class Val> QDataStream& operator>>(QDataStream&, Container&)
 inline QDataStream& operator>> (QDataStream& in, Container& args)
                            ^~~~~~~~
  template argument deduction/substitution failed:
  couldn't deduce template parameter 'Val'

Can someone tell me what am doing wrong or suggest a better way to serialize stl using Qt ? Thank you very much in advance .

Christian Ehrlicher

@Eukaryote said in Using QDataStream with STL containers:

template<typename Container,typename Val>
inline QDataStream& operator<< (QDataStream& out, const Container& args)
{
out << (int)args.size();
for (const Val& val : args )
{
out << val;
}
return out;
}

The compiler can't detmerine what Val is/should be. And it's also not needed. Either use auto

for (const auto &val : args)

or use std::set value_type

typename Container::value_type Val;
for (const Val &val : args)

Eukaryote

Thanks for your reply, but this my original implementation was using auto , it didn't help.
And I don't think using auto is much different from using template .

Christian Ehrlicher

This is working perfectly fine for me...

template<typename Container>
inline QDataStream& operator<< (QDataStream& out, const Container& args)
{
    out << (int)args.size();
    for (const auto& val : args )
    {
        out << val;
    }
    return out;
}
template<typename Container>
inline QDataStream& operator>> (QDataStream& in, Container& args)
{
    int length=0;
    in >> length;
    args.clear();
    args.reserve(length);
    for (int i = 0 ; i < length ; ++i)
    {
        typedef typename Container::value_type valType;
        valType obj;
        in >> obj;
        args.push_back(obj);
    }
    return in;
}

int main(int argc, char **argv)
{
    QDataStream ds;
    std::vector<int> cont;
    ds << cont;
    ds >> cont;
}

Eukaryote

Adding typedef typename Container::value_type valType; to the loop helped with this issue . Thank you very much .