将QPair转换为QVariant

Question

I have the following problem: I want to transmitt data via TCP, and wrote a function for that. For maximum reusability the function template is f(QPair<QString, QVariant> data) . The first value (aka QString ) is used by the receiver as target address, the second contains the data. Now I want to transfer a QPair<int, int> -value, but unfortunately I can not convert a QPair to a QVariant . The optimum would be to be able to transfer a pair of int -values without having to write a new function (or to overload the old one). What is the best alternative for QPair in this case?

Answer 1

You have to use the special macro Q_DECLARE_METATYPE() to make custom types available to QVariant system. Please read the doc carefully to understand how it works.

For QPair though it's quite straightforward:

#include <QPair>
#include <QDebug>

typedef QPair<int,int> MyType;    // typedef for your type
Q_DECLARE_METATYPE(MyType);       // makes your type available to QMetaType system

int main(int argc, char *argv[])
{
    // ...

    MyType pair_in(1,2);
    QVariant variant = QVariant::fromValue(pair_in);
    MyType pair_out = variant.value<MyType>();
    qDebug() << pair_out;

    // ...
}

Answer 2

Note: this answer uses another functions to convert them, something you may consider.

You could use QDataStream to serialize the QPair to QByteArray and then convert it to QVariant , and you can the inverse process to get the QPair from a QVariant .

Example:

//Convert the QPair to QByteArray first and then
//convert it to QVariant
QVariant tovariant(const QPair<int, int> &value)
{
    QByteArray ba;
    QDataStream stream(&ba, QIODevice::WriteOnly);
    stream << value;
    return QVariant(ba);
}

//Convert the QVariant to QByteArray first and then
//convert it to QPair
QPair<int, int> topair(const QVariant &value)
{
    QPair<int, int> pair;
    QByteArray ba = value.toByteArray();
    QDataStream stream(&ba, QIODevice::ReadOnly);
    stream >> pair;
    return pair;
}