Qt and Templates

[jsmax]

Hi.

First, sorry for my bad knowledge of C++ templates

I have 2 classes: Street and Manager.

class Street
{
public:
    Street();
    ~Street();
 
    QMap<QString, QString> values; // ex: "id" => "1", "name" => "Paris"
    ....
};

class Street
{
public:
    Street();
    ~Street();

    QMap<QString, QString> values; // ex: "id" => "1", "name" => "Paris"
    ....
};

To copy to clipboard, switch view to plain text mode 

class Manager
{
public:
    Manager();
    ~Manager();
 
    QMap<QString, QString> values; // ex: "id" => "1", "name" => "John"
    ....
};

class Manager
{
public:
    Manager();
    ~Manager();

    QMap<QString, QString> values; // ex: "id" => "1", "name" => "John"
    ....
};

To copy to clipboard, switch view to plain text mode 

Now i have 2 QAbstractTableModel subclasses: one that works with streets and one for managers.

class StreetTableModel : public QAbstractTableModel
{
    Q_OBJECT
 
public:
    explicit StreetTableModel(QObject *parent = 0);
    ....
 
private:
    QList<Street*> records; // Here we store the streets
    ....
};

class StreetTableModel : public QAbstractTableModel
{
    Q_OBJECT

public:
    explicit StreetTableModel(QObject *parent = 0);
    ....

private:
    QList<Street*> records; // Here we store the streets
    ....
};

To copy to clipboard, switch view to plain text mode 

class ManagerTableModel : public QAbstractTableModel
{
    Q_OBJECT
 
public:
    explicit ManagerTableModel(QObject *parent = 0);
    ....
 
private:
    QList<Manager*> records; // Here we store the managers
    ....
};

class ManagerTableModel : public QAbstractTableModel
{
    Q_OBJECT

public:
    explicit ManagerTableModel(QObject *parent = 0);
    ....

private:
    QList<Manager*> records; // Here we store the managers
    ....
};

To copy to clipboard, switch view to plain text mode 

In the future there will be a lot of objects like street and manager and i don't want to create each time a subclass of QAbstractTableModel.

Can you please show me a way to define a subclass of QAbstractTableModel using templates ? I tried some variants, but a don't get it how it must work.

I want to be able for example to create a model like:

TableModel<Street> *model = new TableModel();

TableModel<Street> *model = new TableModel();

To copy to clipboard, switch view to plain text mode 

Thanks a lot.

[franz]

Try something like

template<typename T>
class TableModel : public QAbstractTableModel
{
    Q_OBJECT
 
public:
    explicit TableModel(QObject *parent = 0);
    ....
 
private:
    typename QList<T *> records;
    ....
};

template<typename T>
class TableModel : public QAbstractTableModel
{
    Q_OBJECT

public:
    explicit TableModel(QObject *parent = 0);
    ....

private:
    typename QList<T *> records;
    ....
};

To copy to clipboard, switch view to plain text mode 

It's dry coded and un-tested, but I'd expect it to behave like you want:

TableModel<Street> *model = new TableModel<Street>();

TableModel<Street> *model = new TableModel<Street>();

To copy to clipboard, switch view to plain text mode 

Note that this might mean a lot of the code needs to be templated, which means you're going to have to put all that implementation in a header file. Be sure to document the requirements on T. Also, given the nature of the example classes you give, you can consider not using pointers in your list. This will probably make management of the records much simpler. If that's possible, use

typename QList<T> records;

typename QList<T> records;

To copy to clipboard, switch view to plain text mode 

instead.

[Santosh Reddy]

I would suggest not to use template for your requirement, and use C++ polymorphism. Here is an example, the main point is to use BasicValue class as a interface for future classes / objects

class BasicValue
{
public:
    BasicValue();
    virtual ~BasicValue();
 
 
    QMap<QString, QString> values; // ex: "id" => "1", "name" => "Paris" (anything)
    ....
};
 
 
class Street     : public BasicValue {};
class Manager    : public BasicValue {};
class FutureItem : public BasicValue {};
 
 
class ValueTableModel : public QAbstractTableModel
{
    Q_OBJECT
 
 
public:
    explicit ValueTableModel(QObject *parent = 0);
    ....
 
 
private:
    QList<BasicValue*> records;
    ....
};

class BasicValue
{
public:
    BasicValue();
    virtual ~BasicValue();


    QMap<QString, QString> values; // ex: "id" => "1", "name" => "Paris" (anything)
    ....
};


class Street     : public BasicValue {};
class Manager    : public BasicValue {};
class FutureItem : public BasicValue {};


class ValueTableModel : public QAbstractTableModel
{
    Q_OBJECT


public:
    explicit ValueTableModel(QObject *parent = 0);
    ....


private:
    QList<BasicValue*> records;
    ....
};

To copy to clipboard, switch view to plain text mode 

[franz]

That is a nicer solution. I gave what OP wants, you gave what OP needs.

[wysota]

Some time ago I was thinking about a template based approach to the problem myself too but the downside was that you can have models with different number of columns or different types of data. Neither of the approaches presented here actually solve the problem. I think it is possiblt to do it both using templates and polymorphism. As for the latter, we go in direction of QStandardItemModel so maybe there is no point in doubling what it already provides (you can see that the solution presented in this thread looks strikingly like QStandardItemModel with BasicValue being QStandardItem and Street being a subclass of the latter).

As for the template based approach I think we'd need to provide information to the template class about ways of accessing data in the model for instance by passing a container holding the names or types of data in each column. An example instantiation could then look like this:

class RecordDescription {
public:
//...
const_iterator begin() const;
const_iterator end() const;
//...
};
 
TemplateBasedModel<RecordType, RecordDescription> model;

class RecordDescription {
public:
//...
const_iterator begin() const;
const_iterator end() const;
//...
};

TemplateBasedModel<RecordType, RecordDescription> model;

To copy to clipboard, switch view to plain text mode 

or:

TemplateBasedModel<RecordType> model(recordDescription);

TemplateBasedModel<RecordType> model(recordDescription);

To copy to clipboard, switch view to plain text mode 

with "modelDescription" being an object that can read and write data to RecordType instances.

[jsmax]

Try something like

template<typename T>
class TableModel : public QAbstractTableModel
{
    Q_OBJECT
 
public:
    explicit TableModel(QObject *parent = 0);
    ....
 
private:
    typename QList<T *> records;
    ....
};

template<typename T>
class TableModel : public QAbstractTableModel
{
    Q_OBJECT

public:
    explicit TableModel(QObject *parent = 0);
    ....

private:
    typename QList<T *> records;
    ....
};

To copy to clipboard, switch view to plain text mode 

I get errors on line 11 in your code:

error: expected nested-name-specifier
error: invalid declarator before 'records'

This is my class (i get the same errors in my class):

template<typename TPL>
class JSONTableModel : public QAbstractTableModel
{
    Q_OBJECT
 
public:
    explicit JSONTableModel(QObject *parent = 0);
 
    int rowCount ( const QModelIndex & parent = QModelIndex() ) const;
    int columnCount ( const QModelIndex & parent = QModelIndex() ) const;
 
    QVariant data ( const QModelIndex & index, int role = Qt::DisplayRole ) const;
    bool setData ( const QModelIndex & index, const QVariant & value, int role = Qt::EditRole );
    QVariant headerData ( int section, Qt::Orientation orientation, int role = Qt::DisplayRole ) const;
    Qt::ItemFlags flags ( const QModelIndex & index ) const;
 
    int getTotalRecordsCount();
    int getFetchedRecordsCount();
    QString getParam(QString);
    QString getHeaderKey(int);
    void setParam(QString, QString);
 
private:
    typename QList<TPL *> records;
    QMap<QString, QString> headers;
    QMap<QString, QString> params;
    QNetworkAccessManager *manager;
    QModelIndex updatingIndex;
    QString url;
    int totalRecords;
 
signals:
    void dataLoaded();
    void requestStarted();
 
public slots:
    void loadRecords();
    void replyFinished(QNetworkReply*);
};

template<typename TPL>
class JSONTableModel : public QAbstractTableModel
{
    Q_OBJECT

public:
    explicit JSONTableModel(QObject *parent = 0);

    int rowCount ( const QModelIndex & parent = QModelIndex() ) const;
    int columnCount ( const QModelIndex & parent = QModelIndex() ) const;

    QVariant data ( const QModelIndex & index, int role = Qt::DisplayRole ) const;
    bool setData ( const QModelIndex & index, const QVariant & value, int role = Qt::EditRole );
    QVariant headerData ( int section, Qt::Orientation orientation, int role = Qt::DisplayRole ) const;
    Qt::ItemFlags flags ( const QModelIndex & index ) const;

    int getTotalRecordsCount();
    int getFetchedRecordsCount();
    QString getParam(QString);
    QString getHeaderKey(int);
    void setParam(QString, QString);

private:
    typename QList<TPL *> records;
    QMap<QString, QString> headers;
    QMap<QString, QString> params;
    QNetworkAccessManager *manager;
    QModelIndex updatingIndex;
    QString url;
    int totalRecords;

signals:
    void dataLoaded();
    void requestStarted();

public slots:
    void loadRecords();
    void replyFinished(QNetworkReply*);
};

To copy to clipboard, switch view to plain text mode 

[stampede]

"typename" is not needed in this case, its needed when the template parameter has a nested type and you want to use it:

template<class T>
class Test{
public:
    Test(){
        // T::NestedClass t; // this wont compile
         typename T::NestedClass t; // this is ok
    }
};

template<class T>
class Test{
public:
    Test(){
        // T::NestedClass t; // this wont compile
         typename T::NestedClass t; // this is ok
    }
};

To copy to clipboard, switch view to plain text mode 

[jsmax]

QObject classes does not support templates

During compilation i get this:

Error: Template classes not supported by Q_OBJECT

[wysota]

They do, only that you can't use Q_OBJECT macro with a template class. You need to subclass twice -- first introduce Q_OBJECT, define all the signals and slots you want and then subclass again and introduce the template.