QModelRoleDataSpan Class

The QModelRoleDataSpan class provides a span over QModelRoleData objects. More...

Header: #include <QModelRoleDataSpan>
CMake: find_package(Qt6 COMPONENTS Core REQUIRED)
target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake: QT += core
Since: Qt 6.0

Public Functions

QModelRoleDataSpan(Container &c)
QModelRoleDataSpan(QModelRoleData *modelRoleData, qsizetype len)
QModelRoleDataSpan(QModelRoleData &modelRoleData)
QModelRoleData *begin() const
QModelRoleData *data() const
QVariant *dataForRole(int role) const
QModelRoleData *end() const
qsizetype length() const
qsizetype size() const
QModelRoleData &operator[](qsizetype index) const

Detailed Description

A QModelRoleDataSpan is used as an abstraction over an array of QModelRoleData objects.

Like a view, QModelRoleDataSpan provides a small object (pointer and size) that can be passed to functions that need to examine the contents of the array. A QModelRoleDataSpan can be constructed from any array-like sequence (plain arrays, QVector, std::vector, QVarLengthArray, and so on). Moreover, it does not own the sequence, which must therefore be kept alive longer than any QModelRoleDataSpan objects referencing it.

Unlike a view, QModelRoleDataSpan is a span, so it allows for modifications to the underlying elements.

QModelRoleDataSpan's main use case is making it possible for a model to return the data corresponding to different roles in one call.

In order to draw one element from a model, a view (through its delegates) will generally request multiple roles for the same index by calling data() as many times as needed:

QVariant text = model->data(index, Qt::DisplayRole);
QVariant decoration = model->data(index, Qt::DecorationRole);
QVariant checkState = model->data(index, Qt::CheckStateRole);
// etc.

QModelRoleDataSpan allows a view to request the same data using just one function call.

This is achieved by having the view prepare a suitable array of QModelRoleData objects, each initialized with the role that should be fetched. The array is then wrapped in a QModelRoleDataSpan object, which is then passed to a model's multiData() function.

std::array<QModelRoleData, 3> roleData = { {
} };

// Usually, this is not necessary: A QModelRoleDataSpan
// will be built automatically for you when passing an array-like
// container to multiData().
QModelRoleDataSpan span(roleData);

model->multiData(index, span);

// Use roleData[0].data(), roleData[1].data(), etc.

Views are encouraged to store the array of QModelRoleData objects (and, possibly, the corresponding span) and re-use it in subsequent calls to the model. This allows to reduce the memory allocations related with creating and returning QVariant objects.

Finally, given a QModelRoleDataSpan object, the model's responsibility is to fill in the data corresponding to each role in the span. How this is done depends on the concrete model class. Here's a sketch of a possible implementation that iterates over the span and uses setData() on each element:

void MyModel::multiData(const QModelIndex &index, QModelRoleDataSpan roleDataSpan) const
    for (QModelRoleData &roleData : roleDataSpan) {
        int role = roleData.role();

        // ... obtain the data for index and role ...


See also Model/View Programming and QAbstractItemModel::multiData().

Member Function Documentation

template <typename Container, if_compatible_container<Container>> QModelRoleDataSpan::QModelRoleDataSpan(Container &c)

Constructs an QModelRoleDataSpan spanning over the container c, which can be any contiguous container of QModelRoleData objects. For instance, it can be a QVector<QModelRoleData>, a std::array<QModelRoleData, 10> and so on.

Note: The container must be kept alive as long as this object has not been destructed.

QModelRoleDataSpan::QModelRoleDataSpan(QModelRoleData *modelRoleData, qsizetype len)

Constructs an QModelRoleDataSpan spanning over the array beginning at modelRoleData and with length len.

Note: The array must be kept alive as long as this object has not been destructed.

QModelRoleDataSpan::QModelRoleDataSpan(QModelRoleData &modelRoleData)

Constructs an QModelRoleDataSpan spanning over modelRoleData, seen as a 1-element array.


Constructs an empty QModelRoleDataSpan. Its data() will be set to nullptr, and its length to zero.

QModelRoleData *QModelRoleDataSpan::begin() const

Returns a pointer to the beginning of the span represented by this object.

QModelRoleData *QModelRoleDataSpan::data() const

Returns a pointer to the beginning of the span represented by this object.

QVariant *QModelRoleDataSpan::dataForRole(int role) const

Returns the data associated with the first QModelRoleData in the span that has its role equal to role. If such a QModelRoleData object does not exist, the behavior is undefined.

Note: Avoid calling this function from the model's side, as a model cannot possibly know in advance which roles are in a given QModelRoleDataSpan. This function is instead suitable for views and delegates, which have control over the roles in the span.

See also QModelRoleData::data().

QModelRoleData *QModelRoleDataSpan::end() const

Returns a pointer to the imaginary element one past the end of the span represented by this object.

qsizetype QModelRoleDataSpan::length() const

Returns the length of the span represented by this object.

qsizetype QModelRoleDataSpan::size() const

Returns the length of the span represented by this object.

QModelRoleData &QModelRoleDataSpan::operator[](qsizetype index) const

Returns a modifiable reference to the QModelRoleData at position index in the span.

Note: index must be a valid index for this span (0 <= index < size()).

© 2021 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.