- Qt 6.8
- Qt Core
- C++ Classes
- QModelRoleDataSpan
- Qt 6.8.3 Reference Documentation

# QModelRoleDataSpan Class

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

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

- List of all members, including inherited members

## Public Functions

| QModelRoleDataSpan()
| QModelRoleDataSpan(Container &c)
| QModelRoleDataSpan(QModelRoleData &modelRoleData)
| QModelRoleDataSpan(QModelRoleData *modelRoleData, qsizetype len)
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 = { {
         QModelRoleData(Qt::DisplayRole),
         QModelRoleData(Qt::DecorationRole),
         QModelRoleData(Qt::CheckStateRole)
     } };

     // 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 ...

             roleData.setData(result);
         }
     }

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

## Member Function Documentation

### `[constexpr noexcept]` QModelRoleDataSpan::QModelRoleDataSpan()

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

### `[constexpr noexcept(...)]` template <typename Container, QModelRoleDataSpan::if_compatible_container<Container> = true> 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.

Note: This function is noexcept when `noexcept(std::data(c)) && noexcept(std::size(c))` is `true`.

### `[constexpr noexcept]` QModelRoleDataSpan::QModelRoleDataSpan(QModelRoleData &modelRoleData)

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

### `[constexpr]` 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.

### `[constexpr noexcept]` QModelRoleData *QModelRoleDataSpan::begin() const

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

### `[constexpr noexcept]` QModelRoleData *QModelRoleDataSpan::data() const

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

### `[constexpr]` 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().

### `[constexpr noexcept]` QModelRoleData *QModelRoleDataSpan::end() const

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

### `[constexpr noexcept]` qsizetype QModelRoleDataSpan::length() const

Returns the length of the span represented by this object.

### `[constexpr noexcept]` qsizetype QModelRoleDataSpan::size() const

Returns the length of the span represented by this object.

### `[constexpr]` 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()).