With current implementation of C++ slint::FilterModel it's easy to trigger undefined behavior, or obtain wrong results, due to invocation of filtering callable before slint::FilterModel children finalize initialization.
Moreover, if not hitting undefined behavior, a spurious additional filtering can be required.
As a comparison, slint::SortModel already implements the deferred policy.
Originally posted by **mous16** March 28, 2024
Playing around with custom models, I found a peculiar behavior of `slint::FilterModel` C++ implementation.
Specifically, when the constructor of `slint::FilterModel` is executed, it performs contextually the filtering on origin model, executing the callable passed as argument.
While this is perfectly fine when using directly `slint::FilterModel`, one should be very careful when subclassing it, because it can lead to accessing child members before their initialization, and potentially undefined behavior.
On the other hand, `slint::SortModel` defers sorting computation at first model usage, preventing the cited scenario.
Here an example:
```Slint
// appwindow.slint
import { ListView, VerticalBox, Switch } from "std-widgets.slint";
export component AppWindow inherits Window {
in property <[int]> model;
in-out property only-evens <=> only-evens-switch.checked;
in-out property ascending <=> ascending-switch.checked;
callback only-evens-changed <=> only-evens-switch.toggled;
callback ascending-changed <=> ascending-switch.toggled;
VerticalBox {
only-evens-switch := Switch {
text: "Only evens";
}
ascending-switch := Switch {
text: "Ascending";
}
ListView {
for item in root.model: Text {
text: item;
}
}
}
}
```
```C++
// main.cpp
#include
#include
#include
#include "appwindow.h"
#include "slint.h"
class MyFilterModel: public slint::FilterModel
{
bool onlyEvens{false};
[[nodiscard]] auto filter(const int& element) const -> bool
{
std::println("Filtering with onlyEvens={}; {} -> {}", onlyEvens, element, (!onlyEvens || element % 2 == 0));
return !onlyEvens || element % 2 == 0;
}
public:
MyFilterModel(std::shared_ptr> source_model):
slint::FilterModel{std::move(source_model), [this](const int& element) { return filter(element); }}
{
std::println("MyFilterModel initialized, onlyEvens={}", onlyEvens);
}
[[nodiscard]] auto getOnlyEvens() const -> bool { return onlyEvens; }
void setOnlyEvens(bool value)
{
std::println("Updating filter; {} -> {}", onlyEvens, value);
onlyEvens = value;
reset();
}
};
class MySortModel: public slint::SortModel
{
bool ascending{true};
[[nodiscard]] auto sort(const int& first, const int& second) const -> bool
{
std::println("Sorting with ascending={}; {} | {} -> {}", ascending, first, second, (ascending ? first < second : first >= second));
return ascending ? first < second : first >= second;
}
public:
MySortModel(std::shared_ptr> source_model):
slint::SortModel{std::move(source_model),
[this](const int& first, const int& second) { return sort(first, second); }}
{
std::println("MySortModel initialized, ascending={}", ascending);
}
[[nodiscard]] auto getAscending() const -> bool { return ascending; }
void setAscending(bool value)
{
std::println("Updating sort; {} -> {}", ascending, value);
ascending = value;
reset();
}
};
auto main([[maybe_unused]] int argc, [[maybe_unused]] char** argv) -> int
{
auto appWindow{AppWindow::create()};
const auto model{std::make_shared>(std::vector{1, 2, 3, 4, 5})};
const auto filterModel{std::make_shared(model)};
const auto sortModel{std::make_shared(filterModel)};
appWindow->set_only_evens(filterModel->getOnlyEvens());
appWindow->on_only_evens_changed([appWindow, filterModel]() {
filterModel->setOnlyEvens(appWindow->get_only_evens());
});
appWindow->set_ascending(sortModel->getAscending());
appWindow->on_ascending_changed([appWindow, sortModel]() { sortModel->setAscending(appWindow->get_ascending()); });
appWindow->set_model(sortModel);
appWindow->run();
return 0;
}
```
The output produced by the `std::println`, is not what one could expect, filtering function is called before initialization of `onlyEvens`, producing wrong results:
```
Filtering with onlyEvens=true; 1 -> false
Filtering with onlyEvens=true; 2 -> true
Filtering with onlyEvens=true; 3 -> false
Filtering with onlyEvens=true; 4 -> true
Filtering with onlyEvens=true; 5 -> false
MyFilterModel initialized, onlyEvens=false
MySortModel initialized, ascending=true
Sorting with ascending=true; 4 | 2 -> false
Sorting with ascending=true; 4 | 2 -> false
```
A possible workaround is to add a `FilterModel::reset()` invocation in `MyFilterModel::MyFilterModel(std::shared_ptr>)` body, but it will perform twice the computation, and still not prevent UB on first invocation.
Could be a good idea to move filtering function invocation at first usage, as is done for `slint::SortModel`.
ogoffart
commented
7 months ago
With current implementation of C++
slint::FilterModelit's easy to trigger undefined behavior, or obtain wrong results, due to invocation of filtering callable beforeslint::FilterModelchildren finalize initialization. Moreover, if not hitting undefined behavior, a spurious additional filtering can be required.As a comparison,
slint::SortModelalready implements the deferred policy.Discussed in https://github.com/slint-ui/slint/discussions/4972