FontEditor: Cannot take reference to local lambda

Under the hood, a lambda is just a struct full of pointers/references/copies and whatever else
the compiler deems necessary. In the case of 'update_demo', the struct lives on the stack
frame of FontEditorWidget::FontEditorWidget(). Hence it is still alive when it's called
during the constructor.

However, when 'fixed_width_checkbox.on_checked' fires, that stack frame is no longer alive,
and thus the *reference* to the (struct of) the lambda is invalid\! This meant that
'update_demo' silently read invalid data, tried to call '.update()' on some innocent arbitrary
memory address, and it crashed somewhere unrelated.

Passing 'update_demo' by value (like with all the other event handlers) fixes this issue.
Note that this solution only works because 'update_demo' itself has no state; otherwise
the various copies of 'update_demo' might notice that they are, in fact, independent copies
of the original lambda. But that doesn't matter here.

1 files changed, 1 insertions, 1 deletions

diff --git a/Applications/FontEditor/FontEditor.cpp b/Applications/FontEditor/FontEditor.cpp
index a7d32a4706..0d8e68340d 100644
--- a/Applications/FontEditor/FontEditor.cpp
+++ b/Applications/FontEditor/FontEditor.cpp
@@ -263,7 +263,7 @@ FontEditorWidget::FontEditorWidget(const String& path, RefPtr<Gfx::Font>&& edite
         info_label.set_text(builder.to_string());
     };
 
-    fixed_width_checkbox.on_checked = [&](bool checked) {
+    fixed_width_checkbox.on_checked = [&, update_demo](bool checked) {
         m_edited_font->set_fixed_width(checked);
         glyph_width_spinbox.set_enabled(!m_edited_font->is_fixed_width());
         glyph_width_spinbox.set_value(m_edited_font->glyph_width(m_glyph_map_widget->selected_glyph()));