Hbox

koto::Hbox is a composite Widget container that owns child widgets on the heap via std::unique_ptr<Widget> and draws them left-to-right using Layout::horizontal (Header: koto/hbox.hpp).

Quick example

Horizontal box container

Hbox row;
row.add<Box>(Constraint::fixed(12)).title("Left").border_style(BorderStyle::Single);
row.add<Box>(Constraint::fill()).title("Right").border_style(BorderStyle::Rounded);
row.draw(frame, hbox_area);

Output

┌── Left ──┐╭────── Right ───────╮
│          ││                    │
└──────────┘╰────────────────────╯

Behavior

• Child ownership and fluent configuration: Hbox::add<T>(Constraint c, Args&&... args) constructs a std::unique_ptr<T>(std::forward<Args>(args)...), appends {std::move(widget), c} to m_children, and returns T& so you can chain widget setter methods directly (include/koto/hbox.hpp:15-25).
• Zero-heap layout path for <= 16 children: When m_children.size() <= LayoutResult::InlineCapacity (16), Hbox::draw() copies child constraints into a stack std::array<Constraint, 16>, calls Layout::horizontal(area, std::span<const Constraint>(...)), and invokes m_children[i].widget->draw(frame, rect[i]). When m_children.size() > 16, it allocates a std::vector<Constraint> before calling Layout::horizontal (src/hbox.cpp:16-42).

Methods

Method	Description	Default
Hbox()	Default-constructs an empty horizontal container (move-only due to std::unique_ptr<Widget>).	m_children empty
add<T>(Constraint c, Args&&... args)	Constructs child widget T(args...) with horizontal constraint c, stores it, and returns T&.	None
draw(Frame& frame, Area area)	Partitions area horizontally and draws each child into its corresponding sub-area.	None

Gotchas

• Heap allocation per child: Although Hbox::draw() avoids heap allocation for the Constraint array when m_children.size() <= 16, every call to Hbox::add<T>(...) allocates the child Widget on the heap via std::make_unique<T>(...). In hot 60 FPS on_render callbacks, partitioning areas directly with Layout::horizontal(area, {...}) and drawing stack-allocated widgets avoids per-frame std::make_unique allocations.
• Move-only container: Because m_children holds std::unique_ptr<Widget>, Hbox cannot be copied.

See also

• Vbox
• Layout
• Layout Recipes Guide