Here are the classes, structs, unions and interfaces with brief descriptions:

Nfcn	Used replacement tokens by configure_file():
Nfont
CFontLoader	Helper class for searching and loading fonts
Nopengl
CGraphics	OpenGL-specific implementation of the Graphics interface
CImage	OpenGL-specific implementation of Image
CImageLoader	OpenGL ImageLoader that loads images with SDL
Nsdl3
CGraphics	SDL3 renderer-specific implementation of the Graphics interface
CImage	SDL3-specific implementation of Image
CImageLoader	SDL3-specific implementation of ImageLoader
CInput	SDL3-specific implementation of Input
CTrueTypeFont	SDL3/FreeType implementation for rendering TrueType fonts
CActionEvent	Represents an action trigger (e.g., button click)
CActionListener	Interface for listening to action events from widgets
CActivityBar	An activity bar widget that displays toggle buttons on the side of a window
CActivityBarItem	A toggle button item for use in an ActivityBar
CAdjustingContainer	Self-adjusting Container class
Capp_version	Static application version and metadata
CBarGraph	Displays data as a rectangular bar graph
CBarSection	A container section for use within HorizontalBar or VerticalBar widgets
CButton	A standard clickable button widget
CCheckBox	An implementation of a check box
CClipRectangle	A rectangle specifically used for clipping rendering regions
CColor	Color
CColumnLayout	Column layout specification passed to items for rendering
CContainer	A composite widget capable of holding and managing child widgets
CContainerEvent	Represents changes within a container (e.g., widget added/removed)
CContainerListener	Interface for listening to container modification events
CCurveGraph	Displays data as a smooth curve based on a vector of points
CDeathListener	Interface for listening to widget destruction events
CDefaultFont	A basic font implementation capable only of drawing rectangles (placeholder)
CDragEvent	Drag and drop event type container
CDragHandler	Manages drag and drop operations for a Gui instance
CDragPayload	Container for arbitrary data carried during a drag operation
CDragPoint	Simple 2D integer point used for drag offsets and positions
CDragRenderConfig	Configuration for how drag visuals are rendered
CDropDown	A dropdown list widget allowing single item selection from a list
CDropTargetListener	Listener interface for widgets that accept dragged payloads
CEvent	Base class for all GUI event objects
CException	An exception class containing a message, a file, and a line number where the exception occurred
Cfloat_traits	Generic template for floating-point type traits
Cfloat_traits< double >	Specialization of float traits for double
Cfloat_traits< float >	Specialization of float traits for float
CFlowContainer	An implementation of a flow container that can contain other widgets
CFocusHandler	Manages focus navigation and assignment among widgets within a Gui instance
CModalScope	Modal scope management (RAII guard)
CModalState	Represents a single modal state level with focus and mouse owners
CFocusListener	Interface for listening to focus gain/loss events
CFont	Abstract interface for font rendering
CGenericInput	A generic input implementation usable with various backends
CGraphics	Abstract interface providing primitive drawing functions (lines, rectangles, etc.)
CGui	The central GUI manager
CGuiDeathListener	Internal listener that forwards death notifications from widgets to the owning Gui so it can clean up references
CHorizontalBar	A horizontal bar container that arranges children from left to right
CIcon	Displays a static image icon
CIconProgressBar	Displays progress using a sequence of icons
CImage	Abstract holder for image data
CImageButton	An implementation of a regular clickable button
CImageFont	A font implementation using an image atlas containing glyph data
CImageFontConfig	Configuration struct for ImageFont constructors
CImageLoader	Abstract interface for loading image assets
CImageProgressBar	Displays progress using images
CInput	Abstract interface for polling user input devices
CInputEvent	Base class for all input-related events (keyboard, mouse)
CKeyEvent	Represents a key event
CKeyInput	Internal class representing raw keyboard input data
CKeyListener	Interface for listening to keyboard events
CLabel	Displays a text caption
CLineGraph	Displays data as a line graph connecting a vector of points
CListBox	A scrollable list box allowing item selection
CListModel	Interface for a data model representing a list (used by ListBox/DropDown)
CMath	Template utility providing math functions and constants for numeric type T
CMenuBar	A menu bar widget that displays menus at the top of a window
CMenuColumns	Aggregated column metrics for all items in a menu
CMenuItem	A menu item widget for use in menus
CMenuItemMetrics	Layout metrics for a single menu item (content only, no padding/borders)
CMenuPopup	A menu popup widget that displays a dropdown menu
CModalBackdrop	A Transparent fullscreen backdrop
CMouseEvent	Represents a mouse event
CMouseInput	Internal class representing raw mouse input data
CMouseListener	Interface for listening to mouse events
CPanel	A Panel widget with a title bar and optional close button
CPasswordField	Password input widget
CPieGraph	Displays data as a circular pie chart
CPieGraphSegment	Represents a single segment of a pie graph (angles and color)
CPoint	Represents a 2D coordinate (X, Y)
CPointGraph	Draws a scatter graph of individual points
CPrimaryPanel	A primary (left-side) panel widget
CRadioButton	An implementation of a radio button where a user can select or deselect the radio button and where the status of the radio button is displayed to the user
CRectangle	Represents a rectangular area (X, Y, Width, Height)
CScrollArea	A scrollable viewport for viewing widgets larger than the visible area
CSecondaryPanel	A secondary (right-side) panel widget
CSelectionEvent	Represents a change in selection state (e.g., list item selected)
CSelectionListener	Interface for listening to selection change events
Csemantic_version	Simple semantic version container
CShortcut	High-level shortcut binding a semantic key with a modifier mask
CSize	Represents dimensions defined by width and height
CSlider	A control for selecting a numeric value by sliding along a scale
CSpacer	Provides empty space to assist with layout management
CStatusBar	A status bar widget that displays information at the bottom of a window
CTab	A single tab label used within a TabbedArea
CTabbedArea	A container organizing content into selectable tabs
CText	Helper class for text manipulation within widgets
CTextBox	A multi-line text input field
CTextField	A single-line text input field
CTextInputEvent	Text input event for IME (input method editor) composition, dead keys, and pasted text
CToggleButton	An implementation of a toggleable button
CTooltip	A tooltip widget
CTooltipModifierBehavior	Tooltip modifier behavior (for an ALT-key extended view)
CTooltipSpec	Tooltip specification (data + behavior)
CUTF8StringEditor	Utility for editing and handling UTF-8 encoded strings
CVisibilityEventHandler	Handles changes in widget visibility states
CWidget	Abstract base class defining the common behavior, properties, and lifecycle of all GUI elements
CWidgetListener	Interface for receiving generic events from widgets
CWindow	A movable container window that can hold other widgets