Modern C++ GUI: Debug Tools & Dashboards with Dear ImGui [#36-1]

Introduction: beyond printf—visible state

Dear ImGui is an immediate-mode GUI library: every frame you call widgets; interaction is returned from those calls. It is small, easy to integrate, and ideal for in-engine overlays, profilers, and tuning panels.

Covers: what ImGui is, minimal window flow, GLFW+OpenGL backends, debugging dashboards, common crashes (frame order, local state), best practices, production notes.

---

Table of contents

1. What is Dear ImGui?
2. Immediate mode
3. Minimal example
4. Backend (GLFW + OpenGL)
5. Complete example
6. Dashboards
7. Common errors
8. Best practices
9. Production patterns

---

1. What is Dear ImGui?

Header-friendly, minimal dependencies, backends for GLFW/SDL/DX/Vulkan/Metal. Best for tools inside an engine—not full desktop app frameworks like Qt for every case.

flowchart TB
    subgraph app["App"]
        loop["Main loop"]
        ui["ImGui Begin…End"]
    end
    subgraph imgui["Dear ImGui"]
        core["imgui core"]
        backend["impl_* backends"]
    end
    loop --> ui --> core --> backend

---

2. Immediate mode vs retained UI

Retained toolkits build widget objects and fire callbacks; ImGui redraws widgets each frame and uses return values (if (Button(...))).

---

3. Minimal frame order

Backend NewFrame → ImGui::NewFrame → Begin/End widgets → ImGui::Render → Backend RenderDrawData

Critical: slider/state must live in static or member storage—not fresh locals reset every frame.

---

4. GLFW + OpenGL

Clone official examples (example_glfw_opengl3), copy backend sources, initialize context, and run the loop—the full main.cpp listing below ties the steps together.

---

5. Complete example

The full main_imgui_minimal.cpp example includes PlotLines, buttons, and a second window—keep comments in your team’s language in your repo copy.

---

6. Dashboards

Bind ServerStats fields to Text, PlotLines, and buttons for live ops panels.

---

7. Common errors

• NewFrame/Render ordering wrong → blank UI or crash.
• Missing backend init → no input / black screen.
• Widgets only called once behind a flag → UI disappears (ImGui needs every frame).
• SliderFloat on a stack local → value resets.

---

8. Best practices

PushID/##suffix to avoid ID clashes; CollapsingHeader for sections; tooltips; keep style setup in one place.

---

9. Production patterns

ImGui is not thread-safe—call only from the main thread; pass stats via queues. Docking/viewports optional. Handle GPU errors from backends explicitly.

---

Keywords

Dear ImGui tutorial, immediate mode GUI, GLFW ImGui, OpenGL debug UI, C++ dashboard

Next: Qt basics (#36-2)
Previous: Wasm (#35-2)