The difference between Building a project and Hot Reloading in Unreal Engine lies in how the changes to your code are compiled and applied to the game or editor during development. Here’s a detailed comparison:

1. Building a Project

  • Definition: Building a project compiles all the C++ code in your project, creating a complete binary (DLLs, EXEs, etc.) that can be run by Unreal Engine.
  • When it Happens:
    • When you click Build in Visual Studio.
    • When you perform a full compile from the Unreal Editor.
    • Automatically triggered during certain workflows (e.g., launching the game or packaging).
  • Characteristics:
    • Complete Compilation:
      • Recompiles all C++ files, even if they haven’t changed.
      • Ensures that everything is linked and compiled correctly.
    • Requires Restart:
      • If the editor is running, the project must be reloaded or restarted after the build.
    • Slower:
      • A full build takes longer than Hot Reload because it processes the entire codebase.
  • Best Use Case:
    • After significant changes (e.g., adding new classes, modifying headers, or including new files).
    • When the project needs a clean build to ensure stability.

2. Hot Reloading

  • Definition: Hot Reloading allows you to recompile only the modified C++ code and apply those changes to the running Unreal Editor without restarting it.
  • When it Happens:
    • Automatically triggered when you press Ctrl + Alt + F11 in Visual Studio (default key for build and reload).
    • Triggered manually via the Compile button in the Unreal Editor.
  • Characteristics:
    • Incremental Compilation:
      • Only recompiles the modified source files and integrates them into the existing binaries.
    • No Restart Needed:
      • The editor remains running, and the new changes are applied directly to the running instance.
    • Faster:
      • Quicker than a full build since it processes only the updated files.
    • Not Always Reliable:
      • May fail if you’ve made significant changes like:
        • Adding new UCLASS, USTRUCT, or UENUM.
        • Modifying header files significantly.
        • Altering certain macros or preprocessor directives.
  • Best Use Case:
    • For small, incremental changes (e.g., logic changes, fixing a bug, or tweaking values).
    • For quick iteration without disrupting the editor’s state.

Key Differences

Feature Building a Project Hot Reloading
Scope Full project compilation. Only modified files are recompiled.
Speed Slower. Faster for small changes.
Editor Restart Requires restarting or reloading editor. No restart needed; integrates on the fly.
Reliability Stable for all changes. Limited; may fail for certain changes.
Changes Supported All changes, including new files/classes. Small changes (e.g., function logic).
When to Use After major changes or a clean build. For minor tweaks or quick iterations.

How to Decide Which to Use

  • Use Building:
    • When adding new C++ classes, structs, or enums.
    • After modifying a .h file with significant structural changes.
    • When debugging issues that might be related to stale binaries.
  • Use Hot Reload:
    • For quick iterations on gameplay logic or bug fixes in .cpp files.
    • When tweaking code that doesn’t require header changes.

Tips to Improve Workflow

  1. Combine Both:
    • Use Hot Reload for rapid iteration.
    • Do a full build periodically to ensure the project is clean and stable.
  2. Enable Live Coding:
    • Unreal Engine’s Live Coding feature enhances Hot Reload by making it more robust.
    • Enable it in Editor Preferences > General > Miscellaneous > Live Coding.
    • Use Ctrl + Alt + F11 to trigger Live Coding in Visual Studio.

Would you like guidance on setting up Live Coding or resolving specific Hot Reload issues?