About UHT generated files and U macros

Introduction

When you need to use U macros in your c++ file you need to include the xxxx.generated.h header file and add GENERATED_BODY() inside the class/struct definition.

Why include xxx.generated.h ?

When you compile your c++ code, UHT (Unreal Header Tool) will generate some code from the U macros (UCLASS, UPROPERTY, UFUNCTION, …). This code is written inside the .generated.h header file. A second file (.gen.cpp) will include it.

Why GENERATED_BODY() ?

It seems that this macro is a marker for UHT, so it knows where to paste some code inside the class/struct.

To put it simply

When you press compile, the following happens:

1. UHT creates .generated.h and .generated.cpp files when needed.
2. U macros are used as markers for UHT or replaced with pasted text.
3. Since the .generated.h file is included in your regular .h file the C++ compiler will compile your regular files with the UHT generated files.

Issues

UHT is a dumb text parser which means it can’t resolve C++ pre-processors.