how does GENERATED_BODY() work?

the UCLASS(), USTRUCT(), and other reflection macros - but especially the GENERATED_BODY() macro might seem a bit magical the first time you see them.
- the thing about it is that it defies the usual rules of how C++ macros work. let’s have a look at it together.
what does it even expand to?
- try looking at what your IDE’s autocomplete suggests - from a cursory glance at the symbols available in a UCLASS()-declared class, you will notice there are a few that are non-standard ones, such as StaticClass() or Super. this is what the GENERATED_BODY() macro ends up expanding to after all the preprocessing is done.
  - but the thing is that Super is a typedef to the parent class - how does it know the parent class without passing it in as a macro argument?
  - and StaticClass() returns a different value for each class. this would require knowing the class beforehand - how does it know?
the thing is - it doesn’t. GENERATED_BODY() by itself is incredibly stupid:
```
#define BODY_MACRO_COMBINE_INNER(A,B,C,D) A##B##C##D
#define BODY_MACRO_COMBINE(A,B,C,D) BODY_MACRO_COMBINE_INNER(A,B,C,D)
#define GENERATED_BODY(...) BODY_MACRO_COMBINE(CURRENT_FILE_ID,_,__LINE__,_GENERATED_BODY);
```
let’s disassemble it piece by piece.
- BODY_MACRO_COMBINE is just a macro combining four identifiers together. no magic here.
- so GENERATED_BODY combines the identifiers CURRENT_FILE_ID, _, __LINE__, and _GENERATED_BODY.
  - CURRENT_FILE_ID is a preprocessor macro defined by the UnrealBuildTool for each file. for simplicity’s sake, let’s assume it’s the filename with dots replaced by underscores. for instance, GameplayAbility_h.
    the actual form it seems to take is FID_{Path} with {Path} being the file path relative to the project root directory, with slashes and dots replaced with underscores. for:
    
    Engine/Source/Runtime/Engine/Classes/Engine/Blueprint.h
    
    the file ID is:
    
    FID_Engine_Source_Runtime_Engine_Classes_Engine_Blueprint_h
    
    I haven’t inspected the UnrealBuildTool/UnrealHeaderTool sources though, so there may be more to it.
  - _ is just an underscore. nothing magical here.
  - __LINE__ is a standard C++ macro which expands to the current line number.
  - and _GENERATED_BODY is just an identifier.
- therefore for a simple file, let’s call it MyClass.h:
  
  #pragma once #include "UObject/Object.h" #include "MyClass.generated.h" UCLASS() class UMyClass : public UObject { GENERATED_BODY() };
  
  after expanding the GENERATED_BODY(), we’ll get this:
  
  // -- snip -- UCLASS() class UMyClass : public UObject { MyClass_h_10_GENERATED_BODY };
  - and this identifier is declared as a macro in the UnrealHeaderTool-generated MyClass.generated.h - and expands to a bunch of declarations, including the declaration of Super and StaticClass, as well as constructors if they’re not already declared.
    you can even inspect the source code of .generated.h files yourself, by <kbd>Ctrl</kbd>+clicking on them (at least in Rider. I haven’t tested Visual Studio.)
that’s all there is to it. incredibly simple, cursed as heck, yet super effective.