treehouse/content/programming/technologies/unreal-engine/generated-body.tree

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%% title = "how does GENERATED_BODY() work?"
% id = "01HV1DGFHMD7KNNX3FWS6R11SF"
- the `UCLASS()`, `USTRUCT()`, and other reflection macros - but especially the `GENERATED_BODY()` macro might seem a bit magical the first time you see them.
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- the thing about it is that it defies the usual rules of how C++ macros work.
let's have a look at it together.
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- _what does it even expand to?_
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- 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.
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- 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?
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- and `StaticClass()` returns a different value for each class. this would require _knowing_ the class beforehand - how does it know?
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- the thing is - it doesn't. `GENERATED_BODY()` by itself is incredibly stupid:
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```cpp
#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);
```
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let's disassemble it piece by piece.
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- `BODY_MACRO_COMBINE` is just a macro combining four identifiers together. no magic here.
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- so `GENERATED_BODY` combines the identifiers `CURRENT_FILE_ID`, `_`, `__LINE__`, and `_GENERATED_BODY`.
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+ `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`.
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- 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:
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```
Engine/Source/Runtime/Engine/Classes/Engine/Blueprint.h
```
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the file ID is:
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```
FID_Engine_Source_Runtime_Engine_Classes_Engine_Blueprint_h
```
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I haven't inspected the UnrealBuildTool/UnrealHeaderTool sources though, so there may be more to it.
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- `_` is just an underscore. nothing magical here.
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- `__LINE__` is a standard C++ macro which expands to the current line number.
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- and `_GENERATED_BODY` is just an identifier.
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- therefore for a simple file, let's call it `MyClass.h`:
```cpp
#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:
```cpp
// -- snip --
UCLASS()
class UMyClass : public UObject
{
MyClass_h_10_GENERATED_BODY
};
```
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- 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.
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- 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.)
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- that's all there is to it.
incredibly simple, cursed as heck, yet super effective.