UE4 Reflection Overview

https://www.unrealengine.com/blog/unreal-property-system-reflection

The type hierarchy for the property system looks like this:

• UField
• UStruct
  • UClass (C++ class)
  • UScriptStruct (C++ struct)
  • UFunction (C++ function)
• UEnum (C++ enumeration)
• UProperty (C++ member variable or function parameter)
  • (Many subclasses for different types)

UStruct is the basic type of aggregate structures (anything that contains other members, such as a C++ class, struct, or function), and shouldn’t be confused with a C++ struct (that's UScriptStruct). UClass can contain functions or properties as their children, while UFunction and UScriptStruct are limited to just properties.

UScriptStructs

Manipulation

The main place to access a lot of manipulation helpers is through UScriptStruct::ICppStructOps. Some useful bits:

C++

UScriptStruct::ICppStructOps& StructCppOps = FES2ArchTypeEntitiesTable::StaticStruct()->GetCppStructOps();
int32 StructSize = StructCppOps->GetSize();
int32 StructAlignment = StructCppOps->GetAlignment();
void* Allocation = FMemory::Malloc(StructSize, StructAlignment);
StructCppOps->Construct(Allocation);
StructCppOps->Serialize(...)
StructCppOps->NetSerialize(...)

Initialize Struct

C++

StructMemory = (uint8*)FMemory::Malloc(ActualStruct->GetStructureSize());
ActualStruct->InitializeStruct(StructMemory);

Copy Struct

C++

if (ProcAnimComponent && OutInitValueStore)
{
     void* RowPtr = Table->FindRowUnchecked(RowName);
     if (RowPtr != NULL)
     {
         UScriptStruct* StructType = Table->RowStruct;
         if (StructType != NULL)
         {
             StructType->CopyScriptStruct(OutRowPtr, RowPtr);
             bFoundRow = true;
         }
     }
}

Destroy Struct

C++

UScriptStruct* structType = FESCompSumType::GetStructType(CompStructType);
            structType->DestroyStruct(StructData);

Reflection through Struct Params apart

C++

FMovieSceneEvalTemplatePtr& operator=(const FMovieSceneEvalTemplatePtr& RHS)
{
  if (RHS.IsValid())
  {
    UScriptStruct::ICppStructOps& StructOps = *RHS->GetScriptStruct().GetCppStructOps();

    void* Allocation = Reserve(StructOps.GetSize(), StructOps.GetAlignment());
    StructOps.Construct(Allocation);
    StructOps.Copy(Allocation, &RHS.GetValue(), 1);
  }

  return *this;
}

Reference: https://forums.unrealengine.com/community/community-content-tools-and-tutorials/27351-tutorial-how-to-accept-wildcard-structs-in-your-ufunctions

Iterate over UStruct members

To iterate over all members of a UStruct, use a TFieldIterator:

C++

for (TFieldIterator<UProperty> PropIt(GetClass()); PropIt; ++PropIt)
{
  UProperty* Property = *PropIt;
  // Do something with the property
}

Each type has a unique set of flags (EClassFlags + HasAnyClassFlags, etc…), as well as a generic metadata storage system inherited from UField.

Look up / Find a property

C++

FindField<UProperty>(Struct, VarDesc->VarName)

Export/Import text from UProperty

C++

Property->ExportText_InContainer(0, PropertyValue, RowData, RowData, nullptr, PPF_None);

Comparison

Compare UProperty Equality

If two properties are identical:

C++

UProperty::Identical( const void* A, const void* B, uint32 PortFlags=0 )
UProperty::Identical_InContainer()

Compare UStruct instances

CompareScriptStruct

Compare UStructs Type Equality

Compare if two UStructs types (not C++ structs but UE4 USTRUCT meaning class type, property, etc) are the same:

C++

FStructUtils::ArePropertiesTheSame(PropA, PropB, false)
FStructUtils::TheSameLayout(const UStruct* StructA, const UStruct* StructB, bool bCheckPropertiesNames)

Hook Into UObject/UProperty change modification delegates

[Listen on property changes and notifies/notifications:]

Special Callbacks

C++

virtual void EditorApplyTranslation(const FVector& DeltaTranslation, bool bAltDown, bool bShiftDown, bool bCtrlDown) override;
virtual void EditorApplyRotation(const FRotator& DeltaRotation, bool bAltDown, bool bShiftDown, bool bCtrlDown) override;
virtual void EditorApplyScale(const FVector& DeltaScale, const FVector* PivotLocation, bool bAltDown, bool bShiftDown, bool bCtrlDown) override;
virtual void PostEditMove(bool bFinished) override;
virtual void PostEditComponentMove(bool bFinished) override;

Export UStruct to String

C++

UScriptStruct* structClass = FBBStadCamManip::StaticStruct();
FString outStr;
structClass->ExportText(outStr, &stadCamManipVal, nullptr, nullptr, PPF_None, nullptr);

UFunctions

Introspect UFunction

Bash

LISTFUNCS
LISTFUNC - List details about a function including parameters and byte sizes

These are helpful functions to look at for examples:

C++

UFunction::IsSignatureCompatibleWith
TFieldIterator<UProperty> IteratorA(ufunc);

Check if a UFunction is latent

C++

bIsLatent = (Function->HasMetaData(FBlueprintMetadata::MD_Latent) != false);

Find No Parameters Event

C++

this->GetClass()->FindFunctionByName("ReceiveBeginPlay", EIncludeSuperFlag::ExcludeSuper)
UFunction* EventTarget = this->FindFunction(EventName);
if( EventTarget && EventTarget->NumParms == 0)
{
  LSA->ProcessEvent(EventTarget, NULL);
  bFoundEvent = true;
}

UFunction Parameters

Calling UFunction Through Reflection

https://answers.unrealengine.com/questions/7732/a-sample-for-calling-a-ufunction-with-reflection.html

Compare UFunctions Signatures

C++

UFunction::IsSignatureCompatibleWith(const UFunction* OtherFunction, uint64 IgnoreFlags) const

Iterate UFunction Parameters

C++

for (TFieldIterator<UFunction> FunctionIt(this->GetClass(), EFieldIteratorFlags::ExcludeSuper); FunctionIt; ++FunctionIt)
{
  UFunction* Function = *FunctionIt;
}

Retrieve UFunction Parameters Default Value

Hi,

This cannot be known in general, but you can check the UFunction's "CPP_Default_" metadata to see if a given parameter has a default argument, and the value of that default.

However, only default arguments of certain types and values can be parsed and, as it's metadata, this information only exists in builds with WITH_METADATA defined, which is basically editor-only.

Hope this helps,

Steve

Reference From https://answers.unrealengine.com/questions/545342/how-can-i-know-ufunction-contain-how-many-paramete.html?sort=oldest&lang=zh-CN

UClass

Iterate Class Inheritance Hierarchy

Finding a specific subclass in a class' hierarchy chain:

C++

// Construct list of non-abstract sound node classes.
for(TObjectIterator<UClass> It; It; ++It)
{
  if(It->IsChildOf(USoundNode::StaticClass())
    && !It->HasAnyClassFlags(CLASS_Abstract))
  {
    SoundNodeClasses.Add(*It);
  }
}

Find Class or UStruct by name

C++

FClass* FClasses::FindClass(const TCHAR* ClassName) const
UObject* ClassPackage = ANY_PACKAGE;
UClass* Result = FindObject<UClass>(ClassPackage, ClassName);

Reference From https://answers.unrealengine.com/questions/92651/get-blueprint-class-by-string-in-c.html

User Defined Structs

Programmatically Create UUserDefinedStruct

C++

return FStructureEditorUtils::CreateUserDefinedStruct(InParent, Name, Flags);
FStructureEditorUtils::AddVariable(StructureDetailsSP->GetUserDefinedStruct(), InitialPinType);

Change UUserDefinedStruct's Parent Struct

C++

FStructureEditorUtils::CreateUserDefinedStruct():
  ((UUserDefinedStructEditorData*)(Struct->EditorData))->NativeBase = FNativeBaseS::StaticStruct();

Programmatically Construct Struct/UScriptStruct

C++

template<typename T>
T ConstructTInlineValue(UScriptStruct* Struct)
static void SetStructurePropertyByName(UObject* Object, FName PropertyName, const T& Value)
UKismetSystemLibrary::Generic_SetStructurePropertyByName(Object, PropertyName, &Value);