how ue4 looks for header files?

I write in my cpp file

#include “Components/ActorComponent.h”

But really, the ActorComponent.h file placed at /UE4/Source/Runtime/Engine/Classes/Components/ActorComponent.h

I understand that “Engine” module placed in the folder /UE4/Source/Runtime/Engine, so ok, if i use “Engine” module, then i can use relative path. But it should be “Classes/Components/ActorComponent.h” (not just a “Components/ActorComponent.h”)

How does VS understand that Components/ActorComponent.h should be searched inside of “Classes” subdirectory? Where is it specified?

The same question with includes of modules. For example FoliageModule.cpp exists in directory Foliage/Private. FoliageModule.h exists in directory Foliage/Public
But .cpp uses header without any directory structure. As if both files placed in the same directory:
#include “FoliageModule.h”

The Build.cs file of that Module declared that “Classes” directory or the path to that “Public” folder as a public include path.

So anything inside those folders will be added to the header search when the build tool is compiling the project.

That’s why “Components/ActorComponent.h” is a valid path. “…/Classes/” is appended to it by the build tool because it’s a public key path (look into Build.cs files for examples).

I thought so too, but i`ve checked file “” (ctrl+f => Classes), but there is no mention of “classes” inside.

The UnrealBuildTool does all that and simply adds the directories to the Project Include Paths.

From UEBuildModuleCPP.cs:

        private void AddDefaultIncludePaths()
            // Add the module's parent directory to the public include paths, so other modules may include headers from it explicitly.

            // Add the base directory to the legacy include paths.

            // Add the 'classes' directory, if it exists
            DirectoryReference ClassesDirectory = DirectoryReference.Combine(ModuleDirectory, "Classes");
            if (DirectoryLookupCache.DirectoryExists(ClassesDirectory))

            // Add all the public directories
            DirectoryReference PublicDirectory = DirectoryReference.Combine(ModuleDirectory, "Public");
            if (DirectoryLookupCache.DirectoryExists(PublicDirectory))

                ReadOnlyHashSet<string> ExcludeNames = UEBuildPlatform.GetBuildPlatform(Rules.Target.Platform).GetExcludedFolderNames();
                EnumerateLegacyIncludePaths(DirectoryItem.GetItemByDirectoryReference(PublicDirectory), ExcludeNames, LegacyPublicIncludePaths);

            // Add the base private directory for this module
            DirectoryReference PrivateDirectory = DirectoryReference.Combine(ModuleDirectory, "Private");

For QT Creator, the information is stored in file UE4Headers.pri (of course generated by UBT) with the format

    "$$unrealRootPath/Engine/Source/Programs/BlankProgram/Private/BlankProgram.h" \
    "$$unrealRootPath/Engine/Source/Programs/BuildPatchTool/Public/BuildPatchTool.h" \
    "$$unrealRootPath/Engine/Source/Programs/BuildPatchTool/Public/Interfaces/ToolMode.h" \
    "$$unrealRootPath/Engine/Source/Programs/BuildPatchTool/Public/ToolModes/AutomationMode.h" \
    "$$unrealRootPath/Engine/Source/Programs/BuildPatchTool/Public/ToolModes/ChunkDeltaOptimiseMode.h" \
    "$$unrealRootPath/Engine/Source/Programs/BuildPatchTool/Public/ToolModes/CompactifyMode.h" \
    "$$unrealRootPath/Engine/Source/Programs/BuildPatchTool/Public/ToolModes/DiffManifestMode.h" \
    "$$unrealRootPath/Engine/Source/Programs/BuildPatchTool/Public/ToolModes/EnumerationMode.h" \
    "$$unrealRootPath/Engine/Source/Programs/CrashReportClient/Private/Linux/LinuxErrorReport.h" \

which tells it where to look for the header files. I have no clue about the “relative path” scheme however.

Built-in engine modules are already set by default.
When creating custom modules you usually have to declare them to be set in the list.

Thanks for answers!