include

Some notes on defining classes with mutual references to instances of each other:

The classes can’t hold the actual other class’s instance data, because that could potentially cause an infinite expansion memory allocation, so by definition, the classes should be defined to contain pointers to the instance of the other class.
The classes C++ header files can’t be including each other (it will cause a compile error), because, this is literally telling the compiler to endlessly read the files, each one including the other.
So only one class header can have an “include” statement pointing to the other.
In order to successfully compile the class header file which doesn’t contain an “include” statement for the other class, a forward declaration of the other class is needed.
I.E. a ‘ghost’ declaration of the class at the beginning of the file, just so the compiler ‘knows’ there is such an entity, and doesn’t throw an error.

Class A code:

#include "B.h"

class A
{
    B* TheB;
};

Class B code:

class A;

class B
{
    A* TheA;
};

Hope this was helpful!
If you find mistakes or inaccuracies in this post I’ll be very grateful of you share them with me in the comments.

Software:
Unreal Engine 4.25
*Also tested on Unreal Engine 5.0.1

Disclaimer:
I’m probably the 10th guy that’s documenting these steps on the web,
I didn’t come up with this solution myself, I learned it from the sources listed below.
The reason I’m documenting this (again) myself is to have a clear source I can come back to for this issue because I’m absolutely incapable of remembering subjects like this…… :-\
If you find inaccuracies in the steps I’m detailing or my explanation, I’ll be very grateful if you share a comment.

In short:
AFAIK since version 4.21 UE doesn’t load custom node shader code from your project/Shaders folder by default anymore, but only from the shaders folder in the engine installation, which makes it less practical for developing shaders for specific projects.

Steps for setting the UE project to load shaders from the project folder in UE 4.22:

> The examples here are for a project named: “Tech_Lab”

A. The project must be a C++ project:

So either create a new project, define as such or just create a new C++ class and compile the project with it to convert it to a C++ project.
Notes:
a. You may need to right click the .uproject file icon and and Generate Visual Studio Project Files for the project to load correctly into Visual Studio and compile.
b. You can delete the unneeded C++ class you added after the new settings took place.

B. Create a folder for the shader files:

Typically, it will be called “Shaders” and will be placed in the project root folder.

C. Add the RenderCore module to the project:

This is done by adding string “RenderCore” to array of public dependency modules in the <project>.build.cs file:

PublicDependencyModuleNames.AddRange(new string[] { "RenderCore", "Core", "CoreUObject", "Engine", "InputCore" });
(see image)

Notes:
a. In UE 4.21 it should be ShaderCore.
b. This addition is needed in-order to compile a new primary project module (next step).

D. Define a custom primary module for your project:

In <project_name>.h file add a new module named F<project_name>Module, with a StartupModule function overrides.
Notes:
a. We have add an include statement for “Modules/ModuleManager”.
b. The <project_name>.h file is located in the /Source/<project_name> folder.
c. Some sources state that you also have to override the ShutdownModule function, with an empty override, it works for me without this (maybe its just a mistake..)

E. Implement the function override,
and set the custom module as the project primary module:

In <project_name>.cpp file, add the StartupModule override,
With the definition of the added shaders path:
FString ShaderDirectory = FPaths::Combine(FPaths::ProjectDir(), TEXT("Shaders"));

and mapping this new path as “/Project” for conveniently including it:
AddShaderSourceDirectoryMapping("/Project", ShaderDirectory);

Last thing to do is to replace “FDefaultGameModuleImpl” with our custom module name in the IMPLEMENT_PRIMARY_GAME_MODULE macro:
IMPLEMENT_PRIMARY_GAME_MODULE(FTech_LabModule, Tech_Lab, "Tech_Lab" );

Notes:
a. We must include “Misc/Paths”
b. Note that the addition of this folder mapping is restricted to versions 4.22 and higher via a compiler directive condition. for version 4.21, you should state “ENGINE_MINOR_VERSION >= 21:

Note for UE5:
Unreal Engine 5 supports this from the get go so this compiler directive condition should be deleted for this to work.

F. Wrapping up:

After taking these steps and compiling the project.
You should be able to include .ush and .usf files stored in <your_ue_project>/Shaders with the “Project” path mapping:
include "/Project/test.usf"

That’s it! 🙂

I hope you found this helpful,
And if you encountered errors, or inaccuracies,
I’ll be grateful if you’ll take the time to comment.

Related:

C++ – Mutually referencing classes

UE4 – Loading shaders from within the project folder