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But there is more! Everything we have said here about Blueprint Classes also applies to C++ Classes, and everything is equally true of Actors and Components. These systems are designed to support large-scale development of extensible functionality, and can scale to projects with 10's or 100's of developers.

Using Blueprint Scripting, C++, or Both

Blueprint Visual Scripting is ideal for simple in-game logic flow and sequencing of actions. It is a great system for designers, artists, and visually-oriented programmers, because it is easy to access and control in-game objects visually. You can even create small standalone-game using just Blueprints; see the Tappy Chicken sample for a full-fledged example.

C++ programming is for larger-scale tasks, such as building gameplay systems, complex AI, and new engine features. If you already have some C++ experience, check out the Introduction to C++ Programming in UE4 page.

Most projects will use a mix of Blueprints and C++. Many developers prototype game functionality using Blueprints, because it is so easy and fun, and later move some or all of it to C++ for performance and engineering rigor.

Blueprint Classes Can Extend C++ Classes

Much of the magic of UE4 game development comes from the interplay between programmers implementing new features in C++, with designers and artists utilizing them in Blueprints and then asking for more! Here's how a team might structure a UE4-based shooter game implementing pickups using a mix of C++ Classes systems programming, and Blueprint Classes for behavior and appearance:

Transform Components

In Unity each GameObject has a Transform component, which gives the GameObject a position, rotation, and scale in the world.

Similarly in UE4, Actors have a Root Component, which can be any subclass of Scene Component. A Scene Component gives the Actor a location, rotation, and scale in the world which is applied hierarchically to all components underneath it. Many of the components that you will use are subclassed from Scene Component, because it is really useful to have a position!

Even if you place an Empty Actor, UE4 will create a 'Default Scene Root' for that Actor, which is just a plain Scene Component. If you drop a new Scene component in of your own, it will replace the Default Scene Root.

Compound Objects

In Unity, you create compound objects by constructing a hierarchy of GameObjects and parenting their transforms together:

In UE4, you create compound game objects by nesting components hierarchically:

As you can see from the diagram, nested hierarchies can be created by attaching Scene Components to one another, since they have a transform - similar to parenting transforms in Unity. Actor Components (the base-class for all components) can only be attached directly to the Actor itself.

Do I build everything out of Components?

Its really up to you, but most often you will use a combination of custom component types along with Actor classes that use those components. We mentioned this earlier, but UE4 has many special types of Actors that guarantee a certain level of capability and always include certain components. For example, a Character always contains a Character Movement Component.

There are several sub-classed Actor types you will quickly encounter in the engine and are useful in almost every type of game. Here is a list of our most common built-in Actors:

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• Pawn - A type of Actor representing a controllable game object, typically the player's avatar. Pawns are moved by Players and AI alike through an owning Controller.

• Character - A more specialized version of Pawn designed for biped avatars that handle a lot of the complexity of those kinds of game objects.

• Controller - Possesses and controls a Pawn. By separating the Pawn from the controller, you can write AI Controllers that can manipulate a pawn using the same interface the player would.

• Player Controller - A more specialized Controller designed for getting input from a player's Gamepad, Touch, or Mouse/Keyboard, and using that input to drive the Pawn or Character they possess.

So is everything an Actor?

Not everything. Actors are most common class used for gameplay in UE4 and the only type that can be Spawned into the World. So everything you place in your level will be an Actor.

The other important type to know about is Object. Object is actually the base class of all Unreal classes, including Actor and many others. This is a much lower-level construct than Actor, but still has features that you will expect from a Unreal class, such as Reflection and Serialization. Object is a very basic class we use when we need to define a new type that just does not fit the mold of an Actor. For example, Actor Component is the base class for all Components, and derives from Object instead of Actor.

What is this Gameplay Framework thing in UE4?

Okay, so this is where things get a little crazy (in sort of an awesome way.) Unity gives you a clean slate to start designing your game, and Unreal does the same. In Unity you can build everything out of basic GameObjects and components, and in Unreal you can build everything out of Actors and components.

However Unreal has this extra layer on top called the Gameplay Framework. This does not really exist in Unity at all. You do not have to use it in Unreal, but it is actually super cool! Basically, if you make use of some basic primitive classes and follow certain established conventions, your game will acquire some awesome features automatically that would be otherwise very difficult to implement or retrofit (such as full multiplayer support!)

Countless awesome games have been designed on top of Unreal's Gameplay Framework, and it is worth taking some time to understand how it works. Yes, you could roll your own version of this. That is totally fine if you want to! But hundreds of awesome Unreal developers have made great use of this framework in UE4 today, so it is worth taking some time to learn.

To use the Gameplay Framework, you really just need to learn the customized built-in Actor classes included with Unreal, such as Pawn, Character, and Player Controller, and eventually learn about how Unreal replication and networking features work. For now, let us get back to the basics though.

How to Write Code in UE4

For MonoDevelop Programmers

For Blueprint scripting, you will only need Unreal Editor -- everything is built in! To write code in C++, download the free version of Visual Studio on Windows, or install Xcode on Mac. When you first create a new project (or add code to an existing project), UE4 will automatically create Visual Studio project files for you. You can open Visual Studio by Double-clicking on a C++ class inside Content Browser, or by clicking the Open Visual Studio button in the main File menu.

One important difference in UE4: You will sometimes have to manually refresh your Visual Studio project files (for example, after downloading a new version of UE4, or when manually making changes to source file locations on disk.) You can do this by clicking Refresh Visual Studio Project in the main menu, or by Right-clicking on the .uproject file in your project's directory and selecting Generate Visual Studio project files.

Writing Event Functions (Start, Update, etc.)

If you are used to working with MonoBehaviors you are used to such methods as Start, Update, and OnDestroy. Here is a comparison between a Unity behavior and its counterpart in UE4 Actors and components.

In Unity, we might have a simple component that looks like this:

But remember, in UE4 you can write code right on the Actor itself rather than only coding new component types. This is actually very common and useful.

Similar to Unity's Start, OnDestroy, and Update functions, we have a similar set of methods for Actors in UE4:

C++ :

Blueprint :

Components in UE4 contain different functions. Here is a simple example:

C++ :

Blueprint :

Remember, in UE4 it is important to call the parent class' version of method.

For example, in Unity C# this would be called base.Update(), but in UE4 C++ we will use Super::TickComponent():

You may have noticed some things begin with 'A' and others with 'U' in C++. The prefix 'A' indicates an Actor sub-class. Where as the prefix 'U' indicates an Object sub-class. There are some other prefixes too, for example 'F' is used for most plain data structures or non-UObject classes.

Writing Gameplay Code in UE4

Okay, things are going to get a bit deeper from here on out. We will be talking about programming topics that are critical for creating games. Because you know Unity, we will be explaining features with slant toward C# users learning Unreal C++, but you can use Blueprint Scripting for pretty much everything if you want! We have added examples in both C++ and Blueprint where possible.

Let us talk about some of the most common gameplay programming patterns and how you would approach them in Unreal. Many functions in Unity have a comparable function in Unreal that should seem familiar. We will go through the most common functions one by one.

Instantiating GameObject / Spawning Actor

In Unity, we use the Instantiate function to create new instances of objects.

This function takes any UnityEngine.Object type (GameObject, MonoBehaviour, etc.), and makes a copy of it.

In UE4, there are a couple different functions to instantiate objects, depending on your needs. NewObject is used for creating new UObject types, and SpawnActor is used for spawning AActor types.

First we will briefly talk about UObjects and NewObject. Subclassing UObject in Unreal is much like subclassing ScriptableObject in Unity. They are useful for gameplay classes that do not need to spawn into the world or have attached components like Actors do.

In Unity, if you created your own subclass of ScriptableObject, you may instantiate it like this:

And in Unreal, if you created your own UObject derived type, you may instantiate it like this:

So what about Actors? Actors are spawned using the SpawnActor method on a World (UWorld in C++) object. How do you get the World object? Some UObjects provide a GetWorld method for you, as an example, all Actors do.

You will notice instead of passing in another Actor, we pass in the 'class' of the Actor we want to spawn. In our example, the class can be any subclass of AMyEnemy.

But what if you want to make a 'copy' of another object, like what Instantiate allows you to do?

The NewObject and SpawnActor functions can also be given a 'template' object to work with. Unreal will make a copy of that object, instead of making one 'from scratch'. This will copy over all of its UPROPERTYs and components.

You might be wondering what 'from scratch' means in this context. Each object class you create has a default template that contain default values for its properties and components. If you do not override these properties and do not provide your own template, Unreal will use these default values to construct your object. To help illustrate this, let us first look at a MonoBehaviour for example:

In the above example, we have an int property that defaults to 42, and a SphereCollider component that defaults to a radius of 20.

We can achieve the same thing in Unreal using the object's constructor.

In the constructor of AMyActor, we have set the default property values for the class. Note the use of the CreateDefaultSubobject function. We can use it to create components and assign default properties to them. All the subobjects we create using this function act as a default template, so we can modify them in a subclass or Blueprint.

Casting from One Type to Another

In this case, we get a component we know we have, then cast it to a specific type and conditionally do something.

Unity C#:

UE4 C++:

Destroying GameObject / Actor

RayCast vs RayTrace

Unity C#:

UE4 C++:

UE4 Blueprint:

Click for full view.

Triggers

Unity C#:

UE4 C++:

UE4 Blueprint:

You can read more about setting up collision responses here.

Kinematic Rigidbodies

Unity C#:

In UE4, The collision component and rigidbody component are one. The base class for this is UPrimitiveComponent, which has many subclasses (USphereComponent, UCapsuleComponent, etc.) to suit your needs.

UE4 C++:

Input events

Unity C#:

UE4 C++:

UE4 Blueprint:

This is what your input properties in your Project Settings might look like:

You can read more about how to set up input here.

FAQ

How do I load my last project automatically?

If you are used to Unity automatically loading the last project you were working on, the same is possible in UE4. To enable, check 'Always load last project on Startup' when opening a project. You can also toggle the setting at any time from the main Edit menu under Edit/Editor Preferences/Loading and Saving/Startup.

Where do I set Input Bindings for my game?

In Unity, you are used to using the Input Manager settings for Project to setup default bindings. In UE4 it works similarly. You will open your Project Settings then select the Input category. There, you can add various buttons (actions) and analog controls (axes). Give each control a name and default binding. Then, you can get callbacks to your game's Pawn when the input events are triggered. Check out the Input documentation page for the details.

How do I change the starting scene of my project?

You can change the startup map of your project from the project settings tab. From the main menu choose Edit/Project Settings->Maps & Modes to change the starting map.

How do I run my game?

The easiest way to run your game is to click the 'Play' button on the the main editor toolbar, which will run the game right inside the editor's process. If you want to run it as a standalone application, click the dropdown arrow next to 'Play' and choose 'Standalone Game'. Finally, if you want to run on a mobile device or in a web browser, you will use the 'Launch' button on the toolbar (after installing any prerequisites that are needed for that platform.)

What units are these?

In Unity, the primary unit of measurement is one meter. In UE4, the primary unit of measurement is one centimeter.

So if you move something 1 unit (meter) in unity that is equivalent to moving something 100 units (centimeters) in UE4.

If you want to move something 2 Feet in Unity that would be 0.61units (meter) and in UE4 is 61 units (centimeters).

What is with this coordinate system? Which way is up?

Both Unity and UE4 use a left-handed coordinate system, but the axes are swapped around. In UE4, positive X is 'forward', positive Y is 'right' and positive Z is 'up'.

How do I see Log Output from my game?

In the UE4 editor, you can open the 'Output Log' from the 'Window -> Developer Tools' menu. You can also run your game with the '-log' command-line parameter to summon a dedicated log window alongside your game, which is really useful!

Speaking of Log Output, where is my Debug.Log?

Logging in UE4 is highly customizable. Read up on how to log messages here.

How do I throw exceptions?

In Unity, you are used to throwing exceptions when things go wrong. UE4 does not use exception handling. Instead, use the 'check()' function to trigger a critical assertion error. You can pass in an error message. If you want to report an error but not halt the program, use 'ensure()' instead. This will log an error with a full call stack, but program execution will continue. If you had a debugger attached, both functions will break into the debugger.

Where is the .NET Framework?

Unlike Unity, UE4 does not use the .NET framework. UE4 has its own set of container classes and libraries. Common container comparisons:

Where To Go From Here

Thanks for reading the guide! This was created for the Unreal community with the help of Unreal developers everywhere, and we very much appreciate any feedback and corrections you can offer. We will try to keep improving this document as we learn more about what is most helpful when transitioning to UE4!

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