Unreal Engine C++ Fundamentals – Structs

Today we are going to take a little break from our player character series and go over a few Unreal Engine C++ Fundamentals. This is going to be a new side series of videos / helpful material that should give you insight into various common Unreal data types, functions, macros, etc etc.

We may even dive into some more basic concepts just around c++ and object oriented programming like loops, conditionals and inheritance.

Lots of stuff for us to play with but let’s kick this off with Structs !

What is a struct ?

A struct is a data structure made up of other data structures …. yes this all makes sense now ….

If you look closely enough though, you have seen them and most likely worked with them already, perhaps in the form of a FVector aka struct FVectoror a FRotator aka struct FRotator.

So why do I care about structs when I have classes ?

Structs allow you to have containers for your object definition without having necessarily carrying the burden of new class definitions and instantiations.

A  class tends to contain a lot more logic, it may carry more data around in it self, it may be used for complex inheritance and it has it’s on constructor / destructor life cycle.

A  struct ends up being much smaller in definition, it tends to carry less data around, it performs less logic and tends not to have various accessors that classes do.

Structs also end up being member variables of a class in order to organize and group certain sets of properties together.

Outside of those small differences they are pretty much the same.

Both allow for member variables, both allow for classes or structs to be nested as member variables and they also allow inheritance.

What about UCLASSand USTRUCTsince you know … Unreal C++ !?

UCLASSand  USTRUCTare pretty much identical too !

UCLASShave their own initialization life cycle and come with constructors and destructors handled by the garbage collection in Unreal. They also allow for variable definition, method signatures, etc etc.

USTRUCT is pretty much a C++ struct but it comes with member reflection. So you can do things like break the FRotator in your Blueprint. Where as this process is a bit more involved with a UCLASS because of how access to member variables is setup.

POINT OF NOTE:

USTRUCTsare not handled by garbage collection so it’s up to the developer to make sure that USTRUCTsbelong to objects, like UObject for example, that will be picked up and destroyed by Unreals garbage collection.

Well if they are so great how do we use them ?

In the realm of C++ a struct is really the same thing as a class, except for a few syntactical differences.

For example structs in C++ default their member variables to public by default while classes have private variables by default.

C++ Struct

struct Character {
    int speed; // speed is public
};

C++ Class

class Character {
    int speed; // speed is private
};

Now how about doing the same in Unreal ?

Below is a basic definition of a UCLASSwhere we define the member variable of Running.

// Header
UCLASS()
class YOURMODULE_API APlayerCharacter: public ACharacter
{
  GENERATED_BODY()

public:
  UFUNCTION(BlueprintCallable, Category="Player")
  bool IsRunning();

private:
        bool Running = false;
};



// Source
bool APlayerCharacter::IsRunning()
{
    return Running;
}

Now let’s build a USTRUCTthat also implements the Running variable.

Since USTRUCTsdon’t require their own class file definitions we can simply put our struct into any accessible header file. Ideally you put structs in their contextually appropriate classes or in meaningful header files ( example: GameStructs.h ).

So let’s re-write the example above using a USTRUCT.

// Header

//Use USTRUCT(BlueprintType) if you would like to include your Struct in Blueprints
USTRUCT()
struct FPlayerStats
{
  GENERATED_BODY()

  // Use UPROPERTY() to decorate member variables as they allow for easier integration with network replication as well as potential garbage collection processing
  UPROPERTY()
  bool Running;	

  FPlayerStats
  {
     Running = false;
  }
};


UCLASS()
class YOURMODULE_API APlayerCharacter: public ACharacter
{
  GENERATED_BODY()

public:
  UFUNCTION(BlueprintCallable, Category="Player")
  bool IsRunning();

private:
  // member variable of FPlayerStats
  FPlayerStats PlayerStats;
};



// Source
bool APlayerCharacter::IsRunning()
{
    return PlayerStats.Running;
}

As you can see it’s pretty easy to convert properties or parts of properties into Structs.

Here are a few helpful links if you want to do more reading about Structs

• C++ Structs
• UClass
• UStruct
• Struct in C Programming
• Structs,_USTRUCTS(),_They’re_Awesome
• Garbage Collection Overview