# Behavior Tree Service in C++

**URL:** <https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626>\
**Category:** AI\
**Tags:** Behavior-Tree, service, unreal-engine, CPP, question\
**Created:** [March 26, 2015, 1:39am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626 "2015-03-26T01:39:40Z")\
**Posts on this page:** 9\
**Page:** 1

<div class="post-metadata">

**Author:** ![eXi](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/exi/32/906583_2.png) [@eXi](https://forums.unrealengine.com/u/eXi)\
**Post date:** [March 26, 2015, 1:39am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626/1 "2015-03-26T01:39:40Z")

</div>

Hey there,

i’m currently creating some VERY basic AI in C++. I only want to use the Behavior Tree, but create the rest in C++.  
I got everything working except the service. In the service, i’m calling the “SearchEnemy()” function that searches and sets the next enemy. This works if i create the logic in Blueprints. If i do the same in the C++ version of the Service, the service shows up with “No Tick” in the Tree.

First one is C++, second one is BP:

[http://puu.sh/gPNlM/956f9dfc49.png](http://puu.sh/gPNlM/956f9dfc49.png)

Here is my C++ code for the service:

Header.h

```
// Fill out your copyright notice in the Description page of Project Settings.

#pragma once

#include "BehaviorTree/Services/BTService_BlueprintBase.h"
#include "MyHeaders.h"
#include "MeeleFollowEnemyService.generated.h"

/**
 * 
 */
UCLASS(BlueprintType, Blueprintable)
class SMALLCOOPTD_API UMeeleFollowEnemyService : public UBTService_BlueprintBase
{
	GENERATED_BODY()

public:

	UMeeleFollowEnemyService(const FObjectInitializer& ObjectInitializer);

	virtual void ReceiveTick(AActor* OwnerActor, float DeltaSeconds) override;
	
};

```

CPP:

```
// Fill out your copyright notice in the Description page of Project Settings.

#include "SmallCoopTD.h"
#include "MeeleFollowEnemyService.h"

UMeeleFollowEnemyService::UMeeleFollowEnemyService(const FObjectInitializer& ObjectInitializer)
	: Super(ObjectInitializer)
{
	
}

void UMeeleFollowEnemyService::ReceiveTick(AActor* OwnerActor, float DeltaSeconds)
{
	ABaseAIController* BotController = Cast<ABaseAIController>(OwnerActor);

	if (BotController)
	{
		BotController->SearchEnemy();
	}
}

```

Same thing in BP:

 ![](https://d3kjluh73b9h9o.cloudfront.net/original/4X/8/3/5/835ee3ebce1b384bbb6732eae5fb1968965a2061.png)

So what exactly am i missing here? Do i need to create services in BP? I don’t think so x)

I can’t find any information on how to setup a service in C++. Do i miss something to let that service tick?

---

<div class="post-metadata">

**Author:** ![Duncan\_Dam](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Duncan\_Dam](https://forums.unrealengine.com/u/Duncan_Dam)\
**Post date:** [March 26, 2015, 7:10am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626/2 "2015-03-26T07:10:03Z")

</div>

This what i found from source code ReceiveTick will be call here

```
else if (ReceiveTickImplementations & FBTNodeBPImplementationHelper::Generic)
{
	ReceiveTick(ActorOwner, DeltaSeconds);
}

```

and the ReceiveTickImplementations only valid if it have the blueprint function name ReceiveTick in blueprint.

```
ReceiveTickImplementations = FBTNodeBPImplementationHelper::CheckEventImplementationVersion(TEXT("ReceiveTick"), TEXT("ReceiveTickAI"), this, StopAtClass);

```

and

```
int32 CheckEventImplementationVersion(FName GenericEventName, FName AIEventName, const UObject* Ob, const UClass* StopAtClass)
	{
		const bool bGeneric = BlueprintNodeHelpers::HasBlueprintFunction(GenericEventName, Ob, StopAtClass);
		const bool bAI = BlueprintNodeHelpers::HasBlueprintFunction(AIEventName, Ob, StopAtClass);
		
		return (bGeneric ? Generic : NoImplementation) | (bAI ? AISpecific : NoImplementation);
	}

```

Seem like ReceiveTick must be a blueprintfunction to able to execute. Maybe u can try put UFUNCTION() on the ReceiveTick, and it will consider that as BlueprintFuction ?

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<div class="post-metadata">

**Author:** ![eXi](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/exi/32/906583_2.png) [@eXi](https://forums.unrealengine.com/u/eXi)\
**Post date:** [March 26, 2015, 10:42am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626/3 "2015-03-26T10:42:32Z")

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Hm ok,

i will have a look at it. The API says it is a BlueprintImplementableEvent. This means i CAN be overriden through a Blueprint version. But i still don’t understand why this Service can’t be created in C++.

I can even choose the C++ file (no BP child) in the Service list. But like i showed, i gives the no tick error.

Let’s see if adding UFUNCTION or just create a child BP class will fix something.

Thanks so far (:

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<div class="post-metadata">

**Author:** ![eXi](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/exi/32/906583_2.png) [@eXi](https://forums.unrealengine.com/u/eXi)\
**Post date:** [March 26, 2015, 10:45am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626/4 "2015-03-26T10:45:13Z")

</div>

Ok, i forgot, an override function can’t have a UFUNCTION() makro. It will just use the original superclass properties. 😕

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<div class="post-metadata">

**Author:** ![eXi](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/exi/32/906583_2.png) [@eXi](https://forums.unrealengine.com/u/eXi)\
**Post date:** [March 26, 2015, 11:03am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626/5 "2015-03-26T11:03:06Z")

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Ok, thanks Duncan Dam.

It just created a BP of my custom Service and added a blanc Event Receive Tick that does nothing. Now the Service works and calls the C++ version.

I don’t understand why this is necessary, but i guess that has some inner logic.

Maybe our AI Lord @MieszkoZ can add some information on this.

Thanks for your help Duncan. For now it is working (:

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<div class="post-metadata">

**Author:** ![Duncan\_Dam](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@Duncan\_Dam](https://forums.unrealengine.com/u/Duncan_Dam)\
**Post date:** [March 26, 2015, 11:06am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626/6 "2015-03-26T11:06:33Z")

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yeah, weird, i think that’s some kind of optimize for the AI, they should have some variable to toggle this.

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<div class="post-metadata">

**Author:** ![eXi](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/exi/32/906583_2.png) [@eXi](https://forums.unrealengine.com/u/eXi)\
**Post date:** [March 26, 2015, 11:48am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626/7 "2015-03-26T11:48:52Z")

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Yop, that would be a good idea. Now i’m questioning around why my AI isn’t moving around a cube, although “MoveTo” should use the NavMesh.

But that’s another problem _-_

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<div class="post-metadata">

**Author:** ![MieszkoZ](https://d3kjluh73b9h9o.cloudfront.net/original/4X/2/8/5/28502927f1d97ba1051b3b47ccfe17233bf41ef0.png) [@MieszkoZ](https://forums.unrealengine.com/u/MieszkoZ)\
**Post date:** [March 30, 2015, 9:07am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626/8 "2015-03-30T09:07:40Z")

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This is all by design. To create a C++ implementation of a service you need to derive from `UBTService` not `UBTService_` **Blueprint** `Base` 🙂 Let me know if you encounter any issues while doing so.

Cheers,

–mieszko

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<div class="post-metadata">

**Author:** ![eXi](https://d1ap1mz92jnks1.cloudfront.net/user_avatar/forums.unrealengine.com/exi/32/906583_2.png) [@eXi](https://forums.unrealengine.com/u/eXi)\
**Post date:** [March 30, 2015, 11:33am UTC](https://forums.unrealengine.com/t/behavior-tree-service-in-c/312626/9 "2015-03-30T11:33:02Z")

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Hey, thanks MieszkoZ (: I didn’t pick the UBTService, because when i looked into the API for it, i missed some function that i thought i would need.

For now i created everything in BP. I will try it with the UBTService when i come to convert this into C++. Thanks for your answer so far. I will report back when i encounter issues!