Unreal Engine 5 C++ Actor Disappearing On Closing A Guide To Persistence

When diving into the world of Unreal Engine 5 (UE5), developers often leverage the power of C++ to create actors that drive gameplay, interactions, and visual elements within their projects. However, a common challenge arises when actors, particularly those created dynamically or as part of specific systems like the Cesium for Unreal Engine plugin, are unexpectedly removed upon closing a discussion category or a specific level within the editor. This issue can lead to frustrating debugging sessions and hinder the development process. This comprehensive guide aims to dissect this problem, offering potential causes, solutions, and best practices to ensure your C++ actors persist as intended within your UE5 projects.

Understanding the Problem: Why Are My Actors Disappearing?

Before diving into solutions, it’s crucial to understand the underlying reasons why actors might be removed unexpectedly. In Unreal Engine, actors are the fundamental building blocks of your game world. They represent objects that can be placed in the level and possess properties, behaviors, and interactions. When an actor is removed upon closing a level or category, it typically indicates an issue with the actor's lifecycle management, referencing, or persistence mechanisms.

1. Garbage Collection

Unreal Engine employs a garbage collection system to automatically manage memory and remove objects that are no longer referenced. If an actor is not properly referenced by any other object in the scene, the garbage collector may identify it as eligible for removal. This is a common culprit for disappearing actors, especially those created dynamically at runtime. Ensuring that your actors are held by a valid reference, such as a UPROPERTY within another actor or game instance, is crucial to prevent premature garbage collection. The engine's garbage collection is designed to optimize memory usage by automatically removing objects that are no longer needed. However, this process can sometimes lead to unintended consequences if actors are not properly referenced. For instance, if an actor is created dynamically and only held by a local variable within a function, it will be marked for garbage collection as soon as the function exits. To prevent this, it's essential to store references to these actors in properties that are part of persistent objects, such as the game instance or another actor that remains in the scene throughout the game's lifecycle. Additionally, understanding the different types of references, such as strong and weak references, is crucial. Strong references prevent objects from being garbage collected, while weak references do not. Using the appropriate type of reference ensures that objects are kept alive as long as they are needed, but can still be garbage collected when they are no longer in use. Another important aspect of garbage collection is the use of the MarkPendingKill() function. This function marks an actor for destruction at the end of the current frame, allowing the engine to perform any necessary cleanup operations before removing the actor from memory. Improper use of MarkPendingKill() can also lead to issues, such as actors being destroyed prematurely or causing crashes if other objects still hold references to them.

2. Level Streaming and Actor Persistence

Level streaming is a technique used in Unreal Engine to divide large game worlds into smaller, manageable chunks that can be loaded and unloaded dynamically. If an actor is placed in a streamed level that is unloaded, the actor will be removed from the world. To ensure actors persist across level transitions, they must be either placed in the persistent level or configured to be persistent using the SetIsTemporarilyHiddenInEditor(false) flag. Understanding how level streaming affects actor persistence is critical for creating seamless open-world experiences. When a level is streamed out, all actors within that level are removed from the world, which can lead to unexpected behavior if not managed properly. One common solution is to use the persistent level as a container for actors that need to exist throughout the game's lifecycle. The persistent level is always loaded, so any actors placed there will not be affected by level streaming. Another approach is to use the SetIsTemporarilyHiddenInEditor(false) flag, which prevents actors from being unloaded when their level is streamed out. This flag is particularly useful for actors that need to maintain their state across level transitions, such as player characters or important gameplay objects. However, it's important to use this flag judiciously, as it can increase memory usage if too many actors are marked as persistent. In addition to these methods, the Unreal Engine provides several other tools for managing actor persistence, such as the SaveGame system and the UWorldPartition system. The SaveGame system allows you to save the state of actors and reload them later, while the UWorldPartition system provides more advanced level streaming capabilities for large, open-world games. Choosing the right approach for managing actor persistence depends on the specific needs of your project and the complexity of your game world. Proper management of actor persistence is essential for creating a smooth and immersive gameplay experience.

3. Editor-Specific Behavior

The Unreal Editor has specific behaviors that can affect actor persistence. For instance, when closing a level tab or switching between editor modes, actors might be temporarily removed or unloaded. This behavior is designed to optimize editor performance but can sometimes lead to confusion. Understanding these editor-specific behaviors is essential for debugging actor persistence issues. The editor's primary goal is to provide a smooth and efficient development environment, which sometimes requires making trade-offs in terms of actor persistence. For example, when you close a level tab in the editor, the engine may unload the actors in that level to free up memory and resources. This behavior can be surprising if you're not aware of it, as it may seem like your actors have disappeared. Similarly, switching between editor modes, such as from the level editor to the blueprint editor, can also cause actors to be temporarily unloaded. To mitigate these issues, it's important to be aware of the editor's behavior and to take steps to ensure that your actors are properly managed. One common technique is to use the Run Construction Script feature in the editor. This feature allows you to re-run the construction script of an actor, which can be useful for restoring its state after it has been unloaded. Another approach is to use the Editor Utility Widget system, which allows you to create custom tools for managing actors in the editor. These tools can be used to save and load actor states, as well as to perform other tasks related to actor persistence. In addition to these techniques, it's also important to follow best practices for actor creation and management. This includes using proper referencing techniques, such as storing references to actors in persistent objects, and avoiding the use of local variables to hold references to dynamically created actors. By understanding the editor's behavior and following best practices, you can minimize the risk of actors disappearing unexpectedly and ensure a smoother development workflow.

4. Cesium for Unreal Engine Specifics

When using Cesium for Unreal Engine, actors that are part of the Cesium World or created within its context might be subject to specific lifecycle rules. For example, if an actor is attached to a Cesium3DTileset, it might be removed when the tileset is unloaded or the Cesium World is reset. Understanding how Cesium for Unreal Engine manages actors is crucial for preventing unexpected removal. Cesium for Unreal Engine is a powerful plugin that allows you to stream high-resolution 3D geospatial data into your Unreal Engine projects. However, it also introduces its own set of rules and considerations for actor management. One key aspect is the concept of tilesets, which are hierarchical structures that represent 3D geospatial data. Actors that are attached to a tileset are considered part of that tileset and may be subject to its lifecycle. For instance, if a tileset is unloaded, all actors attached to it will also be removed from the world. This behavior is designed to optimize performance by only loading the data that is currently visible to the user. To prevent actors from being removed when a tileset is unloaded, you can detach them from the tileset or place them in a persistent level. Another important consideration is the Cesium World, which is a special actor that manages the overall geospatial context of your scene. The Cesium World is responsible for handling the loading and unloading of tilesets, as well as other geospatial operations. If the Cesium World is reset or destroyed, all actors that are part of its context will also be removed. To avoid this, it's important to ensure that the Cesium World is properly managed and that actors are not directly dependent on its lifecycle. In addition to these considerations, Cesium for Unreal Engine also provides several tools and features for managing actors, such as the Cesium Ion Integration and the CesiumGeoreference. The Cesium Ion Integration allows you to stream geospatial data from Cesium Ion, a cloud-based platform for storing and serving 3D geospatial data. The CesiumGeoreference is an actor that allows you to georeference your Unreal Engine scene, which means aligning it with real-world coordinates. By understanding these tools and features, you can effectively manage actors within the Cesium for Unreal Engine environment and prevent unexpected removal.

Diagnosing the Issue: Steps to Identify the Cause

Before implementing solutions, it’s essential to accurately diagnose the cause of the actor removal. Here’s a step-by-step approach:

1. Check the Output Log: The Unreal Editor output log is your first line of defense. It often contains valuable information about why an actor was removed, including garbage collection messages or level streaming events. Analyzing the output log can provide clues about the root cause of the problem. The output log is a comprehensive record of all events that occur within the editor, including warnings, errors, and informational messages. When an actor is removed unexpectedly, the output log may contain messages related to garbage collection, level streaming, or other relevant events. For example, if an actor is garbage collected, the output log will typically display a message indicating that the actor was destroyed due to a lack of references. Similarly, if an actor is removed due to level streaming, the output log will show messages related to the unloading of the level. To effectively analyze the output log, it's important to understand the different types of messages that can appear. Warnings indicate potential issues that may need to be addressed, while errors indicate critical problems that are preventing the engine from functioning correctly. Informational messages provide general information about the engine's state and can be useful for debugging purposes. In addition to examining the output log, you can also use the Unreal Engine's debugging tools to gain further insights into the cause of actor removal. The debugger allows you to step through your code, inspect variables, and set breakpoints to identify the exact point at which an actor is being destroyed. By combining the information from the output log with the debugger, you can effectively diagnose the root cause of actor removal issues.
2. Inspect Actor References: Use the Unreal Editor's reference viewer to see which objects are referencing the actor in question. If there are no references, it’s a prime candidate for garbage collection. The reference viewer is a powerful tool that allows you to visualize the relationships between objects in your Unreal Engine project. It displays a graph showing all objects that reference a particular actor, as well as all actors that are referenced by that actor. This information is invaluable for understanding how actors are connected and for identifying potential issues with referencing. To use the reference viewer, simply right-click on an actor in the editor and select "Reference Viewer." The reference viewer will then display a graph showing all references to and from the actor. Each node in the graph represents an object, and the lines connecting the nodes represent references. By examining the graph, you can quickly identify which objects are holding references to the actor and which objects the actor is referencing. If an actor has no incoming references, it is likely to be garbage collected, as there are no other objects keeping it alive. In this case, you need to ensure that the actor is referenced by a persistent object, such as the game instance or another actor that remains in the scene throughout the game's lifecycle. The reference viewer can also help you identify circular dependencies, which can prevent objects from being garbage collected. A circular dependency occurs when two or more objects reference each other, creating a cycle that prevents the garbage collector from freeing their memory. To resolve circular dependencies, you need to break the cycle by removing one or more of the references. In addition to identifying referencing issues, the reference viewer can also be used to optimize memory usage. By understanding how actors are referenced, you can minimize the number of unnecessary references and reduce the overall memory footprint of your project. The reference viewer is an essential tool for any Unreal Engine developer, providing valuable insights into the complex relationships between objects in your project.
3. Debug Gameplay Code: If the actor is created dynamically, step through your C++ code to ensure the actor is being properly created, attached to the scene, and referenced. Debugging gameplay code is a critical step in identifying the cause of actor removal issues, especially when actors are created dynamically at runtime. By stepping through your code, you can observe the exact sequence of events that lead to the actor's creation and destruction, allowing you to pinpoint any potential problems. To effectively debug your gameplay code, you can use the Unreal Engine's built-in debugger, which provides a range of features for inspecting variables, setting breakpoints, and stepping through code execution. Before you start debugging, it's important to ensure that your code is properly compiled and that you have enabled debugging symbols. Debugging symbols provide additional information that allows the debugger to map the compiled code back to the original source code, making it easier to understand the code's execution flow. Once you have set up your debugging environment, you can start stepping through your code line by line. As you step through the code, pay close attention to the values of variables and the execution path of the code. If you encounter an unexpected value or a deviation from the expected execution path, this may indicate a bug in your code. One common issue that can lead to actor removal is improper referencing. If an actor is not properly referenced by any other object in the scene, it may be garbage collected prematurely. To prevent this, you need to ensure that the actor is referenced by a persistent object, such as the game instance or another actor that remains in the scene throughout the game's lifecycle. Another potential issue is improper destruction. If an actor is destroyed before it is no longer needed, it can lead to crashes or other unexpected behavior. To prevent this, you need to ensure that actors are only destroyed when they are no longer required. By carefully debugging your gameplay code, you can identify and resolve a wide range of actor removal issues, ensuring the stability and reliability of your Unreal Engine project.
4. Check Level Streaming: If the actor is part of a streamed level, ensure the level is loaded and that the actor is not being inadvertently unloaded. Verifying level streaming configurations is essential when troubleshooting actor removal issues, particularly in large or open-world games that utilize level streaming to manage memory and performance. Level streaming allows you to divide your game world into smaller, manageable chunks that can be loaded and unloaded dynamically as the player moves through the environment. However, if not configured correctly, level streaming can lead to actors being removed unexpectedly when their associated levels are unloaded. To verify your level streaming configurations, you can use the Unreal Engine's level streaming tools, which provide a visual representation of your levels and their streaming status. These tools allow you to see which levels are currently loaded, which levels are being streamed in or out, and which levels are persistent. By examining the level streaming status, you can identify potential issues, such as levels that are being unloaded prematurely or levels that are not being loaded when they should be. One common issue is that actors may be placed in levels that are streamed out when the player moves away from a certain area. If these actors are essential for gameplay, they will disappear when the level is unloaded, leading to unexpected behavior. To prevent this, you need to ensure that these actors are placed in a persistent level or that their levels are configured to remain loaded. Another potential issue is that levels may not be streamed in when they should be. This can happen if the streaming distance is set too low or if the streaming volume is not properly configured. To resolve this, you need to adjust the streaming settings to ensure that levels are loaded in a timely manner. In addition to verifying the level streaming status, you should also check the code that is responsible for managing level streaming. This code may contain bugs that are causing levels to be loaded or unloaded incorrectly. By carefully examining your level streaming configurations and code, you can identify and resolve a wide range of actor removal issues, ensuring a smooth and seamless gameplay experience.
5. Cesium Debugging: If using Cesium for Unreal Engine, check the Cesium World settings and any code that interacts with the Cesium 3D Tilesets to ensure actors are properly attached and managed. Performing Cesium-specific debugging is crucial when dealing with actor removal issues in projects that utilize the Cesium for Unreal Engine plugin. Cesium for Unreal Engine integrates high-precision geospatial data into your Unreal Engine environment, allowing you to create realistic and immersive virtual worlds. However, the plugin's unique architecture and data handling mechanisms require specific debugging techniques to ensure actors are properly managed within the Cesium ecosystem. One of the first steps in Cesium debugging is to inspect the Cesium World settings. The Cesium World is the central actor that manages the connection to the Cesium Ion platform and handles the streaming of geospatial data. Incorrect settings in the Cesium World, such as invalid API keys or incorrect georeference parameters, can lead to issues with actor visibility and persistence. You should also examine the code that interacts with Cesium 3D Tilesets. Cesium 3D Tilesets are hierarchical data structures that represent 3D geospatial data, such as terrain, buildings, and other features. Actors that are attached to a Tileset are considered part of that tileset and their lifecycle is managed by the tileset. If an actor is incorrectly attached to a tileset or if the tileset is unloaded prematurely, the actor may be removed from the scene. To debug these issues, you can use the Cesium Debugging Tools, which provide a range of features for inspecting tileset loading, actor attachments, and other Cesium-specific events. These tools allow you to visualize the tileset hierarchy, track the loading status of tiles, and identify any errors or warnings that may be related to actor removal. In addition to the Cesium Debugging Tools, you can also use the standard Unreal Engine debugging tools, such as the debugger and the output log, to diagnose Cesium-related issues. By setting breakpoints in your code and examining the values of variables, you can gain insights into how actors are being managed within the Cesium environment. The output log may also contain valuable information about Cesium-specific errors or warnings, such as issues with tileset streaming or georeference calculations. By combining Cesium-specific debugging techniques with the standard Unreal Engine debugging tools, you can effectively identify and resolve actor removal issues in your Cesium for Unreal Engine projects.

Solutions: Ensuring Actor Persistence in UE5

Once you’ve identified the cause, you can implement the appropriate solution. Here are some common strategies:

1. Proper Referencing

Ensure that your actors are held by a valid reference. This can be a UPROPERTY within another actor, a container in the Game Instance, or any other object that persists throughout the game's lifecycle. Proper referencing is a cornerstone of actor persistence in Unreal Engine. Without a valid reference, the garbage collector will identify an actor as eligible for removal, leading to its untimely destruction. Understanding how to establish and maintain references is therefore essential for ensuring that your actors remain in the game world as intended. A UPROPERTY is a macro that exposes a variable to the Unreal Engine's reflection system, making it accessible from the editor, blueprints, and other parts of the engine. When a UPROPERTY is used to hold a reference to an actor, the engine automatically manages the lifetime of that reference, preventing the actor from being garbage collected as long as the UPROPERTY remains valid. This is the most common and recommended way to hold references to actors in Unreal Engine. In addition to UPROPERTIES, you can also use containers such as TArrays and TMaps to hold references to actors. These containers can be stored within other actors or in the Game Instance, providing a centralized location for managing actor references. The Game Instance is a singleton object that persists throughout the entire game session, making it an ideal place to store references to actors that need to be accessible across multiple levels or scenes. However, it's important to note that containers themselves can be garbage collected if they are not properly referenced. Therefore, you need to ensure that the container is also held by a valid reference, such as a UPROPERTY or another container. Another important aspect of proper referencing is the use of weak pointers. A weak pointer is a special type of pointer that does not prevent an object from being garbage collected. This can be useful in situations where you need to hold a reference to an actor but don't want to prevent it from being destroyed if it's no longer needed. For example, you might use a weak pointer to hold a reference to a target actor, but if the target actor is destroyed, you want the reference to be automatically invalidated. By using a combination of strong and weak pointers, you can effectively manage actor references and ensure that your actors persist in the game world for as long as they are needed. Proper referencing is a fundamental concept in Unreal Engine development, and mastering it is essential for creating stable and performant games.

2. Level Persistence

If the actor should persist across level transitions, place it in the persistent level or use SetIsTemporarilyHiddenInEditor(false) to prevent it from being unloaded. Configuring level persistence is crucial for actors that need to exist across multiple levels or throughout the entire game session. Without proper level persistence, actors may be unloaded when their associated levels are streamed out, leading to unexpected behavior and gameplay issues. Unreal Engine provides several mechanisms for ensuring actor persistence, each with its own advantages and disadvantages. The persistent level is the main level that is always loaded in the game world. Actors placed in the persistent level will remain in the scene throughout the game session, regardless of which other levels are loaded or unloaded. This is the simplest way to ensure actor persistence, but it's not always the most appropriate solution. If you have a large number of actors that need to persist, placing them all in the persistent level can lead to performance issues. Another approach is to use the SetIsTemporarilyHiddenInEditor(false) function. This function prevents an actor from being unloaded when its level is streamed out. This can be useful for actors that need to maintain their state across level transitions, such as player characters or important gameplay objects. However, it's important to use this function judiciously, as it can increase memory usage if too many actors are marked as persistent. A more flexible approach is to use the SaveGame system. The SaveGame system allows you to save the state of actors and reload them later. This is particularly useful for actors that need to persist between game sessions or for actors that need to be dynamically created and destroyed. To use the SaveGame system, you need to create a SaveGame object and store the relevant data for your actors in this object. When you want to save the game, you can serialize the SaveGame object to disk. When you want to load the game, you can deserialize the SaveGame object and use the data to restore the state of your actors. In addition to these methods, you can also use the UWorldPartition system for managing actor persistence in large, open-world games. The UWorldPartition system provides more advanced level streaming capabilities, allowing you to divide your game world into smaller, more manageable chunks. By using the UWorldPartition system, you can ensure that only the necessary actors are loaded at any given time, improving performance and reducing memory usage. Choosing the right approach for configuring level persistence depends on the specific needs of your project. By understanding the different options available, you can effectively manage actor persistence and ensure a seamless gameplay experience.

3. Editor Awareness

Be mindful of editor-specific behaviors. If an actor disappears when closing a level tab, it might be expected behavior. Re-running the game or using the