Ошибка	Последствие
Хранить `UObject* Cached;` без `UPROPERTY` в C++-классе	Висячий указатель после следующего цикла GC; падение в случайном кадре
Полагать, что GC работает как `std::shared_ptr`	Ожидание подсчёта ссылок; недоумение, почему объект жив
Считать `UPROPERTY` ссылку на цикл утечкой	Mark-and-Sweep свободно собирает циклы; утечки нет
Звать `AddToRoot` «на всякий случай»	Постоянная утечка: укоренённый объект и всё, на что он ссылается, никогда не собираются
Сравнивать с `nullptr` вместо `IsValid`	`UObject* Ptr` ненулевой, но цель уже `PendingKill` — крэш в `->Method()`
Звать `delete` на `UObject`	Поломка реестра GC; падения и повреждение памяти
Использовать Actor сразу после `Destroy()`	`Destroy` лишь помечает объект; реальная зачистка позже — типичная ловушка
Хранить сырой `UObject*` в не-`UObject` классе (FMyStruct, FCallback)	GC не видит ссылку; объект погибает, указатель висит
Использовать `TWeakObjectPtr` чтобы «разорвать цикл»	Циклы и так разрываются Mark-and-Sweep; `TWeakObjectPtr` нужен для семантики «не владею»
Полагаться на `IsValidLowLevel` в шиппинг-коде	Это диагностика, не валидатор; в release ведёт себя иначе и не заменяет `IsValid`

intermediate

UObject Garbage Collection

Unreal's GC — Mark-and-Sweep over the UPROPERTY graph, root set, strong vs weak references, IsValid, cycles, and leaks.

ue-garbage-collection

Sections

Contents

Detailed explanation

Unreal GC — Mark-and-Sweep over the UPROPERTY graph

Unreal's garbage collector is fundamentally different from what a regular C++ programmer expects. There is no reference counting, no std::shared_ptr, no destructor at the moment the last reference disappears. There is a periodic tracing collector: every few seconds the engine stops the world, walks the reachability graph from a root set along fields marked with the UPROPERTY macro, and sweeps everything it did not reach. That's Mark-and-Sweep.

Every other rule flows from that model. A strong reference is a UPROPERTY field holding UObject* or TObjectPtr<>: the GC sees it during tracing and keeps the target alive. A raw UObject* Cached inside a plain C++ class is invisible to the collector: it does not keep the object alive, and after one GC cycle it becomes a dangling pointer. A weak reference TWeakObjectPtr does not keep the object alive at all, but unlike a raw pointer it knows the target has died and honestly answers IsValid() == false.

The third key point: cycles are handled automatically. Two UObjects holding each other via UPROPERTY do not leak: if no path from the root set reaches the cycle, both vertices are unreachable and both are reaped in the same sweep. That's the main advantage of a tracing GC over reference counting, and the reason TWeakObjectPtr in UE is a tool for "non-owning" semantics, not a workaround for cycles the way std::weak_ptr is in the STL.

Topic map

Object Lifetime — NewObject, the Outer chain, Destroy, MarkAsGarbage, BeginDestroy, FinishDestroy, GC cycle phases.
GC Algorithm — mark phase, sweep phase, walking the graph through UPROPERTY, the GUObjectArray registry.
GC Root Set — what's in the root set by default, AddToRoot/RemoveFromRoot, the danger of "permanent" rooting.
Strong References — UPROPERTY, TObjectPtr<>, TArray<TObjectPtr<>>, auto-nulling when the target dies.
Weak References — TWeakObjectPtr, IsValid, caches, and back-pointers to an owner.
Validity Check — IsValid vs IsValidLowLevel vs nullptr comparison, what each checks and when each lies.
UObject GC — UE GC vs std::shared_ptr, incremental GC, clustering, what the collector does not manage.

Common traps

Mistake	Consequence
Storing `UObject* Cached;` without `UPROPERTY` in a C++ class	Dangling pointer after the next GC cycle; crash in a random frame
Assuming GC works like `std::shared_ptr`	Expecting reference counting; confusion about why an object is still alive
Treating a `UPROPERTY` cycle as a leak	Mark-and-Sweep collects cycles freely; no leak
Calling `AddToRoot` "just in case"	Permanent leak: the rooted object and everything it references are never collected
Comparing to `nullptr` instead of `IsValid`	`UObject* Ptr` is non-null, but the target is already `PendingKill` — crash on `->Method()`
Calling `delete` on a `UObject`	Corrupts the GC registry; crashes and memory corruption
Using an Actor right after `Destroy()`	`Destroy` only marks the object; actual cleanup happens later — a classic trap
Holding a raw `UObject*` inside a non-`UObject` class (FMyStruct, FCallback)	GC cannot see the reference; the object dies, the pointer dangles
Reaching for `TWeakObjectPtr` to "break a cycle"	Mark-and-Sweep already breaks cycles; `TWeakObjectPtr` is for "non-owning" semantics
Trusting `IsValidLowLevel` in shipping code	It's diagnostic, not a validator; in release builds it behaves differently and doesn't replace `IsValid`

Interview relevance

Garbage collection is mandatory middle-level material for any UE5 interview. Checks:

That UE's GC is Mark-and-Sweep, not reference counting, and that cycles collect automatically.
How UPROPERTY fundamentally differs from a raw UObject* in a C++ class.
What the root set is, and why AddToRoot without RemoveFromRoot is a permanent leak.
The difference between TObjectPtr<> (strong) and TWeakObjectPtr<> (weak).
What IsValid checks and why comparing to nullptr is not enough.
Why Destroy() on an Actor is not delete, and what happens between the two and actual memory release.

Common wrong answer: "Unreal's GC works like smart pointers: as long as someone holds a reference, the object is alive." Wrong twice over — first, the GC is tracing, not counting; second, "someone holds a reference" through a raw UObject* does not exist for the GC. You need a UPROPERTY reference or a root-set entry.

Why it matters

Unreal's GC is **not `std::shared_ptr`**. It's a tracing Mark-and-Sweep that walks the graph of `UPROPERTY` fields from a root set. Developers who miss that write `UObject* Cached;` without `UPROPERTY`, end up with a dangling pointer, and crash in a random frame.

Ошибка	Последствие
Хранить `UObject* Cached;` без `UPROPERTY` в C++-классе	Висячий указатель после следующего цикла GC; падение в случайном кадре
Полагать, что GC работает как `std::shared_ptr`	Ожидание подсчёта ссылок; недоумение, почему объект жив
Считать `UPROPERTY` ссылку на цикл утечкой	Mark-and-Sweep свободно собирает циклы; утечки нет
Звать `AddToRoot` «на всякий случай»	Постоянная утечка: укоренённый объект и всё, на что он ссылается, никогда не собираются
Сравнивать с `nullptr` вместо `IsValid`	`UObject* Ptr` ненулевой, но цель уже `PendingKill` — крэш в `->Method()`
Звать `delete` на `UObject`	Поломка реестра GC; падения и повреждение памяти
Использовать Actor сразу после `Destroy()`	`Destroy` лишь помечает объект; реальная зачистка позже — типичная ловушка
Хранить сырой `UObject*` в не-`UObject` классе (FMyStruct, FCallback)	GC не видит ссылку; объект погибает, указатель висит
Использовать `TWeakObjectPtr` чтобы «разорвать цикл»	Циклы и так разрываются Mark-and-Sweep; `TWeakObjectPtr` нужен для семантики «не владею»
Полагаться на `IsValidLowLevel` в шиппинг-коде	Это диагностика, не валидатор; в release ведёт себя иначе и не заменяет `IsValid`

intermediate

UObject Garbage Collection

Unreal's GC — Mark-and-Sweep over the UPROPERTY graph, root set, strong vs weak references, IsValid, cycles, and leaks.

ue-garbage-collection

Practice: 16 questions →

Sections

Contents

Detailed explanation

Unreal GC — Mark-and-Sweep over the UPROPERTY graph

Topic map

Object Lifetime — NewObject, the Outer chain, Destroy, MarkAsGarbage, BeginDestroy, FinishDestroy, GC cycle phases.
GC Algorithm — mark phase, sweep phase, walking the graph through UPROPERTY, the GUObjectArray registry.
GC Root Set — what's in the root set by default, AddToRoot/RemoveFromRoot, the danger of "permanent" rooting.
Strong References — UPROPERTY, TObjectPtr<>, TArray<TObjectPtr<>>, auto-nulling when the target dies.
Weak References — TWeakObjectPtr, IsValid, caches, and back-pointers to an owner.
Validity Check — IsValid vs IsValidLowLevel vs nullptr comparison, what each checks and when each lies.
UObject GC — UE GC vs std::shared_ptr, incremental GC, clustering, what the collector does not manage.

Common traps

Mistake	Consequence
Storing `UObject* Cached;` without `UPROPERTY` in a C++ class	Dangling pointer after the next GC cycle; crash in a random frame
Assuming GC works like `std::shared_ptr`	Expecting reference counting; confusion about why an object is still alive
Treating a `UPROPERTY` cycle as a leak	Mark-and-Sweep collects cycles freely; no leak
Calling `AddToRoot` "just in case"	Permanent leak: the rooted object and everything it references are never collected
Comparing to `nullptr` instead of `IsValid`	`UObject* Ptr` is non-null, but the target is already `PendingKill` — crash on `->Method()`
Calling `delete` on a `UObject`	Corrupts the GC registry; crashes and memory corruption
Using an Actor right after `Destroy()`	`Destroy` only marks the object; actual cleanup happens later — a classic trap
Holding a raw `UObject*` inside a non-`UObject` class (FMyStruct, FCallback)	GC cannot see the reference; the object dies, the pointer dangles
Reaching for `TWeakObjectPtr` to "break a cycle"	Mark-and-Sweep already breaks cycles; `TWeakObjectPtr` is for "non-owning" semantics
Trusting `IsValidLowLevel` in shipping code	It's diagnostic, not a validator; in release builds it behaves differently and doesn't replace `IsValid`

Interview relevance

Garbage collection is mandatory middle-level material for any UE5 interview. Checks:

That UE's GC is Mark-and-Sweep, not reference counting, and that cycles collect automatically.
How UPROPERTY fundamentally differs from a raw UObject* in a C++ class.
What the root set is, and why AddToRoot without RemoveFromRoot is a permanent leak.
The difference between TObjectPtr<> (strong) and TWeakObjectPtr<> (weak).
What IsValid checks and why comparing to nullptr is not enough.
Why Destroy() on an Actor is not delete, and what happens between the two and actual memory release.

Why it matters

Сборка мусора UObject

Сборка мусора Unreal — Mark-and-Sweep по графу UPROPERTY

Карта темы

Частые ошибки и ловушки

Значение для собеседований

UObject Garbage Collection

Unreal GC — Mark-and-Sweep over the UPROPERTY graph

Topic map

Common traps

Interview relevance

Сборка мусора UObject

Сборка мусора Unreal — Mark-and-Sweep по графу UPROPERTY

Карта темы

Частые ошибки и ловушки

Значение для собеседований

UObject Garbage Collection

Unreal GC — Mark-and-Sweep over the UPROPERTY graph

Topic map

Common traps

Interview relevance