coding-standards

Unreal coding standards

Match Epic's C++ conventions so code reads like the engine and the rest of the project. The overriding rule: match the surrounding code first; these are the defaults when nothing else is established.

Related skills: cpp-fundamentals, core-types-and-containers, module-and-build-system.

When to use this skill

• Writing any new UE C++ header or source file.
• Reviewing or cleaning up code for style consistency.
• Choosing names for types, members, functions, or files.
• Structuring includes and forward declarations in a header.

Type prefixes (mandatory for reflected types)

UHT enforces prefixes for reflected types; mismatches are a compile error.

Prefix	Type	Engine example
U	UObject subclass (non-actor)	UActorComponent (Components/ActorComponent.h:160)
A	AActor subclass	AActor (GameFramework/Actor.h:282)
F	Plain struct or non-UObject class	FAttachmentTransformRules (Engine/EngineTypes.h:75)
E	Enum / enum class	EAttachmentRule (Engine/EngineTypes.h:62)
I	Abstract interface class	IInterface_AssetUserData (Interfaces/Interface_AssetUserData.h:22)
T	Class template	TArray (Containers/Array.h:767)
S	Slate widget	SWidget, SCompoundWidget
b	Boolean variable	bReplicates (GameFramework/Actor.h:593)

The word after the prefix is PascalCase. The class name without its prefix must match the filename: AMyPawn → MyPawn.h. Typedefs take the prefix appropriate to their underlying type.

Full prefix rules, interface pairing, enum value style, and template parameter conventions: references/naming-conventions.md.

Naming

• PascalCase for every identifier — types, functions, member variables, local variables, and parameters. No m_ prefix, no camelCase, no snake_case.
• Booleans carry the b prefix: bIsDead, bHasKey, bReplicates.
• Functions with a bool return ask a question: IsAlive(), ShouldClearBuffer().
• Output reference parameters carry Out: void GetItems(TArray<FItem>& OutItems).
• Type and variable names are nouns; function names are verb phrases.
• Be descriptive; avoid abbreviations except established ones (AI, HUD, LOD, GC).
• Macros are UE_ALL_CAPS_WITH_UNDERSCORES.

Formatting

Allman braces — opening brace on its own line for every construct. Always brace single-statement blocks:

void AMyActor::BeginPlay()
{
    Super::BeginPlay();
    if (bIsReady)
    {
        DoThing();
    }
}

• Tabs (4-character width) for indentation; spaces only for alignment within a line.
• One statement per line.
• Pointer/reference spacing: FType* Ptr; and const FType& Ref; — */& bind to the type.
• No variable shadowing across scopes.

Switch statements must have an explicit default: branch and document intentional fall-through with // falls through.

Full formatting rules, switch style, and namespace rules: references/formatting-and-includes.md.

Language conventions

• nullptr — never NULL or 0 for pointers.
• override on every overriding virtual. Add final where the class or function should not be further overridden.
• const correctness — const member functions for non-mutating methods; const& for non-trivial parameters not being copied; never const a by-value return.
• TEXT("...") around every string literal that constructs an FString or FName.
• enum class over plain enums; back with uint8 if exposed to Blueprints. Values are PascalCase. Use ENUM_CLASS_FLAGS(EFoo) for bitfield enums with a None = 0 sentinel.
• Engine containers (TArray, TMap, TSet, FString, FName) over std:: equivalents in engine-facing code.
• auto only where the type is either a lambda, a verbose iterator, or genuinely indiscernible from context. Always apply const, &, or * explicitly with auto.
• Range-based for is preferred. For TMap, iterate as for (TPair<K,V>& Kvp : Map).
• Move semantics — use MoveTemp(X) (UE's std::move) when transferring ownership of containers or FString into a member or return.
• Lambdas — prefer explicit captures over [=] or [&]. Captured UObject* pointers are invisible to the GC. Use CreateWeakLambda / BindWeakLambda for deferred lambdas.
• UObjects via pointer — pass by pointer, not reference. Null is the signal for "absent".
• Portable integer types: int32, uint32, uint8, float, double, TCHAR; avoid bare int in serialized or replicated data.

Headers and includes

Header structure (in order)

// Copyright Epic Games, Inc. All Rights Reserved.

#pragma once

#include "CoreMinimal.h"          // 1. CoreMinimal (or fine-grained core headers)
#include "GameFramework/Actor.h"  // 2. Engine / module headers this type needs
#include "MyActor.generated.h"   // 3. generated.h — ALWAYS LAST

Source file structure

#include "MyActor.h"                        // matching header first
#include "Components/StaticMeshComponent.h" // then any implementation deps

• #pragma once at the top of every header (all target compilers support it).
• generated.h must be the last include — UHT requires it. Missing it or putting it in the middle causes broken generated code.
• IWYU (Include What You Use) — include every header you directly depend on; do not rely on transitive includes through another header.
• Forward declare in headers where you only need a pointer or reference. In .cpp, include the full header. This reduces compile times and dependency coupling.

// Header — forward declare only
class UStaticMeshComponent;

UCLASS()
class MYGAME_API AMyActor : public AActor
{
    GENERATED_BODY()
    UPROPERTY(VisibleAnywhere) TObjectPtr<UStaticMeshComponent> Mesh;
};

// Source — full include
#include "Components/StaticMeshComponent.h"

Engine evidence: Actor.h lines 5–32 use IWYU-style fine-grained includes ending with "Actor.generated.h" at line 32; Character.h lines 5–19 show CoreMinimal.h first and "Character.generated.h" at line 19.

Reflection style

Every reflected class or struct needs GENERATED_BODY() as its first body member. Every public class in a module needs the module *_API export macro:

UCLASS(Blueprintable, BlueprintType, config=Game)
class MYGAME_API AWeapon : public AActor
{
    GENERATED_BODY()
public:
    UFUNCTION(BlueprintCallable, Category="Weapon")
    void Fire();

    UPROPERTY(EditDefaultsOnly, BlueprintReadOnly, Category="Stats",
              meta=(ClampMin="0"))
    float Damage = 25.f;

protected:
    UPROPERTY(VisibleAnywhere, Category="Components")
    TObjectPtr<UStaticMeshComponent> Mesh;
};

Key rules:

• Always set a Category on any UPROPERTY/UFUNCTION the editor will show. Without it, properties land in an uncategorized root group.
• Put meta=(...) last in the specifier list.
• Use TObjectPtr<T> for UPROPERTY members holding UObject-derived pointers (UE5+ modern form). Raw T* still works and appears in older code.
• The *_API macro (e.g. MYGAME_API, ENGINE_API) must appear between class and the class name for any non-inline public API. UBT expands it to __declspec(dllexport/import).

See references/reflection-and-uht.md for the full specifier reference, USTRUCT/UENUM/UFUNCTION details, and common UHT errors.

Comments and documentation

• // for inline comments; /** ... */ JavaDoc-style comments on public API declarations — these surface as tooltips for Blueprint-exposed members and in the generated API docs.
• Comment why, not what. Keep comments accurate and current with the code.
• Class comments describe the problem the class solves. Multi-line method comments document purpose, parameter units/ranges, return value, and any @warning/@see/@deprecated.

/** Maximum health this actor can have. Modified by difficulty at BeginPlay. */
UPROPERTY(EditDefaultsOnly, Category="Health", meta=(ClampMin="1"))
float MaxHealth = 100.f;

Logging and errors

• Log through a named category with UE_LOG (logging-and-assertions).
• check(Condition) for invariants — aborts in all builds if violated. Never put side effects inside a check.
• ensure(Condition) for recoverable "shouldn't happen" — fires once in non-shipping builds, returns bool so you can handle the failure.
• Remove debug prints before submitting.

Gotchas

• Wrong or missing type prefix — UHT build error; check U/A/F/E/I/T/S.
• generated.h not last — UHT mis-generates or fails outright.
• Missing GENERATED_BODY() — compile errors from undefined generated symbols.
• Missing Category on editor-exposed properties — unorganized Details panel.
• m_ prefix or snake_case — not Unreal style; use PascalCase.
• Missing b on booleans — style violation; also breaks naming-based tooling.
• std:: containers in engine-facing code — use TArray/TMap/FString instead.
• Omitting override — silent non-override when a virtual signature drifts.
• Bare NULL/0 for pointers — use nullptr.
• No TEXT() around string literals — produces an undesirable narrow-to-wide conversion.
• auto overuse — hide types from readers; use only for lambdas, verbose iterators, or template-context expressions where the type is genuinely unwriteable.
• [=]/[&] lambda captures — UObject pointers captured by [=] are invisible to the GC; deferred [&] lambdas dangle. Use explicit captures and weak wrappers.

References and source material

Engine source (UE 5.8, under Engine/Source/):

• Runtime/Engine/Classes/GameFramework/Actor.h:281–282, 288, 306, 593, 891, 1019, 1024
• Runtime/Engine/Classes/GameFramework/Character.h:3, 5, 19, 337–338
• Runtime/Engine/Classes/Components/ActorComponent.h:3, 23, 27–38, 159–162, 177, 340
• Runtime/Engine/Classes/Engine/EngineTypes.h:62, 75
• Runtime/Engine/Classes/Interfaces/Interface_AssetUserData.h:3, 16–22
• Runtime/CoreUObject/Public/UObject/Object.h:97–100, 106, 129
• Runtime/Core/Public/Containers/Array.h:767
• Runtime/Core/Public/Windows/WindowsPlatform.h:209–210
• Runtime/Core/Public/HAL/Platform.h:1063–1065

Official docs (UE 5.8):

• Epic C++ Coding Standard: https://dev.epicgames.com/documentation/unreal-engine/epic-cplusplus-coding-standard-for-unreal-engine

Deep-dive references in this skill:

• references/naming-conventions.md — all prefix rules, PascalCase details, boolean/enum/function/macro naming, interface pairing.
• references/formatting-and-includes.md — Allman braces, tabs, switch style, const correctness detail, #pragma once, include order, IWYU, forward declarations, API export macros.
• references/reflection-and-uht.md — UCLASS/USTRUCT/ UENUM/UPROPERTY/UFUNCTION specifier reference, TObjectPtr, GENERATED_BODY(), and common UHT errors.