🎛️ Lesson 2.3: UPROPERTY & UFUNCTION Specifiers

Edit vs Visible

The first choice for any property: can it be edited in the Details panel, or only viewed? And does that apply to the class defaults, to placed instances, or both? Two axes, combined into keywords:

// A tunable designers set on the Blueprint default and per placed actor:
UPROPERTY(EditAnywhere, Category = "Combat")
float FireRate = 5.0f;

// A component pointer: shown for inspection, but you don't reassign it by hand:
UPROPERTY(VisibleAnywhere, Category = "Components")
UStaticMeshComponent* Mesh;

// A value that should be identical for every instance of a weapon type —
// edit it on the Blueprint default, not per-instance:
UPROPERTY(EditDefaultsOnly, Category = "Combat")
int32 MagazineSize = 30;

Blueprint Access

Separately from the editor, you control whether Blueprint graphs can read or write a property. These stack with the edit/visible specifiers:

• BlueprintReadWrite — Blueprint can get and set the value
• BlueprintReadOnly — Blueprint can get but not set (good for state C++ owns)
• (neither) — invisible to Blueprint entirely

// Designers tune it in Details AND Blueprint logic can change it at runtime:
UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Combat")
float Damage = 25.0f;

// Blueprint can read current health for a HUD, but only C++ changes it:
UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "State")
float CurrentHealth = 100.0f;

Category & Meta

As your classes grow, the Details panel gets crowded. Category groups properties into collapsible sections; nested categories use |.

UPROPERTY(EditAnywhere, Category = "Combat|Ranged")
float Range = 5000.0f;

UPROPERTY(EditAnywhere, Category = "Combat|Ranged")
float Spread = 2.0f;

meta unlocks finer control. A few you'll use constantly:

// Clamp the slider AND the typed value between 0 and 100:
UPROPERTY(EditAnywhere, Category = "Stats",
    meta = (ClampMin = "0.0", ClampMax = "100.0", UIMin = "0.0", UIMax = "100.0"))
float HealthPercent = 100.0f;

// Only show this property when bUseSpread is true:
UPROPERTY(EditAnywhere, Category = "Combat",
    meta = (EditCondition = "bUseSpread"))
float SpreadAngle = 2.0f;

UPROPERTY(EditAnywhere, Category = "Combat")
bool bUseSpread = false;

UFUNCTION Specifiers

UFUNCTION exposes a C++ function to the reflection system — most often to make it callable from Blueprint. The everyday specifiers:

// An action a designer can call from a Blueprint graph:
UFUNCTION(BlueprintCallable, Category = "Combat")
void Fire();

// A pure getter — shows as a node with only a return value, no exec pins:
UFUNCTION(BlueprintPure, Category = "State")
bool IsAlive() const;

// A handy editor button, e.g. to reset or randomize during level design:
UFUNCTION(CallInEditor, Category = "Debug")
void ResetToDefaults();

The Silent GC Trap

Here's a subtle but critical reason UPROPERTY matters beyond the editor. Unreal garbage-collects UObjects. The garbage collector only "sees" a pointer as keeping an object alive if that pointer is a reflected UPROPERTY. A raw, un-reflected UObject* is invisible to the GC.

UCLASS()
class MYPROJECT_API AWeapon : public AActor
{
    GENERATED_BODY()

    // SAFE: the GC sees this pointer and keeps the ammo object alive.
    UPROPERTY()
    UAmmoData* CurrentAmmo;

    // DANGER: no UPROPERTY — the GC can't see this. The object it points to
    // may be collected out from under you, leaving a dangling pointer.
    UAmmoData* CachedAmmo;   // ⚠ do not do this for UObject pointers
};

Hands-on Exercise & Quiz

UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Combat",
    meta = (ClampMin = "0.0", ClampMax = "1000.0"))
float Damage = 25.0f;

UPROPERTY(EditDefaultsOnly, Category = "Combat")
int32 MagazineSize = 30;

UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "State")
int32 CurrentAmmo = 30;

UFUNCTION(BlueprintCallable, Category = "Combat")
void Fire();

UPROPERTY()   // GC visibility — no editor exposure needed
UAmmoData* AmmoData;

Summary

📚 Additional Resources

• UProperties (Epic)
• BenUI — the community UPROPERTY/UFUNCTION specifier reference

🚀 What's Next?

You can shape how any member surfaces. Next we meet the types those members hold — Unreal's own strings (FString/FName/FText) and containers (TArray/TMap/TSet) — and why you use them instead of the C++ standard library.