Structs🔗

Classes declared in script are always types of UObject, and are part of unreal's normal object system and garbage collector.

You can also make structs in script, which behave as value types:

struct FExampleStruct
{
    /* Properties with UPROPERTY() in a struct will be accessible in blueprint. */
    UPROPERTY()
    float ExampleNumber = 4.0;

    UPROPERTY()
    FString ExampleString = "Example String";

    /* Properties without UPROPERTY() will still be in the struct, but cannot be seen by blueprint. */
    float ExampleHiddenNumber = 3.0;
};

Note: Unlike classes, structs cannot have UFUNCTION()s. They can have plain script methods on them however, although they will not be usable from blueprint.

Passing and Returning Structs🔗

Structs can be passed and returned from script functions and UFUNCTIONs as normal:

UFUNCTION()
FExampleStruct CreateExampleStruct(float Number)
{
    FExampleStruct ResultStruct;
    ResultStruct.ExampleNumber = Number;
    ResultStruct.ExampleString = f"{Number}";

    return ResultStruct;
}

UFUNCTION(BlueprintPure)
bool IsNumberInStructEqual(FExampleStruct Struct, float TestNumber)
{
    return Struct.ExampleNumber == TestNumber;
}

Struct References🔗

By default, argument values in script functions are read-only. That means properties of a struct parameter cannot be changed, and non-const methods cannot be called on it.

If needed, you can take a reference to a struct to modify it:

// Change the parameter struct so its number is randomized between 0.0 and 1.0
UFUNCTION()
void RandomizeNumberInStruct(FExampleStruct& Struct)
{
    Struct.ExampleNumber = Math::RandRange(0.0, 1.0);
}

Declaring Out Parameters🔗

When a function with a struct reference is called from a blueprint node, the struct will be passed as an input value:

When you want a struct parameter to be an ouput value only, declare the reference as &out in script. This works to create output pins for primitives as well:

UFUNCTION()
void OutputRandomizedStruct(FExampleStruct&out OutputStruct, bool&out bOutSuccessful)
{
    FExampleStruct ResultStruct;
    ResultStruct.ExampleNumber = Math::RandRange(0.0, 1.0);

    OutputStruct = ResultStruct;
    bOutSuccessful = true;
}

Automatic References for Function Parameters🔗

As an implementation detail: script functions never take struct parameters by value.
When you declare a struct parameter, it is internally implemented as a const reference, as if you added const &.

This means there is no difference between a FVector parameter and a const FVector& parameter. Both behave exactly the same in performance and semantics.

This choice was made to improve script performance and avoid having to instruct gameplay scripters to write const & on all their parameters.