Constucting a struct variable in c++

Development Programming & Scripting Blueprint
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1

Given a function such as one below

UFUNCTION(BlueprintCallable)
void Func( UStruct* Struct );

How can I construct a struct from the UStruct passed in as parameter? The struct will be passed in from Blueprint.

Edit :
I am trying to construct a struct in the scope of the function as though I am creating a new object, for example, calling

NewObject<SomeObjectClass>( nullptr )

However, even though UStruct is derived from UObject, I cannot use NewObject() to construct a new struct based on the UStruct* param.

2

I am not sure what you are asking, but I’ll give it a try.
Are you trying to define a new UStruct? Or you have one already?

If you are trying to define a ustruct then on on your .h file add a definition like so:

USTRUCT(BlueprintType)
struct FMyUStruct
{
    GENERATED_BODY()
    UPROPERTY()
    int32 MyInt;
}

then you will have to pass a variable like this to your function:

FMyStruct myStruct;
Func(myStruct);

but the definition must be as follows:

     UFUNCTION(BlueprintCallable)
     void Func( FMyStruct& Struct ); 
    // notice I am using a reference because 
    // pointers to structs are not supported in blueprint

Now, if the function is hardcoded and you already have the struct you should try to cast your pointer:

Cast<UStruct>(myStruct)

but I am not sure that would work. Also you can try a reinterpret_cast()

3

Take a look at FStructOnScope. This handles creating and initialising a struct instance from a UStruct.

If you need the struct instance to outlive your current scope then you’ll either have to put FStructOnScope into a smart pointer, or copy what it does internally and make it work for what you need.

1 Like
4

I want to keep my function generic. Thanks for trying though :slight_smile:

5

Thanks for the prompt reply! Looking at the documentation of FStructOnScope, I do not understand how to access the properties in the struct instance. Is it right of me to assume hat SampleStructMemory is the struct instance? If so, how can I access the properties in the struct instance?

6

Correct.

The UStruct defines the properties on the struct, and you can use that information to modify the correct memory within the struct instance. FStructOnScope::GetStruct() can get you the UStruct it was created with, and FStructOnScope::GetStructMemory() will get you the instance data.

You’re dealing with completely type-erased data, so you’re reliant on the properties to let you interpret it correctly. See UProperty::ContainerPtrToValuePtr, along with the various forms of ExportText and ImportText that are on UProperty. Property sub-classes also often have extra functions to let you more easily deal with the type of data that the property represents.

You can use TFieldIterator<UProperty> to iterate all the properties on a struct.