Issue with copying of derived class

[golden marten]

I'm currently having an issue when I'm memcpying the data from a derived class. The result is way off. In the screenshot you can see the data when it's uploaded to the GPU.

class Light
{
private:
    virtual void Fill() = 0;
public:
    int id = -1;
    Vector3 color;
    float strenght = 1;
    Light() = default;
};

class PointLight : public Light
{
private:
    virtual void Fill() override {};
public:
    PointLight()
        :Light() {};
    Vector4 position;
};

Won't the PointLight class and its data match this struct on the GPU in the HLSL shader?

struct PointLight
{
    int id;
    float3 color;
    float strenght;
    float4 position;
};

[winged stump]

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[errant berry]

are doing doing a straight up memcpy?

what exactly are you doing?

[golden marten]

It's very complex, a lot of code. But here is how I'm doing the memcpy. ```
numBytes == 48, _frameIndex == 1, _offset == 0

[errant berry]

cause those 2 classes are not identical in memory

there is a vtable etc

[golden marten]

I'm using tripple buffering, thats why i have a subresource offset

[errant berry]

it's not just POD

[golden marten]

Ah, so the vtable takes actual space? I knew about the vtable, but didnt think it added space

[errant berry]

ofc, it needs to know what function to take

[golden marten]

So how does it look in memory? And why can I cast it, and get the derived class?

[manic summit]

The vtable pointer takes the space yep

[golden marten]

But if it was a non-virtual method it would, right?

[errant berry]

propably, in all fairness I would be highly dubious to simply memcopy any non POD class

[golden marten]

Ok. How should I do this then? I want to be able to use dynamic cast, but the base class has to be abstract i think

[golden marten]

is there a way to be able to use dynamic cast without affecting the memory layout?

since I have to make the base class abstract

[errant berry]

are you casting a pointer or?

[golden marten]

to be able to use dynamic

[errant berry]

why does the base need to be abstract?

[golden marten]

LightComponent pLight;
pLight.light = new PointLight;
PointLight* actualLight = (PointLight*)pLight.light;
actualLight->color = Vector3(0, 1, 0);
actualLight->position = Vector4(0, 1, 0, 1);
actualLight->strenght = 2;

PointLight* copy = dynamic_cast<PointLight*>(pLight.light);
                                
if (copy != nullptr)
{
graphics.scene->AddComponentToEntity<LightComponent>(graphics.scene->GetEntity(selectedEntityID), pLight);

PointLight* comp = (PointLight*)graphics.scene->GetComponentForEntity<LightComponent>(graphics.scene->GetEntity(selectedEntityID))->light;
                                    graphics.lManager.AddLight(graphics.device, graphics.directCmdList, *comp, graphics.frameIndex);

[errant berry]

(PointLight*) this is not a dynamic cast btw

honestly I would simply just threat all the lights as different components

[golden marten]

since a component can have 1 light of multiple different types I want to dynamically check which light it has.

[golden marten]

PointLight* copy = dynamic_cast<PointLight*>(pLight.light);
thats dynamic cast, isnt it?

[errant berry]

yeah that is

keep in mind dynamic casts can fail

but why not just have a pointlight component?

[golden marten]

yeah, was thinking about that. makes the components to be located contigously in mem aswell, which is good for cache

[errant berry]

just completly get rid of casts

[manic summit]

I think it is legal to do so when the class is trivially copyable

[golden marten]

I want to avoid duplicating code for each type of light.

[manic summit]

But im on phone and looking at the standard is impossible

[errant berry]

yes that is essentially what I am saying :)

[golden marten]

I'm not so sure. dynamic cast returns nullptr if the class isnt deferred

which is why im checking for it

I guess I'll just add 1 component for each light :/

Works now once I've changed them to individual components 🙂 The more you know.

!solved