Bug 64479 – RenderObject::containingBlock is a const method but it returns a non-const object

• Status: RESOLVED
• Resolution: WONTFIX
• Component: Layout and Rendering

[Julien Chaffraix]

See https://bugs.webkit.org/show_bug.cgi?id=64318#c2

This leads to ugly code like the following:

RenderBlock* RenderView::containingBlock() const
{
    return const_cast<RenderView*>(this);
}

Also it is a violation of const-ness.

[Ahmad Saleem]

https://searchfox.org/wubkat/source/Source/WebCore/rendering/RenderObject.cpp#727

^ Anything else needed here?


RenderBlock* RenderObject::containingBlock() const
{
    if (is<RenderText>(*this))
        return containingBlockForPositionType(PositionType::Static, *this);

    auto containingBlockForRenderer = [](const auto& renderer) -> RenderBlock* {
        if (isInTopLayerOrBackdrop(renderer.style(), renderer.element()))
            return &renderer.view();
        return containingBlockForPositionType(renderer.style().position(), renderer);
    };

    if (!parent() && is<RenderScrollbarPart>(*this)) {
        if (auto* scrollbarPart = downcast<RenderScrollbarPart>(*this).rendererOwningScrollbar())
            return containingBlockForRenderer(*scrollbarPart);
        return nullptr;
    }
    return containingBlockForRenderer(downcast<RenderElement>(*this));
}

[Darin Adler]

I am not sure this should be changed. The exact meaning of const for pointers into a tree and tree traversal is unclear to me and I definitely don’t want to create two of each tree traversal function. Many, many other functions like Node::parent and Node::document also work this way. I don’t agree with the premise of this bug report.

[zalan]

(In reply to Darin Adler from comment #2)
> I am not sure this should be changed. The exact meaning of const for
> pointers into a tree and tree traversal is unclear to me and I definitely
> don’t want to create two of each tree traversal function. Many, many other
> functions like Node::parent and Node::document also work this way. I don’t
> agree with the premise of this bug report.
I agree. I don't think we should change this. (Though in LFC we managed to achieve const correctness but we did it by turning the layout tree (render tree) immutable as a whole)