Generic Inheritance-tree misbehavior

[dustinlacewell]

Preface

Assuming you have the sdl2 and perlin packages installed:

• install commit 46e66e41f7205b9f7f292d5fb27d68961517abeb of https://github.com/dustinlacewell/dadren with nimble install
• from dadren/examples/bsp2 execute make run
• use the h, v, and d keys along with the mouse to interact with the demo.

The demo implements a 2D binary-space-partition graph. That is for a given 2D rectangle, split it either vertically or horizontally and get two new rectangles. Repeat.

The implementation of the binary tree is here: https://github.com/dustinlacewell/dadren/blob/46e66e41f7205b9f7f292d5fb27d68961517abeb/dadren/bsp.nim

It implements a simple inheritance tree:

BSPNode[T] -> Leaf[T]   # contains actual T content
BSPNode[T] -> ParentNode[T] -> VSplit[T] # has two BSPNode children
BSPNode[T] -> ParentNode[T] -> HSplit[T] # has two BSPNode children

Intended Implementation

The example that uses this BSP implementation is here: https://github.com/dustinlacewell/dadren/blob/46e66e41f7205b9f7f292d5fb27d68961517abeb/examples/bsp2/main.nim

In that example Color a simple ref object type is used as the T content for building the BSPTree. This all works as expected.

The BSPNode base type defines the parent field as BSPNode. This is not the ideal expression of the program's design. Only a ParentNode should ever be a parent in the graph. Therefore it would be ideal to have the BSPNode.parent field by of type ParentNode. This would avoid the otherwise unnecessary casts anytime the parent field is accessed:

https://github.com/dustinlacewell/dadren/blob/46e66e41f7205b9f7f292d5fb27d68961517abeb/dadren/bsp.nim#L134
https://github.com/dustinlacewell/dadren/blob/46e66e41f7205b9f7f292d5fb27d68961517abeb/dadren/bsp.nim#L160
https://github.com/dustinlacewell/dadren/blob/46e66e41f7205b9f7f292d5fb27d68961517abeb/dadren/bsp.nim#L172
https://github.com/dustinlacewell/dadren/blob/46e66e41f7205b9f7f292d5fb27d68961517abeb/dadren/bsp.nim#L177

Problem

When changing the BSPNode.parent field from type BSPNode to ParentNode the following error is generated:

bsp.nim(30, 20) Error: inheritance only works with non-final objects

With some exploration an unusual result was discovered. Make the following changes:

• change the base type of Color from ref object => ref object of RootObj
• add a field to color: parent: float

The compiler error is now:

bsp.nim(31, 17) Error: type mismatch: got (BSPNode[]) but expected 'float'

This would imply that, even though no type in the BSPNode hierarchy inherits from T, the compiler is some how conflating T into the resolution of BSPNode fields.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. If you think it is still a valid issue, write a comment below; otherwise it will be closed. Thank you for your contributions.