ERC 721: canTransfer should modify _transfer function

[lknix]

ERC 721: canTransfer should modify _transfer function

[MoMannn]

_transfer is a private function so it cannot be overridden or called if a contracts inherits from NFToken contract... If anyone is modifying NFToken as it is he can change anything anyway..

[lknix]

Here is my reasoning and possible bug scenarios (all of them dealing with user doing something wrong):

1. User inherits and overrides safeTransferFrom w/o adding modifiers -> open acess.
2. User inherits and adds its own specialTransfer which uses _transfer. Here he has to explicitly check for state change permissions which happen in _transfer. If he fails to do it, then we have open access again.
3. User modifies our existing implementation and does some stupid things like changing _transfer to external or other weird things.

I'm not saying this would "guard" against all stupidity but I think it's a good design if you keep permission checks coupled with the code that does states changes. Wdyt? I don't see any drawbacks but potentially we shrink surface for introducing some bugs.

[MoMannn]

If a user overriders safeTransferFrom he cannot call _transfer from it because it is private.. So neither 1 or 2 can happen and there is no protection for 3.

[lknix]

You're right. Just checked the docs and private functions aren't visible to derived contracts - my bad. OK, this invalidates the 1 and 2 which were the most significant ones. Closing this issue.