You can't turn a capturing C++ lambda into a WNDPROC, which is an ordinary function pointer. You'd still have to ferry the lambda via a context pointer, which is what this blog post and the other solutions in the comments are all about.

You kind of can, that is one of their design points, naturally you need to move the context into the body and know what to cast back from.

I guess I need to prove a point on my Github during next week.

If you mean that you can call a C++ lambda from a static C callback via a context pointer, of course you can do that, it's not very mind-boggling. Rust FFI libraries similarly have to do that trick all the time to turn a closure into a C callback. The primary problem with WNDPROC is how to get that context pointer in the first place, which is the part that OP and everyone in the comments are talking about.

No need for context pointer,

https://github.com/pjmlp/LambdaWndProc/blob/main/LambdaWndPr...

I assume by "move the context into the body" you mean using GetWindowLongPtr? Why not just use a static wndproc at that point?

I mean using a static C++ lambda that moves the context into the lambda body via capture specifier.

C++ lambdas are basically old style C++ functors that are compiled generated, with the calling address being the operator().

That doesn't sound like a valid wndproc

It certainly is,

https://github.com/pjmlp/LambdaWndProc/blob/main/LambdaWndPr...

It looks like you missed the part where you "move the context into the lambda body via capture specifier."