We investigated public domain, but that has international legal problems with it. Boost was the best solution.

Do you have any comment son international legal issues with public domain? Is that not recognized in some places?

The main problem with the public domain is that there is no such thing as "the public domain". Each country has its own copyright law, with its own terms and conditions. What the public domain means in the US is not what the public domain means elsewhere i.e. Germany. In Germany an author has Moral Rights which the author is incapable of giving up and may only transfer as apart of their will. So rather than require all German contributors to be dead and have a will that grants their moral rights to the project, using a permissive license gets the same effect.

SQLite ran into trouble by using public domain, due to it not being recognized and for other reasons, and as a result will sell you a copy under a different license that guarantees your rights.

https://www.sqlite.org/copyright.html

It was not recognized in some countries, as I recall. Maybe things have changed since we decided on Boost a decade ago, but who cares. Boost works.

Several European countries do not recognize the ability of copyright holders to completely give up their copyright. So it needs a license of some kind.

And that's why the Unlicense has a BSD-style "Anyone is free to…" clause.

What about CC0? I thought it was explicitly made to be a public domain replacement where there is no "public domain"?