--enable-thread-safe-observer-pattern doesn't work with --enable-std-pointers

[lballabio]

Passing both flags to configure result in a number of compilation errors, due primarily to the use of boost::enable_shared_from_this (which should be imported into the ext namespace in <ql/shared_ptr.hpp>) and secondarily by a few methods in the implementation whose call doesn't work.

[klausspanderen]

Hi Luigi,

I think the thread safe observer pattern will only work with C++17. The implementation uses
boost::enable_shared_from_this::weak_from_this() aka
std::enable_shared_from_this::weak_from_this(),
which is only available in C++17 onward. Shall we add a check for both flags in the configure script / userconfig.hpp?

[lballabio]

There's also a call to _empty() that only works with Boost, and possibly something else that I don't remember.

I don't know about the check in configure. I'd try not to make it overly complex...

[klausspanderen]

The C++17 port of the observer pattern can be found here
https://github.com/haozhangphd/QuantLib-noBoost
The call to _empty() is replaced by expired().

[lballabio]

Ok. Then I'd make sure it works, and then I would just document in configure --help that in order to use both switches C++17 is required.