interop: add weak_virtual_ptr for std::weak_ptr

[jll63]

Closes #73.

Summary

Adds <boost/openmethod/interop/std_weak_ptr.hpp>, providing weak_virtual_ptr<Class>, an alias for virtual_ptr<std::weak_ptr<Class>>. It tracks an object with a std::weak_ptr and remembers its v-table pointer, so lock() returns a shared_virtual_ptr with no hash table lookup — it costs exactly std::weak_ptr::lock(). Remembering the vptr is safe: the weak pointer keeps the control block alive, so once the object is destroyed it stays expired for good.

It is a storage facility only:

• constructed/assigned from a shared_virtual_ptr (copies the vptr; the class need not be polymorphic), from another weak_virtual_ptr (upcasts), or from a std::shared_ptr / std::weak_ptr (acquires the vptr; expired ⇒ null vptr);
• lock(), expired(), use_count(), reset(), pointer(), vptr();
• deliberately no get(), operator*, operator->, cast<>() or make_weak_virtual;
• not usable as a virtual parameter, neither as virtual_<std::weak_ptr<T>> nor as weak_virtual_ptr<T> (by value, &, const&): validate_method_parameter specializations reject both with "a weak pointer cannot be a virtual parameter; call lock() first".

shared_virtual_ptr<Animal> animal = make_shared_virtual<Dog>();
weak_virtual_ptr<Animal> observer = animal;

std::cout << poke(observer.lock()) << "\n"; // bark

animal = nullptr;
std::cout << std::boolalpha << observer.expired() << "\n"; // true

Design notes

• std::weak_ptr does not fit the generic smart-pointer specialization of virtual_ptr (it needs get(), operator* and a conversion to bool), so the specialization is written by hand. virtual_traits is deliberately not specialized for std::weak_ptr: that keeps IsSmartPtr false, which is what lets the hand-written specialization win over the generic one.
• One generic change in core.hpp: the plain virtual_ptr's converting constructor and assignment from another virtual_ptr now also require the source to have get() (detail::has_get, a partially specialized constexpr bool variable template). A weak source is then a clean substitution failure instead of a hard error in the body, and std::is_constructible_v reports it correctly.

Tests and docs

• test/test_weak_virtual_ptr.cpp: 7 cases over the 4 direct/indirect × vector/map registries, plus static_asserts on the allowed and rejected conversions.
• test/compile_fail_weak_ptr_parameter.cpp and test/compile_fail_weak_virtual_ptr_parameter.cpp, one per rejected spelling.
• "Weak Pointers" section in smart_pointers.adoc, ref_headers.adoc entries, and weak_lock / weak_virtual_ptr_alias tags in the smart_pointers.cpp snippet (referenced by the MrDocs comments).

Verified locally: gcc 13.3 Debug -Werror, full suite 162/162; clang 22 on the new tests, compile-fail tests and snippet. Both compile-fail tests report the intended static_assert as the first error on both compilers. Not verified: a MrDocs build (not installed here).

🤖 Generated with Claude Code

[cppalliance-bot]

An automated preview of the documentation is available at https://100.openmethod.prtest3.cppalliance.org/libs/openmethod/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2026-09-05 20:26:23 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.77%. Comparing base (5ec487c) to head (9ab25da).
⚠️ Report is 8 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #100      +/-   ##
===========================================
+ Coverage    93.46%   93.77%   +0.31%     
===========================================
  Files           22       23       +1     
  Lines         1653     1752      +99     
  Branches       500      512      +12     
===========================================
+ Hits          1545     1643      +98     
  Misses          64       64              
- Partials        44       45       +1     

Files with missing lines	Coverage Δ
include/boost/openmethod/core.hpp	93.09% <ø> (ø)
include/boost/openmethod/interop/std_weak_ptr.hpp	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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