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Alias template weak_proxy
Class template weak_facade

Header: proxy.h
Module: proxy
Namespace: pro::inline v4
Since: 3.3.0

template <facade F> struct weak_facade; template <facade F> using weak_proxy = proxy<weak_facade<F>>;

weak_proxy<F> is a non-owning handle that participates in the weak ownership model of an object that (when alive) models proxiable_target<T, F>. It is analogous to std::weak_ptr relative to std::shared_ptr: it never extends the lifetime of the underlying object, but it can attempt to produce a (strong) proxy<F> via lock().

weak_facade<F> adapts the original facade F so that:

  • A lock() member (direct convention) is provided, returning a proxy<F> that contains a value if and only if the referenced object is still alive at the time of the call.
  • All direct substitution conversions that would have produced a proxy<G> become conversions that produce a weak_proxy<G> instead (so that "weak-ness" is preserved across facade-substitution).
  • No reflections from F are preserved.

Member Types of weak_facade

Name Description
convention_types A tuple-like type transformed from typename F::convention_types. Specifically:
- It always prepends a direct convention whose dispatch type denotes the member function lock and whose single overload has signature proxy<F>() const noexcept. Calling this overload attempts to obtain a strong proxy<F>; it returns an empty proxy<F> if the object has expired.
- For any direct convention C in F whose dispatch_type is substitution_dispatch, a transformed convention C' is included, whose
* is_direct is true and dispatch_type is still substitution_dispatch.
* For every overload O in typename C::overload_types with signature (after cv/ref/noexcept qualifiers) returning a proxy<G>, replace its return type with weak_proxy<G> while preserving qualifiers and noexcept.
- All other conventions from F are discarded.
reflection_types A tuple-like type that contains no types.

Member Constants of weak_facade

Name Definition
max_size [static] [constexpr] F::max_size
max_align [static] [constexpr] F::max_align
copyability [static] [constexpr] F::copyability
relocatability [static] [constexpr] F::relocatability
destructibility [static] [constexpr] F::destructibility

Example

#include <iostream> #include <proxy/proxy.h> struct Formattable : pro::facade_builder //  ::add_skill<pro::skills::format> //  ::build {}; int main() {  std::shared_ptr<int> val = std::make_shared<int>(123);  pro::weak_proxy<Formattable> wp = std::weak_ptr{val};  pro::proxy<Formattable> p = wp.lock();  std::cout << std::boolalpha << p.has_value() << "\n"; // Prints "true"  std::cout << std::format("{}\n", *p); // Prints "123"  p.reset();  val.reset();  p = wp.lock();  std::cout << p.has_value() << "\n"; // Prints "false" }

See Also