basic_facade_builder::support_weakusing support_weak = basic_facade_builder</* see below */>;
The member type support_weak of basic_facade_builder<Cs, Rs, C> adds necessary convention types to allow implicit conversion from proxy<F> to weak_proxy<F> where F is a facade type built from basic_facade_builder.
Let p be a value of type proxy<F>, ptr of type P be the contained value of p (if any), the conversion from type const proxy<F>& to type weak_proxy<F> is equivalent to return typename P::weak_type{ptr} if p contains a value, or otherwise equivalent to return nullptr.
support_weak is compatible with std::weak_ptr, and may generate more efficient code when working with make_proxy_shared or allocate_proxy_shared. It is also compatible with any custom shared/weak ownership implementations if typename P::weak_type{ptr} is well-formed.
#include <iostream>
#include "proxy.h"
struct Formattable : pro::facade_builder
::support_format
::support_weak
::build {};
int main() {
pro::proxy<Formattable> p1 = pro::make_proxy_shared<Formattable>(123);
pro::weak_proxy<Formattable> wp = p1;
pro::proxy<Formattable> p2 = wp.lock();
std::cout << std::boolalpha << p2.has_value() << "\n"; // Prints "true"
std::cout << std::format("{}\n", *p2); // Prints "123"
p1.reset();
p2.reset();
p2 = wp.lock();
std::cout << p2.has_value() << "\n"; // Prints "false"
}