new test_variant_unique_shared with vptr_holder prototype

tests/test_variant_unique_shared.cpp

+#include <memory>
+#include <variant>
+
+#include "pybind11_tests.h"
+
+namespace pybind11_tests {
+
+// Could this be a holder for a `class_`-like `vclass`?
+// To enable passing of unique_ptr as in pure C++.
+template <typename T> class vptr_holder {
+  public:
+    explicit vptr_holder(T *ptr = nullptr) : vptr_{std::unique_ptr<T>(ptr)} {}
+    explicit vptr_holder(std::unique_ptr<T> u) : vptr_{std::move(u)} {}
+    explicit vptr_holder(std::shared_ptr<T> s) : vptr_{s} {}
+
+    int ownership_type() const {
+        if (std::get_if<0>(&vptr_)) {
+            return 0;
+        }
+        if (std::get_if<1>(&vptr_)) {
+            return 1;
+        }
+        return -1;
+    }
+
+    T *get() {
+        auto u = std::get_if<0>(&vptr_);
+        if (u) {
+            return u->get();
+        }
+        auto s = std::get_if<1>(&vptr_);
+        if (s) {
+            return s->get();
+        }
+        return nullptr;
+    }
+
+    std::unique_ptr<T> get_unique() {
+        auto u = std::get_if<0>(&vptr_);
+        if (u) {
+            return std::move(*u);
+        }
+        throw std::runtime_error("get_unique failure.");
+    }
+
+    std::shared_ptr<T> get_shared() {
+        auto s = std::get_if<1>(&vptr_);
+        if (s) {
+            return *s;
+        }
+        auto u = std::get_if<0>(&vptr_);
+        if (u) {
+            auto result = std::shared_ptr<T>(std::move(*u));
+            vptr_ = result;
+            return result;
+        }
+        throw std::runtime_error("get_shared failure.");
+    }
+
+  private:
+    std::variant<std::unique_ptr<T>, std::shared_ptr<T>> vptr_;
+};
+
+vptr_holder<double> from_raw() { return vptr_holder<double>{new double{3}}; }
+
+vptr_holder<double> from_unique() {
+    return vptr_holder<double>{std::unique_ptr<double>(new double{5})};
+}
+
+vptr_holder<double> from_shared() {
+    return vptr_holder<double>{std::shared_ptr<double>(new double{7})};
+}
+
+TEST_SUBMODULE(variant_unique_shared, m) {
+
+    m.def("from_raw", from_raw);
+    m.def("from_unique", from_unique);
+    m.def("from_shared", from_shared);
+
+    py::class_<vptr_holder<double>>(m, "vptr_holder_double")
+        .def(py::init<>())
+        .def("ownership_type", &vptr_holder<double>::ownership_type)
+        .def("get_value",
+             [](vptr_holder<double> &v) {
+                 auto p = v.get();
+                 if (p)
+                     return *p;
+                 return -1.;
+             })
+        .def("get_unique",
+             [](vptr_holder<double> &v) {
+                 v.get_unique();
+                 return;
+             })
+        .def("get_shared", [](vptr_holder<double> &v) {
+            v.get_shared();
+            return;
+        });
+}
+
+} // namespace pybind11_tests

tests/test_variant_unique_shared.py

+# -*- coding: utf-8 -*-
+import pytest
+
+from pybind11_tests import variant_unique_shared as m
+
+
+def test_default_constructed():
+    v = m.vptr_holder_double()
+    assert v.ownership_type() == 0
+    assert v.get_value() == -1
+
+
+def test_from_raw():
+    v = m.from_raw()
+    assert v.ownership_type() == 0
+    assert v.get_value() == 3
+
+
+def test_from_unique():
+    v = m.from_unique()
+    assert v.ownership_type() == 0
+    assert v.get_value() == 5
+
+
+def test_from_shared():
+    v = m.from_shared()
+    assert v.ownership_type() == 1
+    assert v.get_value() == 7
+
+
+def test_promotion_to_shared():
+    v = m.from_raw()
+    v.get_unique()
+    assert v.ownership_type() == 0
+    v.get_shared()  # Promotion to shared_ptr.
+    assert v.ownership_type() == 1
+    v.get_shared()  # Existing shared_ptr.
+    with pytest.raises(RuntimeError) as exc_info:
+        v.get_unique()
+    assert str(exc_info.value) == "get_unique failure."
+    v.get_shared()  # Still works.
+
+
+def test_shared_from_birth():
+    v = m.from_shared()
+    assert v.ownership_type() == 1
+    with pytest.raises(RuntimeError) as exc_info:
+        v.get_unique()
+    assert str(exc_info.value) == "get_unique failure."
+    v.get_shared()  # Still works.