Test and document binding protected member functions

docs/advanced/classes.rst

@@ -916,3 +916,78 @@ Python type created elsewhere.
 
     The file :file:`tests/test_local_bindings.cpp` contains additional examples
     that demonstrate how ``py::module_local()`` works.
+
+Binding protected member functions
+==================================
+
+It's normally not possible to expose ``protected`` member functions to Python:
+
+.. code-block:: cpp
+
+    class A {
+    protected:
+        int foo() const { return 42; }
+    };
+
+    py::class_<A>(m, "A")
+        .def("foo", &A::foo); // error: 'foo' is a protected member of 'A'
+
+On one hand, this is good because non-``public`` members aren't meant to be
+accessed from the outside. But we may want to make use of ``protected``
+functions in derived Python classes.
+
+The following pattern makes this possible:
+
+.. code-block:: cpp
+
+    class A {
+    protected:
+        int foo() const { return 42; }
+    };
+
+    class Publicist : public A { // helper type for exposing protected functions
+    public:
+        using A::foo; // inherited with different access modifier
+    };
+
+    py::class_<A>(m, "A") // bind the primary class
+        .def("foo", &Publicist::foo); // expose protected methods via the publicist
+
+This works because ``&Publicist::foo`` is exactly the same function as
+``&A::foo`` (same signature and address), just with a different access
+modifier. The only purpose of the ``Publicist`` helper class is to make
+the function name ``public``.
+
+If the intent is to expose ``protected`` ``virtual`` functions which can be
+overridden in Python, the publicist pattern can be combined with the previously
+described trampoline:
+
+.. code-block:: cpp
+
+    class A {
+    public:
+        virtual ~A() = default;
+
+    protected:
+        virtual int foo() const { return 42; }
+    };
+
+    class Trampoline : public A {
+    public:
+        int foo() const override { PYBIND11_OVERLOAD(int, A, foo, ); }
+    };
+
+    class Publicist : public A {
+    public:
+        using A::foo;
+    };
+
+    py::class_<A, Trampoline>(m, "A") // <-- `Trampoline` here
+        .def("foo", &Publicist::foo); // <-- `Publicist` here, not `Trampoline`!
+
+.. note::
+
+    MSVC 2015 has a compiler bug (fixed in version 2017) which
+    requires a more explicit function binding in the form of
+    ``.def("foo", static_cast<int (A::*)() const>(&Publicist::foo));``
+    where ``int (A::*)() const`` is the type of ``A::foo``.

tests/test_class.cpp

@@ -229,6 +229,57 @@ TEST_SUBMODULE(class_, m) {
     // This test is actually part of test_local_bindings (test_duplicate_local), but we need a
     // definition in a different compilation unit within the same module:
     bind_local<LocalExternal, 17>(m, "LocalExternal", py::module_local());
+
+    // test_bind_protected_functions
+    class ProtectedA {
+    protected:
+        int foo() const { return value; }
+
+    private:
+        int value = 42;
+    };
+
+    class PublicistA : public ProtectedA {
+    public:
+        using ProtectedA::foo;
+    };
+
+    py::class_<ProtectedA>(m, "ProtectedA")
+        .def(py::init<>())
+#if !defined(_MSC_VER) || _MSC_VER >= 1910
+        .def("foo", &PublicistA::foo);
+#else
+        .def("foo", static_cast<int (ProtectedA::*)() const>(&PublicistA::foo));
+#endif
+
+    class ProtectedB {
+    public:
+        virtual ~ProtectedB() = default;
+
+    protected:
+        virtual int foo() const { return value; }
+
+    private:
+        int value = 42;
+    };
+
+    class TrampolineB : public ProtectedB {
+    public:
+        int foo() const override { PYBIND11_OVERLOAD(int, ProtectedB, foo, ); }
+    };
+
+    class PublicistB : public ProtectedB {
+    public:
+        using ProtectedB::foo;
+    };
+
+    py::class_<ProtectedB, TrampolineB>(m, "ProtectedB")
+        .def(py::init<>())
+#if !defined(_MSC_VER) || _MSC_VER >= 1910
+        .def("foo", &PublicistB::foo);
+#else
+        .def("foo", static_cast<int (ProtectedB::*)() const>(&PublicistB::foo));
+#endif
 }
 
 template <int N> class BreaksBase { public: virtual ~BreaksBase() = default; };

tests/test_class.py

@@ -176,3 +176,22 @@ def test_operator_new_delete(capture):
         "C delete " + sz_noalias + "\n" +
         "C delete " + sz_alias + "\n"
     )
+
+
+def test_bind_protected_functions():
+    """Expose protected member functions to Python using a helper class"""
+    a = m.ProtectedA()
+    assert a.foo() == 42
+
+    b = m.ProtectedB()
+    assert b.foo() == 42
+
+    class C(m.ProtectedB):
+        def __init__(self):
+            m.ProtectedB.__init__(self)
+
+        def foo(self):
+            return 0
+
+    c = C()
+    assert c.foo() == 0