Support pointers to member functions in def_buffer.

Closes #857, by adding overloads to def_buffer that match pointers to
member functions and wrap them in lambdas.

include/pybind11/pybind11.h

@@ -1020,6 +1020,16 @@ public:
         return *this;
     }
 
+    template <typename Return, typename Class, typename... Args>
+    class_ &def_buffer(Return (Class::*func)(Args...)) {
+        return def_buffer([func] (type &obj) { return (obj.*func)(); });
+    }
+
+    template <typename Return, typename Class, typename... Args>
+    class_ &def_buffer(Return (Class::*func)(Args...) const) {
+        return def_buffer([func] (const type &obj) { return (obj.*func)(); });
+    }
+
     template <typename C, typename D, typename... Extra>
     class_ &def_readwrite(const char *name, D C::*pm, const Extra&... extra) {
         cpp_function fget([pm](const C &c) -> const D &{ return c.*pm; }, is_method(*this)),

tests/test_buffers.cpp

@@ -74,6 +74,32 @@ private:
     float *m_data;
 };
 
+struct PTMFBuffer {
+    int32_t value = 0;
+
+    py::buffer_info get_buffer_info() {
+        return py::buffer_info(&value, sizeof(value),
+                               py::format_descriptor<int32_t>::format(), 1);
+    }
+};
+
+class ConstPTMFBuffer {
+    std::unique_ptr<int32_t> value;
+
+public:
+    int32_t get_value() const { return *value; }
+    void set_value(int32_t v) { *value = v; }
+
+    py::buffer_info get_buffer_info() const {
+        return py::buffer_info(value.get(), sizeof(*value),
+                               py::format_descriptor<int32_t>::format(), 1);
+    }
+
+    ConstPTMFBuffer() : value(new int32_t{0}) { };
+};
+
+struct DerivedPTMFBuffer : public PTMFBuffer { };
+
 test_initializer buffers([](py::module &m) {
     py::class_<Matrix> mtx(m, "Matrix", py::buffer_protocol());
 
@@ -114,4 +140,21 @@ test_initializer buffers([](py::module &m) {
             );
         })
         ;
+
+    py::class_<PTMFBuffer>(m, "PTMFBuffer", py::buffer_protocol())
+        .def(py::init<>())
+        .def_readwrite("value", &PTMFBuffer::value)
+        .def_buffer(&PTMFBuffer::get_buffer_info);
+
+    py::class_<ConstPTMFBuffer>(m, "ConstPTMFBuffer", py::buffer_protocol())
+        .def(py::init<>())
+        .def_property("value", &ConstPTMFBuffer::get_value, &ConstPTMFBuffer::set_value)
+        .def_buffer(&ConstPTMFBuffer::get_buffer_info);
+
+    // Tests that passing a pointer to member to the base class works in
+    // the derived class.
+    py::class_<DerivedPTMFBuffer>(m, "DerivedPTMFBuffer", py::buffer_protocol())
+        .def(py::init<>())
+        .def_readwrite("value", (int32_t DerivedPTMFBuffer::*) &DerivedPTMFBuffer::value)
+        .def_buffer(&DerivedPTMFBuffer::get_buffer_info);
 });

tests/test_buffers.py

@@ -1,5 +1,6 @@
+import struct
 import pytest
-from pybind11_tests import Matrix, ConstructorStats
+from pybind11_tests import Matrix, ConstructorStats, PTMFBuffer, ConstPTMFBuffer, DerivedPTMFBuffer
 
 pytestmark = pytest.requires_numpy
 
@@ -60,3 +61,12 @@ def test_to_python():
     # assert cstats.move_constructions >= 0  # Don't invoke any
     assert cstats.copy_assignments == 0
     assert cstats.move_assignments == 0
+
+
+@pytest.unsupported_on_pypy
+def test_ptmf():
+    for cls in [PTMFBuffer, ConstPTMFBuffer, DerivedPTMFBuffer]:
+        buf = cls()
+        buf.value = 0x12345678
+        value = struct.unpack('i', bytearray(buf))[0]
+        assert value == 0x12345678