std::function<void(Foo &)> reports error when Foo is non-copyable and non-movable

[qianyizh]

This might be related to #765 and #1041

My problem is that I have a non-copyable and non-movable class Foo that I want to change in a callback function.

The C++ code is:

struct Foo {
	int a = 0;
	Foo() = default;
	Foo(const Foo &) = delete;
	Foo &operator=(const Foo &) = delete;
	Foo(Foo &&o) = delete;
	Foo &operator=(Foo &&o) = delete;
};

py::class_<Foo> foo(m, "Foo");
foo.def_readwrite("a", &Foo::a);
m.def("test_foo", [](std::function<void(Foo &)> f) {
	Foo x;
	f(x);
	return x.a;
});

The python code is:

def test_obj():
    def change_value(x):
        x.a = 1
    assert m.test_foo(change_value) == 1

This gives an error:

RuntimeError: return_value_policy = copy, but the object is non-copyable!

The real problem is that I want to change the return policy of std::function<void(Foo &)> f to return_policy_type::reference but it does not seem possible.

[qianyizh]

There is indeed a workaround. If test_foo() is a member function of Foo then everything works as expected. But this is ugly and not ideal.

C++ code:

struct Foo {
	int a = 0;
	Foo() = default;
	Foo(const Foo &) = delete;
	Foo &operator=(const Foo &) = delete;
	Foo(Foo &&o) = delete;
	Foo &operator=(Foo &&o) = delete;
};
py::class_<Foo> foo(m, "Foo");
foo.def("__init__", [](Foo &t) {
	new (&t)Foo();
	});
foo.def_readwrite("a", &Foo::a);
foo.def("test_foo", [](Foo &x, std::function<void(Foo &)> f) {
	f(x);				// x.a = 1
	return x.a;
}, "f"_a);

Python code:

def test_obj():
    def change_value(x):
        x.a = 1
    x = m.Foo()
    assert x.test_foo(change_value) == 1

[qianyizh]

Actually there is an easy workaround.
Change std::function<void(Foo &)> to std::function<void(Foo *)>.