operators: Explicitly expose `py::hash(py::self)`

Add warnings about extending STL

include/pybind11/operators.h

@@ -147,6 +147,9 @@ PYBIND11_INPLACE_OPERATOR(ixor,     operator^=,   l ^= r)
 PYBIND11_INPLACE_OPERATOR(ior,      operator|=,   l |= r)
 PYBIND11_UNARY_OPERATOR(neg,        operator-,    -l)
 PYBIND11_UNARY_OPERATOR(pos,        operator+,    +l)
+// WARNING: This usage of `abs` should only be done for existing STL overloads.
+// Adding overloads directly in to the `std::` namespace is advised against:
+// https://en.cppreference.com/w/cpp/language/extending_std
 PYBIND11_UNARY_OPERATOR(abs,        abs,          std::abs(l))
 PYBIND11_UNARY_OPERATOR(hash,       hash,         std::hash<L>()(l))
 PYBIND11_UNARY_OPERATOR(invert,     operator~,    (~l))
@@ -160,6 +163,8 @@ PYBIND11_UNARY_OPERATOR(float,      float_,       (double) l)
 NAMESPACE_END(detail)
 
 using detail::self;
+// Add named operators so that they are accessible via `py::`.
+using detail::hash;
 
 NAMESPACE_END(PYBIND11_NAMESPACE)
 

tests/test_operator_overloading.cpp

@@ -43,6 +43,13 @@ public:
     friend Vector2 operator-(float f, const Vector2 &v) { return Vector2(f - v.x, f - v.y); }
     friend Vector2 operator*(float f, const Vector2 &v) { return Vector2(f * v.x, f * v.y); }
     friend Vector2 operator/(float f, const Vector2 &v) { return Vector2(f / v.x, f / v.y); }
+
+    bool operator==(const Vector2 &v) const {
+        return x == v.x && y == v.y;
+    }
+    bool operator!=(const Vector2 &v) const {
+        return x != v.x || y != v.y;
+    }
 private:
     float x, y;
 };
@@ -55,6 +62,11 @@ int operator+(const C2 &, const C2 &) { return 22; }
 int operator+(const C2 &, const C1 &) { return 21; }
 int operator+(const C1 &, const C2 &) { return 12; }
 
+// Note: Specializing explicit within `namespace std { ... }` is done due to a
+// bug in GCC<7. If you are supporting compilers later than this, consider
+// specializing `using template<> struct std::hash<...>` in the global
+// namespace instead, per this recommendation:
+// https://en.cppreference.com/w/cpp/language/extending_std#Adding_template_specializations
 namespace std {
     template<>
     struct hash<Vector2> {
@@ -63,6 +75,11 @@ namespace std {
     };
 }
 
+// Not a good abs function, but easy to test.
+std::string abs(const Vector2&) {
+    return "abs(Vector2)";
+}
+
 // MSVC warns about unknown pragmas, and warnings are errors.
 #ifndef _MSC_VER
   #pragma GCC diagnostic push
@@ -107,7 +124,13 @@ TEST_SUBMODULE(operators, m) {
         .def(float() / py::self)
         .def(-py::self)
         .def("__str__", &Vector2::toString)
-        .def(hash(py::self))
+        .def("__repr__", &Vector2::toString)
+        .def(py::self == py::self)
+        .def(py::self != py::self)
+        .def(py::hash(py::self))
+        // N.B. See warning about usage of `py::detail::abs(py::self)` in
+        // `operators.h`.
+        .def("__abs__", [](const Vector2& v) { return abs(v); })
         ;
 
     m.attr("Vector") = m.attr("Vector2");

tests/test_operator_overloading.py

@@ -6,6 +6,9 @@ from pybind11_tests import ConstructorStats
 def test_operator_overloading():
     v1 = m.Vector2(1, 2)
     v2 = m.Vector(3, -1)
+    v3 = m.Vector2(1, 2)  # Same value as v1, but different instance.
+    assert v1 is not v3
+
     assert str(v1) == "[1.000000, 2.000000]"
     assert str(v2) == "[3.000000, -1.000000]"
 
@@ -24,7 +27,11 @@ def test_operator_overloading():
     assert str(v1 * v2) == "[3.000000, -2.000000]"
     assert str(v2 / v1) == "[3.000000, -0.500000]"
 
+    assert v1 == v3
+    assert v1 != v2
     assert hash(v1) == 4
+    # TODO(eric.cousineau): Make this work.
+    # assert abs(v1) == "abs(Vector2)"
 
     v1 += 2 * v2
     assert str(v1) == "[7.000000, 0.000000]"
@@ -40,14 +47,17 @@ def test_operator_overloading():
     assert str(v2) == "[2.000000, 8.000000]"
 
     cstats = ConstructorStats.get(m.Vector2)
-    assert cstats.alive() == 2
+    assert cstats.alive() == 3
     del v1
-    assert cstats.alive() == 1
+    assert cstats.alive() == 2
     del v2
+    assert cstats.alive() == 1
+    del v3
     assert cstats.alive() == 0
     assert cstats.values() == [
         '[1.000000, 2.000000]',
         '[3.000000, -1.000000]',
+        '[1.000000, 2.000000]',
         '[-3.000000, 1.000000]',
         '[4.000000, 1.000000]',
         '[-2.000000, 3.000000]',