Allow perfect forwarding of method args (#2048)

Author: rhaschke

include/pybind11/pybind11.h

@@ -75,7 +75,7 @@ class cpp_function : public function {
     /// Construct a cpp_function from a class method (non-const, no ref-qualifier)
     template <typename Return, typename Class, typename... Arg, typename... Extra>
     cpp_function(Return (Class::*f)(Arg...), const Extra&... extra) {
-        initialize([f](Class *c, Arg... args) -> Return { return (c->*f)(args...); },
+        initialize([f](Class *c, Arg... args) -> Return { return (c->*f)(std::forward<Arg>(args)...); },
                    (Return (*) (Class *, Arg...)) nullptr, extra...);
     }
 
@@ -91,7 +91,7 @@ class cpp_function : public function {
     /// Construct a cpp_function from a class method (const, no ref-qualifier)
     template <typename Return, typename Class, typename... Arg, typename... Extra>
     cpp_function(Return (Class::*f)(Arg...) const, const Extra&... extra) {
-        initialize([f](const Class *c, Arg... args) -> Return { return (c->*f)(args...); },
+        initialize([f](const Class *c, Arg... args) -> Return { return (c->*f)(std::forward<Arg>(args)...); },
                    (Return (*)(const Class *, Arg ...)) nullptr, extra...);
     }
 

tests/test_methods_and_attributes.cpp

@@ -21,6 +21,7 @@ class ExampleMandA {
     ExampleMandA() { print_default_created(this); }
     ExampleMandA(int value) : value(value) { print_created(this, value); }
     ExampleMandA(const ExampleMandA &e) : value(e.value) { print_copy_created(this); }
+    ExampleMandA(std::string&&) {}
     ExampleMandA(ExampleMandA &&e) : value(e.value) { print_move_created(this); }
     ~ExampleMandA() { print_destroyed(this); }
 
@@ -43,6 +44,8 @@ class ExampleMandA {
     void add9(int *other) { value += *other; }                      // passing by pointer
     void add10(const int *other) { value += *other; }               // passing by const pointer
 
+    void consume_str(std::string&&) {}
+
     ExampleMandA self1() { return *this; }                          // return by value
     ExampleMandA &self2() { return *this; }                         // return by reference
     const ExampleMandA &self3() { return *this; }                   // return by const reference
@@ -150,6 +153,7 @@ TEST_SUBMODULE(methods_and_attributes, m) {
     py::class_<ExampleMandA> emna(m, "ExampleMandA");
     emna.def(py::init<>())
         .def(py::init<int>())
+        .def(py::init<std::string&&>())
         .def(py::init<const ExampleMandA&>())
         .def("add1", &ExampleMandA::add1)
         .def("add2", &ExampleMandA::add2)
@@ -161,6 +165,7 @@ TEST_SUBMODULE(methods_and_attributes, m) {
         .def("add8", &ExampleMandA::add8)
         .def("add9", &ExampleMandA::add9)
         .def("add10", &ExampleMandA::add10)
+        .def("consume_str", &ExampleMandA::consume_str)
         .def("self1", &ExampleMandA::self1)
         .def("self2", &ExampleMandA::self2)
         .def("self3", &ExampleMandA::self3)

tests/test_methods_and_attributes.py

@@ -58,8 +58,8 @@ def test_methods_and_attributes():
     assert cstats.alive() == 0
     assert cstats.values() == ["32"]
     assert cstats.default_constructions == 1
-    assert cstats.copy_constructions == 3
-    assert cstats.move_constructions >= 1
+    assert cstats.copy_constructions == 2
+    assert cstats.move_constructions >= 2
     assert cstats.copy_assignments == 0
     assert cstats.move_assignments == 0