Use private virtual key_function() to address -Wweak-vtables (#1487)

Author: eramongodb

src/bsoncxx/include/bsoncxx/v1/exception.hpp

@@ -97,14 +97,10 @@ BSONCXX_PRIVATE_WARNINGS_DISABLE(MSVC(4275));
 ///
 class exception : public std::system_error {
    public:
-    ~exception() override;
-
-    exception(exception&&) noexcept = default;
-    exception& operator=(exception&&) noexcept = default;
-    exception(exception const&) = default;
-    exception& operator=(exception const&) = default;
-
     using std::system_error::system_error;
+
+   private:
+    BSONCXX_ABI_NO_EXPORT virtual void key_function() const;
 };
 
 BSONCXX_PRIVATE_WARNINGS_POP();

src/bsoncxx/lib/bsoncxx/v1/exception.cpp

@@ -85,7 +85,14 @@ std::error_category const& type_error_category() {
 namespace bsoncxx {
 namespace v1 {
 
-exception::~exception() = default;
+// Prevent vague linkage of the vtable and type_info object (-Wweak-vtables).
+// - https://itanium-cxx-abi.github.io/cxx-abi/abi.html#vague-vtable
+//   > The key function is the first non-pure virtual function that is not inline at the point of class definition.
+// - https://lld.llvm.org/missingkeyfunction:
+//   > It’s always advisable to ensure there is at least one eligible function that can serve as the key function.
+// - https://gcc.gnu.org/onlinedocs/gcc/Vague-Linkage.html
+//   > For polymorphic classes (classes with virtual functions), the ‘type_info’ object is written out along with the vtable.
+void exception::key_function() const {}
 
 } // namespace v1
 } // namespace bsoncxx

src/mongocxx/include/mongocxx/v1/exception.hpp

@@ -100,14 +100,10 @@ BSONCXX_PRIVATE_WARNINGS_DISABLE(MSVC(4275));
 ///
 class exception : public std::system_error {
    public:
-    ~exception() override;
-
-    exception(exception&&) noexcept = default;
-    exception& operator=(exception&&) noexcept = default;
-    exception(exception const&) = default;
-    exception& operator=(exception const&) = default;
-
     using std::system_error::system_error;
+
+   private:
+    MONGOCXX_ABI_NO_EXPORT virtual void key_function() const;
 };
 
 BSONCXX_PRIVATE_WARNINGS_POP();

src/mongocxx/lib/mongocxx/v1/exception.cpp

@@ -83,7 +83,14 @@ std::error_category const& type_error_category() {
     return instance.value();
 }
 
-exception::~exception() = default;
+// Prevent vague linkage of the vtable and type_info object (-Wweak-vtables).
+// - https://itanium-cxx-abi.github.io/cxx-abi/abi.html#vague-vtable
+//   > The key function is the first non-pure virtual function that is not inline at the point of class definition.
+// - https://lld.llvm.org/missingkeyfunction:
+//   > It’s always advisable to ensure there is at least one eligible function that can serve as the key function.
+// - https://gcc.gnu.org/onlinedocs/gcc/Vague-Linkage.html
+//   > For polymorphic classes (classes with virtual functions), the ‘type_info’ object is written out along with the vtable.
+void exception::key_function() const {}
 
 } // namespace v1
 } // namespace mongocxx