This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

261. When is a deallocation function "used?"

[Moved to DR at October 2002 meeting.]

6.3 [basic.def.odr] paragraph 2 says that a deallocation function is "used" by a new-expression or delete-expression appearing in a potentially-evaluated expression. 6.3 [basic.def.odr] paragraph 3 requires only that "used" functions be defined.

This wording runs afoul of the typical implementation technique for polymorphic delete-expressions in which the deallocation function is invoked from the virtual destructor of the most-derived class. The problem is that the destructor must be defined, because it's virtual, and if it contains an implicit reference to the deallocation function, the deallocation function must also be defined, even if there are no relevant new-expressions or delete-expressions in the program.

For example:

        struct B { virtual ~B() { } };

        struct D: B {
            void operator delete(void*);
            ~D() { }
        };

Is it required that D::operator delete(void*) be defined, even if no B or D objects are ever created or deleted?

Suggested resolution: Add the words "or if it is found by the lookup at the point of definition of a virtual destructor (11.4.7 [class.dtor])" to the specification in 6.3 [basic.def.odr] paragraph 2.

Notes from 04/01 meeting:

The consensus was in favor of requiring that any declared non-placement operator delete member function be defined if the destructor for the class is defined (whether virtual or not), and similarly for a non-placement operator new if a constructor is defined.

Proposed resolution (10/01):

In 6.3 [basic.def.odr] paragraph 2, add the indicated text:

An allocation or deallocation function for a class is used by a new expression appearing in a potentially-evaluated expression as specified in 7.6.2.8 [expr.new] and 11.4.11 [class.free]. A deallocation function for a class is used by a delete expression appearing in a potentially-evaluated expression as specified in 7.6.2.9 [expr.delete] and 11.4.11 [class.free]. A non-placement allocation or deallocation function for a class is used by the definition of a constructor of that class. A non-placement deallocation function for a class is used by the definition of the destructor of that class, or by being selected by the lookup at the point of definition of a virtual destructor (11.4.7 [class.dtor]). [Footnote: An implementation is not required to call allocation and deallocation functions from constructors or destructors; however, this is a permissible implementation technique.]