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Updated C.35 to address Issue 375

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hsutter
2015-12-02 11:25:32 -08:00
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@@ -3695,14 +3695,19 @@ Also, that may affect ABIs.
* A class with a reference data member is suspect. * A class with a reference data member is suspect.
* A class with an `owner<T>` reference should define its default operations. * A class with an `owner<T>` reference should define its default operations.
### <a name="Rc-dtor-virtual"></a> C.35: A base class with a virtual function needs a virtual destructor ### <a name="Rc-dtor-virtual"></a> C.35: A base class destructor should be either public and virtual, or protected and nonvirtual
##### Reason ##### Reason
To prevent undefined behavior. To prevent undefined behavior.
If an application attempts to delete a derived class object through a base class pointer, the result is undefined if the base class's destructor is non-virtual. If the destructor is public, then calling code can attempt to destroy a derived class object through a base class pointer, and the result is undefined if the base class's destructor is non-virtual.
If the destructor is protected, then calling code cannot destroy through a base class pointer and the destructor does not need to be virtual; it does need to be protected, not private, so that derived destructors can invoke it.
In general, the writer of a base class does not know the appropriate action to be done upon destruction. In general, the writer of a base class does not know the appropriate action to be done upon destruction.
##### Discussion
See [this in the Discussion section](#Sd-dtor).
##### Example, bad ##### Example, bad
struct Base { // BAD: no virtual destructor struct Base { // BAD: no virtual destructor
@@ -3724,11 +3729,11 @@ In general, the writer of a base class does not know the appropriate action to b
##### Note ##### Note
A virtual function defines an interface to derived classes that can be used without looking at the derived classes. A virtual function defines an interface to derived classes that can be used without looking at the derived classes.
Someone using such an interface is likely to also destroy using that interface. If the interface allows destroying, it should be safe to do so.
##### Note ##### Note
A destructor must be `public` or it will prevent stack allocation and normal heap allocation via smart pointer (or in legacy code explicit `delete`): A destructor must be nonprivate or it will prevent using the type :
class X { class X {
~X(); // private destructor ~X(); // private destructor
@@ -3741,9 +3746,14 @@ A destructor must be `public` or it will prevent stack allocation and normal hea
auto p = make_unique<X>(); // error: cannot destroy auto p = make_unique<X>(); // error: cannot destroy
} }
##### Exception
We can imagine one case where you could want a protected virtual destructor: When an object of a derived type (and only of such a type) should be allowed to destroy *another* object (not itself) through a pointer to base. We haven't seen such a case in practice, though.
##### Enforcement ##### Enforcement
(Simple) A class with any virtual functions should have a virtual destructor. * A class with any virtual functions should have a destructor that is either public and virtual or else protected and nonvirtual.
### <a name="Rc-dtor-fail"></a> C.36: A destructor may not fail ### <a name="Rc-dtor-fail"></a> C.36: A destructor may not fail