Title
Abstract base constructors and virtual base initialization
Status
cd2
Section
11.9.3 [class.base.init]
Submitter
Mike Miller

Created on 2000-11-01.00:00:00 last changed 139 months ago

Messages

Date: 2009-10-15.00:00:00

[Voted into WP at October, 2009 meeting.]

Date: 2009-07-15.00:00:00

Proposed resolution (July, 2009):

  1. Add the indicated text (moved from paragraph 11) to the end of 11.9.3 [class.base.init] paragraph 7:

  2. ...The initialization of each base and member constitutes a full-expression. Any expression in a mem-initializer is evaluated as part of the full-expression that performs the initialization. A mem-initializer where the mem-initializer-id names a virtual base class is ignored during execution of a constructor of any class that is not the most derived class.
  3. Change 11.9.3 [class.base.init] paragraph 8 as follows:

  4. If a given non-static data member or base class is not named by a mem-initializer-id (including the case where there is no mem-initializer-list because the constructor has no ctor-initializer) and the entity is not a virtual base class of an abstract class (11.7.4 [class.abstract]), then

    • if the entity is a non-static data member that has a brace-or-equal-initializer, the entity is initialized as specified in 9.4 [dcl.init];

    • otherwise, if the entity is a variant member (11.5 [class.union]), no initialization is performed;

    • otherwise, the entity is default-initialized (9.4 [dcl.init]).

    [Note: An abstract class (11.7.4 [class.abstract]) is never a most derived class, thus its constructors never initialize virtual base classes, therefore the corresponding mem-initializers may be omitted. —end note] After the call to a constructor for class X has completed...

  5. Change 11.9.3 [class.base.init] paragraph 10 as follows:

  6. Initialization shall proceed proceeds in the following order:

    • First, and only for the constructor of the most derived class as described below (6.7.2 [intro.object]), virtual base classes shall be are initialized in the order they appear on a depth-first left-to-right traversal of the directed acyclic graph of base classes, where “left-to-right” is the order of appearance of the base class names in the derived class base-specifier-list.

    • Then, direct base classes shall be are initialized in declaration order as they appear in the base-specifier-list (regardless of the order of the mem-initializers).

    • Then, non-static data members shall be are initialized in the order they were declared in the class definition (again regardless of the order of the mem-initializers).

    • Finally, the compound-statement of the constructor body is executed.

    [Note: the declaration order is mandated to ensure that base and member subobjects are destroyed in the reverse order of initialization. —end note]

  7. Remove all normative text in 11.9.3 [class.base.init] paragraph 11, keeping the example:

  8. All subobjects representing virtual base classes are initialized by the constructor of the most derived class (6.7.2 [intro.object]). If the constructor of the most derived class does not specify a mem-initializer for a virtual base class V, then V's default constructor is called to initialize the virtual base class subobject. If V does not have an accessible default constructor, the initialization is ill-formed. A mem-initializer naming a virtual base class shall be ignored during execution of the constructor of any class that is not the most derived class. [Example:...
Date: 2004-09-10.00:00:00

Must a constructor for an abstract base class provide a mem-initializer for each virtual base class from which it is directly or indirectly derived? Since the initialization of virtual base classes is performed by the most-derived class, and since an abstract base class can never be the most-derived class, there would seem to be no reason to require constructors for abstract base classes to initialize virtual base classes.

It is not clear from the Standard whether there actually is such a requirement or not. The relevant text is found in 11.9.3 [class.base.init] paragraph 6:

All sub-objects representing virtual base classes are initialized by the constructor of the most derived class (6.7.2 [intro.object]). If the constructor of the most derived class does not specify a mem-initializer for a virtual base class V, then V's default constructor is called to initialize the virtual base class subobject. If V does not have an accessible default constructor, the initialization is ill-formed. A mem-initializer naming a virtual base class shall be ignored during execution of the constructor of any class that is not the most derived class.

This paragraph requires only that the most-derived class's constructor have a mem-initializer for virtual base classes. Should the silence be construed as permission for constructors of classes that are not the most-derived to omit such mem-initializers?

Christopher Lester, on comp.std.c++, March 19, 2004: If any of you reading this posting happen to be members of the above working group, I would like to encourage you to review the suggestion contained therein, as it seems to me that the final tenor of the submission is both (a) correct (the silence of the standard DOES mandate the omission) and (b) describes what most users would intuitively expect and desire from the C++ language as well.

The suggestion is to make it clearer that constructors for abstract base classes should not be required to provide initialisers for any virtual base classes they contain (as only the most-derived class has the job of initialising virtual base classes, and an abstract base class cannot possibly be a most-derived class).

For example:

struct A {
  A(const int i, const int j) {};
};

struct B1 : virtual public A {
  virtual void moo()=0;
  B1() {};   // (1) Look! not "B1() : A(5,6) {};"
};

struct B2 : virtual public A {
  virtual void cow()=0;
  B2() {};   // (2) Look! not "B2() : A(7,8) {};"
};

struct C : public B1, public B2 {
  C() : A(2,3) {};
  void moo() {};
  void cow() {};
};

int main() {
  C c;
  return 0;
};

I believe that, by not expressly forbidding it, the standard does (and should!) allow the above code. However, as the standard doesn't expressly allow it either (have I missed something?) there appears to be room for misunderstanding. For example, g++ version 3.2.3 (and maybe other versions as well) rejects the above code with messages like:

	In constructor `B1::B1()':
	no matching function for call to `A::A()'
	candidates are: A::A(const A&)
         	        A::A(int, int)

Fair enough, the standard is perhaps not clear enough. But it seems to be a shame that although this issue was first raised in 2000, we are still living with it today.

Note that we can work-around, and persuade g++ to compile the above by either (a) providing a default constructor A() for A, or (b) supplying default values for i and j in A(i,j), or (c) replace the construtors B1() and B2() with the forms shown in the two comments in the above example.

All three of these workarounds may at times be appropriate, but equally there are other times when all of these workarounds are particularly bad. (a) and (b) may be very bad if you are trying to enforce string contracts among objects, while (c) is just barmy (I mean why did I have to invent random numbers like 5, 6, 7 and 8 just to get the code to compile?).

So to to round up, then, my plea to the working group is: "at the very least, please make the standard clearer on this issue, but preferrably make the decision to expressly allow code that looks something like the above"

History
Date User Action Args
2010-03-29 00:00:00adminsetstatus: dr -> cd2
2009-11-08 00:00:00adminsetmessages: + msg2467
2009-11-08 00:00:00adminsetstatus: ready -> dr
2009-08-03 00:00:00adminsetstatus: review -> ready
2008-05-18 00:00:00adminsetmessages: + msg1637
2008-05-18 00:00:00adminsetstatus: open -> review
2000-11-01 00:00:00admincreate