Is it at all possible to have the second instantiation work somehow? I've tried combinations of using and = default but to no avail.
struct A
{
int i;
};
struct B : A
{};
A a{11}; // This works.
B b{22}; // "No matching constructor".
According to the C++ 2014 Standard (8.5.1 Aggregates):
1 An aggregate is an array or a class (Clause 9) with no user-provided
constructors (12.1), no private or protected non-static data members
(Clause 11), no base classes (Clause 10), and no virtual functions
(10.3).
However According to the C++ 2017 Standard (8.6.1 Aggregates):
1 An aggregate is an array or a class (Clause 9) with
(1.1) — no user-provided, explicit, or inherited constructors (12.1),
(1.2) — no private or protected non-static data members (Clause 11),
(1.3) — no virtual functions (10.3), and
(1.4) — no virtual, private, or protected base classes (10.1).
As you can see, the C++ 2017 Standard removes the restriction that aggregates shall not have base classes. So the code will not be compiled by a compiler that does not support the C++ 2017 Standard.
You need to declare explicitly a constructor that excepts an argument for the class B yourself.
That's not a constructor for A. That's aggregate initialization.
Anyway, you are doomed. While B is-a A, there is no way to pass initialization through B to A without a constructor. There may be some voodoo and witchcraft to make it look like you are initializing A through B, but it would be better to just write some nice, clean code like a B constructor that initializes A
struct B : A
{
B(int val):A{val}
{
}
};
Now obviously B has a constructor and
B b{22};
is valid.
Related
Let's say I have a type and I want to make its default constructor private. I write the following:
class C {
C() = default;
};
int main() {
C c; // error: C::C() is private within this context (g++)
// error: calling a private constructor of class 'C' (clang++)
// error C2248: 'C::C' cannot access private member declared in class 'C' (MSVC)
auto c2 = C(); // error: as above
}
Great.
But then, the constructor turns out to not be as private as I thought it was:
class C {
C() = default;
};
int main() {
C c{}; // OK on all compilers
auto c2 = C{}; // OK on all compilers
}
This strikes me as very surprising, unexpected, and explicitly undesired behavior. Why is this OK?
The trick is in C++14 8.4.2/5 [dcl.fct.def.default]:
... A function is user-provided if it is user-declared and not explicitly defaulted or
deleted on its first declaration. ...
Which means that C's default constructor is actually not user-provided, because it was explicitly defaulted on its first declaration. As such, C has no user-provided constructors and is therefore an aggregate per 8.5.1/1 [dcl.init.aggr]:
An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or
protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).
You're not calling the default constructor, you're using aggregate initialization on an aggregate type. Aggregate types are allowed to have a defaulted constructor, so long as it's defaulted where it's first declared:
From [dcl.init.aggr]/1:
An aggregate is an array or a class (Clause [class]) with
no user-provided constructors ([class.ctor]) (including those inherited ([namespace.udecl]) from a base class),
no private or protected non-static data members (Clause [class.access]),
no virtual functions ([class.virtual]), and
no virtual, private, or protected base classes ([class.mi]).
and from [dcl.fct.def.default]/5
Explicitly-defaulted functions and implicitly-declared functions are collectively called defaulted functions, and the implementation shall provide implicit definitions for them ([class.ctor] [class.dtor], [class.copy]), which might mean defining them as deleted. A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration. A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed. [ Note: Declaring a function as defaulted after its first declaration can provide efficient execution and concise definition while enabling a stable binary interface to an evolving code base. — end note ]
Thus, our requirements for an aggregate are:
no non-public members
no virtual functions
no virtual or non-public base classes
no user-provided constructors inherited or otherwise, which allows only constructors which are:
implicitly declared, or
explicitly declared and defined as defaulted at the same time.
C fulfills all of these requirements.
Naturally, you may be rid of this false default construction behavior by simply providing an empty default constructor, or by defining the constructor as default after declaring it:
class C {
C(){}
};
// --or--
class C {
C();
};
inline C::C() = default;
Angew's and jaggedSpire's' answers are excellent and apply to c++11. And c++14. And c++17.
However, in c++20, things change a bit and the example in the OP will no longer compile:
class C {
C() = default;
};
C p; // always error
auto q = C(); // always error
C r{}; // ok on C++11 thru C++17, error on C++20
auto s = C{}; // ok on C++11 thru C++17, error on C++20
As pointed out by the two answers, the reason the latter two declarations work is because C is an aggregate and this is aggregate-initialization. However, as a result of P1008 (using a motivating example not too dissimilar from the OP), the definition of aggregate changes in C++20 to, from [dcl.init.aggr]/1:
An aggregate is an array or a class ([class]) with
no user-declared or inherited constructors ([class.ctor]),
no private or protected direct non-static data members ([class.access]),
no virtual functions ([class.virtual]), and
no virtual, private, or protected base classes ([class.mi]).
Emphasis mine. Now the requirement is no user-declared constructors, whereas it used to be (as both users cite in their answers and can be viewed historically for C++11, C++14, and C++17) no user-provided constructors. The default constructor for C is user-declared, but not user-provided, and hence ceases to be an aggregate in C++20.
Here is another illustrative example of aggregate changes:
class A { protected: A() { }; };
struct B : A { B() = default; };
auto x = B{};
B was not an aggregate in C++11 or C++14 because it has a base class. As a result, B{} just invokes the default constructor (user-declared but not user-provided), which has access to A's protected default constructor.
In C++17, as a result of P0017, aggregates were extended to allow for base classes. B is an aggregate in C++17, which means that B{} is aggregate-initialization that has to initialize all the subobjects - including the A subobject. But because A's default constructor is protected, we don't have access to it, so this initialization is ill-formed.
In C++20, because of B's user-declared constructor, it again ceases to be an aggregate, so B{} reverts to invoking the default constructor and this is again well-formed initialization.
Let's say I have a type and I want to make its default constructor private. I write the following:
class C {
C() = default;
};
int main() {
C c; // error: C::C() is private within this context (g++)
// error: calling a private constructor of class 'C' (clang++)
// error C2248: 'C::C' cannot access private member declared in class 'C' (MSVC)
auto c2 = C(); // error: as above
}
Great.
But then, the constructor turns out to not be as private as I thought it was:
class C {
C() = default;
};
int main() {
C c{}; // OK on all compilers
auto c2 = C{}; // OK on all compilers
}
This strikes me as very surprising, unexpected, and explicitly undesired behavior. Why is this OK?
The trick is in C++14 8.4.2/5 [dcl.fct.def.default]:
... A function is user-provided if it is user-declared and not explicitly defaulted or
deleted on its first declaration. ...
Which means that C's default constructor is actually not user-provided, because it was explicitly defaulted on its first declaration. As such, C has no user-provided constructors and is therefore an aggregate per 8.5.1/1 [dcl.init.aggr]:
An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or
protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).
You're not calling the default constructor, you're using aggregate initialization on an aggregate type. Aggregate types are allowed to have a defaulted constructor, so long as it's defaulted where it's first declared:
From [dcl.init.aggr]/1:
An aggregate is an array or a class (Clause [class]) with
no user-provided constructors ([class.ctor]) (including those inherited ([namespace.udecl]) from a base class),
no private or protected non-static data members (Clause [class.access]),
no virtual functions ([class.virtual]), and
no virtual, private, or protected base classes ([class.mi]).
and from [dcl.fct.def.default]/5
Explicitly-defaulted functions and implicitly-declared functions are collectively called defaulted functions, and the implementation shall provide implicit definitions for them ([class.ctor] [class.dtor], [class.copy]), which might mean defining them as deleted. A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration. A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed. [ Note: Declaring a function as defaulted after its first declaration can provide efficient execution and concise definition while enabling a stable binary interface to an evolving code base. — end note ]
Thus, our requirements for an aggregate are:
no non-public members
no virtual functions
no virtual or non-public base classes
no user-provided constructors inherited or otherwise, which allows only constructors which are:
implicitly declared, or
explicitly declared and defined as defaulted at the same time.
C fulfills all of these requirements.
Naturally, you may be rid of this false default construction behavior by simply providing an empty default constructor, or by defining the constructor as default after declaring it:
class C {
C(){}
};
// --or--
class C {
C();
};
inline C::C() = default;
Angew's and jaggedSpire's' answers are excellent and apply to c++11. And c++14. And c++17.
However, in c++20, things change a bit and the example in the OP will no longer compile:
class C {
C() = default;
};
C p; // always error
auto q = C(); // always error
C r{}; // ok on C++11 thru C++17, error on C++20
auto s = C{}; // ok on C++11 thru C++17, error on C++20
As pointed out by the two answers, the reason the latter two declarations work is because C is an aggregate and this is aggregate-initialization. However, as a result of P1008 (using a motivating example not too dissimilar from the OP), the definition of aggregate changes in C++20 to, from [dcl.init.aggr]/1:
An aggregate is an array or a class ([class]) with
no user-declared or inherited constructors ([class.ctor]),
no private or protected direct non-static data members ([class.access]),
no virtual functions ([class.virtual]), and
no virtual, private, or protected base classes ([class.mi]).
Emphasis mine. Now the requirement is no user-declared constructors, whereas it used to be (as both users cite in their answers and can be viewed historically for C++11, C++14, and C++17) no user-provided constructors. The default constructor for C is user-declared, but not user-provided, and hence ceases to be an aggregate in C++20.
Here is another illustrative example of aggregate changes:
class A { protected: A() { }; };
struct B : A { B() = default; };
auto x = B{};
B was not an aggregate in C++11 or C++14 because it has a base class. As a result, B{} just invokes the default constructor (user-declared but not user-provided), which has access to A's protected default constructor.
In C++17, as a result of P0017, aggregates were extended to allow for base classes. B is an aggregate in C++17, which means that B{} is aggregate-initialization that has to initialize all the subobjects - including the A subobject. But because A's default constructor is protected, we don't have access to it, so this initialization is ill-formed.
In C++20, because of B's user-declared constructor, it again ceases to be an aggregate, so B{} reverts to invoking the default constructor and this is again well-formed initialization.
Let's say I have a type and I want to make its default constructor private. I write the following:
class C {
C() = default;
};
int main() {
C c; // error: C::C() is private within this context (g++)
// error: calling a private constructor of class 'C' (clang++)
// error C2248: 'C::C' cannot access private member declared in class 'C' (MSVC)
auto c2 = C(); // error: as above
}
Great.
But then, the constructor turns out to not be as private as I thought it was:
class C {
C() = default;
};
int main() {
C c{}; // OK on all compilers
auto c2 = C{}; // OK on all compilers
}
This strikes me as very surprising, unexpected, and explicitly undesired behavior. Why is this OK?
The trick is in C++14 8.4.2/5 [dcl.fct.def.default]:
... A function is user-provided if it is user-declared and not explicitly defaulted or
deleted on its first declaration. ...
Which means that C's default constructor is actually not user-provided, because it was explicitly defaulted on its first declaration. As such, C has no user-provided constructors and is therefore an aggregate per 8.5.1/1 [dcl.init.aggr]:
An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or
protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).
You're not calling the default constructor, you're using aggregate initialization on an aggregate type. Aggregate types are allowed to have a defaulted constructor, so long as it's defaulted where it's first declared:
From [dcl.init.aggr]/1:
An aggregate is an array or a class (Clause [class]) with
no user-provided constructors ([class.ctor]) (including those inherited ([namespace.udecl]) from a base class),
no private or protected non-static data members (Clause [class.access]),
no virtual functions ([class.virtual]), and
no virtual, private, or protected base classes ([class.mi]).
and from [dcl.fct.def.default]/5
Explicitly-defaulted functions and implicitly-declared functions are collectively called defaulted functions, and the implementation shall provide implicit definitions for them ([class.ctor] [class.dtor], [class.copy]), which might mean defining them as deleted. A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration. A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed. [ Note: Declaring a function as defaulted after its first declaration can provide efficient execution and concise definition while enabling a stable binary interface to an evolving code base. — end note ]
Thus, our requirements for an aggregate are:
no non-public members
no virtual functions
no virtual or non-public base classes
no user-provided constructors inherited or otherwise, which allows only constructors which are:
implicitly declared, or
explicitly declared and defined as defaulted at the same time.
C fulfills all of these requirements.
Naturally, you may be rid of this false default construction behavior by simply providing an empty default constructor, or by defining the constructor as default after declaring it:
class C {
C(){}
};
// --or--
class C {
C();
};
inline C::C() = default;
Angew's and jaggedSpire's' answers are excellent and apply to c++11. And c++14. And c++17.
However, in c++20, things change a bit and the example in the OP will no longer compile:
class C {
C() = default;
};
C p; // always error
auto q = C(); // always error
C r{}; // ok on C++11 thru C++17, error on C++20
auto s = C{}; // ok on C++11 thru C++17, error on C++20
As pointed out by the two answers, the reason the latter two declarations work is because C is an aggregate and this is aggregate-initialization. However, as a result of P1008 (using a motivating example not too dissimilar from the OP), the definition of aggregate changes in C++20 to, from [dcl.init.aggr]/1:
An aggregate is an array or a class ([class]) with
no user-declared or inherited constructors ([class.ctor]),
no private or protected direct non-static data members ([class.access]),
no virtual functions ([class.virtual]), and
no virtual, private, or protected base classes ([class.mi]).
Emphasis mine. Now the requirement is no user-declared constructors, whereas it used to be (as both users cite in their answers and can be viewed historically for C++11, C++14, and C++17) no user-provided constructors. The default constructor for C is user-declared, but not user-provided, and hence ceases to be an aggregate in C++20.
Here is another illustrative example of aggregate changes:
class A { protected: A() { }; };
struct B : A { B() = default; };
auto x = B{};
B was not an aggregate in C++11 or C++14 because it has a base class. As a result, B{} just invokes the default constructor (user-declared but not user-provided), which has access to A's protected default constructor.
In C++17, as a result of P0017, aggregates were extended to allow for base classes. B is an aggregate in C++17, which means that B{} is aggregate-initialization that has to initialize all the subobjects - including the A subobject. But because A's default constructor is protected, we don't have access to it, so this initialization is ill-formed.
In C++20, because of B's user-declared constructor, it again ceases to be an aggregate, so B{} reverts to invoking the default constructor and this is again well-formed initialization.
Let's say I have a type and I want to make its default constructor private. I write the following:
class C {
C() = default;
};
int main() {
C c; // error: C::C() is private within this context (g++)
// error: calling a private constructor of class 'C' (clang++)
// error C2248: 'C::C' cannot access private member declared in class 'C' (MSVC)
auto c2 = C(); // error: as above
}
Great.
But then, the constructor turns out to not be as private as I thought it was:
class C {
C() = default;
};
int main() {
C c{}; // OK on all compilers
auto c2 = C{}; // OK on all compilers
}
This strikes me as very surprising, unexpected, and explicitly undesired behavior. Why is this OK?
The trick is in C++14 8.4.2/5 [dcl.fct.def.default]:
... A function is user-provided if it is user-declared and not explicitly defaulted or
deleted on its first declaration. ...
Which means that C's default constructor is actually not user-provided, because it was explicitly defaulted on its first declaration. As such, C has no user-provided constructors and is therefore an aggregate per 8.5.1/1 [dcl.init.aggr]:
An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or
protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).
You're not calling the default constructor, you're using aggregate initialization on an aggregate type. Aggregate types are allowed to have a defaulted constructor, so long as it's defaulted where it's first declared:
From [dcl.init.aggr]/1:
An aggregate is an array or a class (Clause [class]) with
no user-provided constructors ([class.ctor]) (including those inherited ([namespace.udecl]) from a base class),
no private or protected non-static data members (Clause [class.access]),
no virtual functions ([class.virtual]), and
no virtual, private, or protected base classes ([class.mi]).
and from [dcl.fct.def.default]/5
Explicitly-defaulted functions and implicitly-declared functions are collectively called defaulted functions, and the implementation shall provide implicit definitions for them ([class.ctor] [class.dtor], [class.copy]), which might mean defining them as deleted. A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration. A user-provided explicitly-defaulted function (i.e., explicitly defaulted after its first declaration) is defined at the point where it is explicitly defaulted; if such a function is implicitly defined as deleted, the program is ill-formed. [ Note: Declaring a function as defaulted after its first declaration can provide efficient execution and concise definition while enabling a stable binary interface to an evolving code base. — end note ]
Thus, our requirements for an aggregate are:
no non-public members
no virtual functions
no virtual or non-public base classes
no user-provided constructors inherited or otherwise, which allows only constructors which are:
implicitly declared, or
explicitly declared and defined as defaulted at the same time.
C fulfills all of these requirements.
Naturally, you may be rid of this false default construction behavior by simply providing an empty default constructor, or by defining the constructor as default after declaring it:
class C {
C(){}
};
// --or--
class C {
C();
};
inline C::C() = default;
Angew's and jaggedSpire's' answers are excellent and apply to c++11. And c++14. And c++17.
However, in c++20, things change a bit and the example in the OP will no longer compile:
class C {
C() = default;
};
C p; // always error
auto q = C(); // always error
C r{}; // ok on C++11 thru C++17, error on C++20
auto s = C{}; // ok on C++11 thru C++17, error on C++20
As pointed out by the two answers, the reason the latter two declarations work is because C is an aggregate and this is aggregate-initialization. However, as a result of P1008 (using a motivating example not too dissimilar from the OP), the definition of aggregate changes in C++20 to, from [dcl.init.aggr]/1:
An aggregate is an array or a class ([class]) with
no user-declared or inherited constructors ([class.ctor]),
no private or protected direct non-static data members ([class.access]),
no virtual functions ([class.virtual]), and
no virtual, private, or protected base classes ([class.mi]).
Emphasis mine. Now the requirement is no user-declared constructors, whereas it used to be (as both users cite in their answers and can be viewed historically for C++11, C++14, and C++17) no user-provided constructors. The default constructor for C is user-declared, but not user-provided, and hence ceases to be an aggregate in C++20.
Here is another illustrative example of aggregate changes:
class A { protected: A() { }; };
struct B : A { B() = default; };
auto x = B{};
B was not an aggregate in C++11 or C++14 because it has a base class. As a result, B{} just invokes the default constructor (user-declared but not user-provided), which has access to A's protected default constructor.
In C++17, as a result of P0017, aggregates were extended to allow for base classes. B is an aggregate in C++17, which means that B{} is aggregate-initialization that has to initialize all the subobjects - including the A subobject. But because A's default constructor is protected, we don't have access to it, so this initialization is ill-formed.
In C++20, because of B's user-declared constructor, it again ceases to be an aggregate, so B{} reverts to invoking the default constructor and this is again well-formed initialization.
I face the well known "dreaded" diamond situation :
A
/ \
B1 B2
\ /
C
|
D
The class A has, say the constructor A::A(int i). I also want to forbid a default instantiation of a A so I declare the default constructor of A as private.
The classes B1 and B2 are virtually derived from A and have some constructors and a protected default constructor.
[edit]
The constructors of B1 and B2 don't call the default constructor of A.
[reedit]
The default constructors of B1 and B2 don't call the default constructor of A either.
[reedit]
[edit]
The class C is an abstract class and has some constructors that don't call any of the A, B1 or B2 constructors.
In the class D, I call the constructor A::A(i) and some constructor of C.
So as expected, when D is created, it first creates a A to solve the dreaded diamond problem, then it creates B1, B2 and C. Therefore there is no call of the default constructor of A in B1, B2 and C because if there was, it would create many instances of A.
The compiler rejects the code because the default constructor of A is private. If I set it to protected it compiles.
What I don't understand is that when I run the code, the default constructor of A is never called (as it should be). So why doesn't the compiler allow me to set it as private?
[edit]
okay I'll write an example... but it hurts ;-)
class A{
public:
A(int i):i_(i){};
virtual ~A(){};
protected:
int i_;
private:
A():i_(0){};/*if private => compilation error, if protected => ok*/
};
class B1: public virtual A{
public:
B1(int i):A(i){};
virtual ~B1(){};
protected:
B1():A(0){};
};
class B2: public virtual A{
public:
B2(int i):A(i){};
virtual ~B2(){};
protected:
B2():A(0){};
};
class C: public B1, public B2{
public:
C(int j):j_(j){};
virtual ~C()=0;
protected:
int j_;
};
C::~C(){};
class D: public C{
public:
D(int i,int j):A(i),C(j){};
~D(){};
};
int main(){
D d(1,2);
}
The compiler says that in constructor of C, A::A() is private. I agree with this, but as C is an abstract class, it can't be instantiated as a complete object (but it can be instantiated as a base class subobject, by instantiating a D).
[edit]
I added the tag `language-lawer' on someone's recommendation.
C++ doesn't have an access control specifier for member functions that can only be called from a derived class, but a constructor for an abstract class can only be called from a derived class by definition of an abstract class.
The compiler cannot know in advance exactly which classes are instantiated (this is a runtime property), and it cannot know which constructors are potentially called before link-time.
The standard text (emphasis mine):
All sub-objects representing virtual base classes are initialized by
the constructor of the most derived class (1.8 [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.
1) It makes no exception for abstract classes and can only be interpreted as saying that all constructors should do a (sometimes fake) attempt at calling virtual base constructors.
2) It says that at runtime such attempts are ignored.
Some committee members have stated a different opinion in DR 257:
Abstract base constructors and virtual base initialization
Section: 12.6.2 [class.base.init] Status: CD2 Submitter: Mike
Miller Date: 1 Nov 2000 [Voted into WP at October, 2009 meeting.]
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 12.6.2
[class.base.init] paragraph 6:
(...quoted above)
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?
There is no "silence". The general rule applies as there is no specific rule for abstract classes.
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).
The suggestion cannot make "clearer" something that doesn't exist now.
Some committee members are taken their desire for reality and it is very wrong.
(snip example and discussion similar to OP's code)
Proposed resolution (July, 2009):
Add the indicated text (moved from paragraph 11) to the end of 12.6.2
[class.base.init] paragraph 7:
...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.
Change 12.6.2 [class.base.init]
paragraph 8 as follows:
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 (10.4
[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
8.5 [dcl.init];
otherwise, if the entity is a variant member (9.5 [class.union]), no
initialization is performed;
otherwise, the entity is default-initialized (8.5 [dcl.init]).
[Note: An abstract class (10.4 [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...
Change 12.6.2 [class.base.init] paragraph 10 as follows:
Initialization shall proceed proceeds in the following order:
First, and only for the constructor of the most derived class as
described below (1.8 [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]
Remove all normative text in 12.6.2 [class.base.init] paragraph 11,
keeping the example:
All subobjects representing virtual base classes are initialized by
the constructor of the most derived class (1.8 [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:...
The DR is marked "CD2": the committee agrees this was an issue and the language definition is changed to fix this issue.