220 39404 <3a9e87f5-de59-4297-8096-f8f887b9f656@isocpp.org> article Path: news.gmane.org!.POSTED!not-for-mail From: rmbeer2@gmail.com Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: new feature: declare data that replaces the previous one, or similar situation.... Date: Wed, 25 Jul 2018 10:16:43 -0700 (PDT) Lines: 718 Approved: news@gmane.org Message-ID: <3a9e87f5-de59-4297-8096-f8f887b9f656@isocpp.org> References: Reply-To: std-proposals@isocpp.org NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="----=_Part_9518_1313330686.1532539003444" X-Trace: blaine.gmane.org 1532538879 30550 195.159.176.226 (25 Jul 2018 17:14:39 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Wed, 25 Jul 2018 17:14:39 +0000 (UTC) Cc: rmbeer2@gmail.com, cppljevans@gmail.com To: ISO C++ Standard - Future Proposals Original-X-From: std-proposals+bncBDI6L6W3SYLBB7HA4LNAKGQE7IPLSQQ@isocpp.org Wed Jul 25 19:14:35 2018 Return-path: Envelope-to: gclcip-std-proposals@m.gmane.org Original-Received: from mail-yb0-f199.google.com ([209.85.213.199]) by blaine.gmane.org with esmtp (Exim 4.84_2) (envelope-from ) id 1fiNMw-0007qM-HT for gclcip-std-proposals@m.gmane.org; Wed, 25 Jul 2018 19:14:34 +0200 Original-Received: by mail-yb0-f199.google.com with SMTP id b12-v6sf4258349ybr.12 for ; Wed, 25 Jul 2018 10:16:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=isocpp-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=eF7Q6VtXHhyWUTeoEjIogD7u/cB2NIPYRDMx7NntMHE=; b=i/7jB9vrRsxHPFEvX+xNFaC9VHieg402z6U7dJ01D2oNtI4QaQFpkH7k895So8qzCU kGQXrBpThriYJGc9hPz+YK16uRt7hFzIOdH9w5Bs0dz293UYOc2PR1+kgO5f1pQpCQZR GJrwtZmfkXA8G1tqNSg8HLLwztfX6NyNxapIUCBobNq6FOEP2jT6zrGfyuupr6lHxqXa +fThHsVQOwjOfWXKGj7J8PPyyBTqYXzu3fH0Lb7CufK/YTYit53+nythidCpIQ54Edb5 QAUhOiDW0wKdWBt8zKZ3pXs+ztScqBXyvlC8ieNqxRJQUsMiZ5d1qNJHykMTBedbpw9y hXag== DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:message-id:in-reply-to:references:subject :mime-version:x-original-sender:reply-to:precedence:mailing-list :list-id:list-post:list-help:list-archive:list-subscribe :list-unsubscribe; bh=eF7Q6VtXHhyWUTeoEjIogD7u/cB2NIPYRDMx7NntMHE=; b=bULGTnSZ5RV3uDNkDnqEUvm3JKYseporuFlT//G/Q+4N4eXAxPH3G0ugcQScZqGi7z 7GmndwGJpXnQHepPcn6DO+ELdkAlYOZg7EmeOvBFL/3PauOPTxOER0oSboQNMhiqczIS QNlBXD/B26iDkdyY47tQLlJrzp5+PhhOPpAN5ySz39BJtBoydt/Ci+TN3jvrUVIqG8Xr tYRm5F5Vdcv9fFjHK6CLeC9wm99YWNYAlC+pkUCH6J0bRLlQpFKm0zSE8bsXSktOhGqc LliNY/8+83uELoZcllYgMQltWRKKuxOj59SQO6Xx4pgN3wCzyIRmxJxwo+YN42X7y5RQ g86g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:message-id:in-reply-to :references:subject:mime-version:x-original-sender:reply-to :precedence:mailing-list:list-id:x-spam-checked-in-group:list-post :list-help:list-archive:list-subscribe:list-unsubscribe; bh=eF7Q6VtXHhyWUTeoEjIogD7u/cB2NIPYRDMx7NntMHE=; b=hD8lhJ5Y1Bo+ArJONrChDO9UA2xzylb5sn/p+rrMW2WbJtWHcTcicM0V/dieE1GRXH J4YdrbSzAu3Ow/N4FcrabA+Nb9ZDEeG9D5WzGKA2ldoY9/2F+M9Ff3RDlGu4UvHfBqmT uxX5jJLI8zQzzj3gKZnTV8gQmuk5AwrHgpr6v7PnIU/szIiV3hr1RlfMfCIaIcFglrvj 5wxC//zDzBN2cfLRlE5ITOr9MYVlIH3mhi0tK1Ygx6mGTNRTANKl9gAblRWx4PMMWw53 rGJ/EahjfyBZdQGhGw85KHPnAu4Eofh/6KnswQYuJvHOusC3WrhMACqxWJYsLBlTHFRU /9Jw== X-Gm-Message-State: AOUpUlFigH3y66RSU0qCu/drwqxOUY7tD9dFz1e9kmZy7cSWziLVudU4 zSDNqvGywtDlioGevFLMaRme6A== X-Google-Smtp-Source: AAOMgpdTMYU/lfxSTdcMEX5xnDjYrJ7D94zmXO+hoZA87qhH0RhdarDObKRTcvONOEv96p88P77JCQ== X-Received: by 2002:a81:f95:: with SMTP id 143-v6mr7197974ywp.188.1532539004980; Wed, 25 Jul 2018 10:16:44 -0700 (PDT) X-BeenThere: std-proposals@isocpp.org Original-Received: by 2002:a25:888b:: with SMTP id d11-v6ls3532874ybl.1.gmail; Wed, 25 Jul 2018 10:16:44 -0700 (PDT) X-Received: by 2002:a5b:60f:: with SMTP id d15-v6mr371765ybq.6.1532539003899; Wed, 25 Jul 2018 10:16:43 -0700 (PDT) In-Reply-To: X-Original-Sender: rmbeer2@gmail.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:39404 Archived-At: ------=_Part_9518_1313330686.1532539003444 Content-Type: multipart/alternative; boundary="----=_Part_9519_957408359.1532539003445" ------=_Part_9519_957408359.1532539003445 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Yes, that's exactly what I was looking for! I did not know that the keyword 'mutable' had to be used, although there is= =20 still the problem of having to use the virtual function to directly extract= =20 the class already converted from a parent class... The class structure that I use is similar, but directly uses a "D2 itsD2;",= =20 a direct statement within the class itself... =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D si, es justamente lo que estaba buscando!... no sabia que habia que usar la palabra clave 'mutable', aunque sigue=20 estando el problema de tener que usar la funcion virtual para extraer=20 directamente la clase ya convertida desde una clase padre... La estructura de clases que yo uso es similar, pero usa directamente un "D2= =20 itsD2;", una declaraci=C3=B3n directa dentro de la propia clase... El mi=C3=A9rcoles, 18 de julio de 2018, 7:04:33 (UTC-3), cpplj...@gmail.com= =20 escribi=C3=B3: > > > > On Monday, June 11, 2018 at 7:17:54 PM UTC-5, rmb...@gmail.com wrote: >> >> We have 2 classes that contain different data sets, one needs to use the= =20 >> functions and variables of another class, so it is included as a member= =20 >> pointing to the other class: >> >> member >> A -------> B >> >> Code: >> --------------------------------------- >> >> class A { >> public: >> A(int z_) : z(z_) { } >> int z; >> }; >> >> class B { >> public: >> B() : a(1) { } >> void f2() { >> std::cout << "A:" << a.z << std::endl; >> } >> A a; >> }; >> >> --------------------------------------- >> >> Then you need to create an inheritance for each class, since you need to= =20 >> cover a new interface layer. Let's call C and D, where C uses the base= =20 >> class A and D uses the base class B. >> >> member >> A ---------> B >> | | >> |inheritance | inheritance >> | | >> v member v >> C ---------> D >> >> Code: >> --------------------------------------- >> >> class A { >> public: >> A(int z_) : z(z_) { } >> int z; >> }; >> class C: public A { >> public: >> C(int y_) : y(y_), A(y_) { } >> long y; >> }; >> >> class B { >> public: >> B() : a(1) { } >> void f2() { >> std::cout << "A:" << a.z << std::endl; >> } >> A a; >> }; >> class D: public B { >> public: >> B() : a(3) { } >> void f2() { >> std::cout << "A:" << a.z << std::endl; >> } >> C a; >> }; >> >> --------------------------------------- >> >> Here where the problems appear, if you create B you can access the data= =20 >> of A with the functions of B. But if you create D, you access the data o= f=20 >> C+A with the functions of D and C, but as soon as you call the functions= =20 >> inherited from B, B can only access the data of A that are totally in=20 >> disuse, that is, create an instance in D generates in memory a variable = "a"=20 >> with C+A, and also a variable "a" with A , being in class D a C+A, A, wh= ere=20 >> the last A is not used, only C+A, in A it ends up using when the functio= ns=20 >> of B. are called. >> It is only possible to create a new virtual type function as the only= =20 >> case to access only C+A, but this generates junk code and in memory it= =20 >> reserves A as garbage: >> >> Code: >> --------------------------------------- >> #include >> >> class A { >> public: >> A(int z_) : z(z_) { } >> int z; >> }; >> >> class C: public A { >> public: >> C(int z_) : A(z_) { } >> }; >> >> class B { >> public: >> B() : a(1) { } >> void f2() { >> std::cout << "A:" << get_C_a().z << std::endl; >> } >> virtual A const& get_C_a() { return a; } >> A a; // Use only from B >> }; >> >> class D : public B { >> public: >> D() : a(3) { } >> void f2(){ >> std::cout << "C+A:" << a.z << std::endl; >> B::f2(); >> } >> C a; // Use C+A only from D >> A const& get_C_a() override { return a; } >> }; >> >> int main() { >> B b; D d; >> b.f2(); d.f2(); >> return 0; >> } >> >> /* OUTPUT: >> A:1 >> C+A:3 >> A:3 >> */ >> >> --------------------------------------- >> >> This would be the best option, but not the ideal one. Ideally, "C a;" of= =20 >> D replace "A a;" of B, in this way D only uses C+A from "C a;" and B onl= y=20 >> uses A from "A a;" >> >> This is my proposal, a keyword like 'virtual' (or can be any other) in= =20 >> the variable of the same name that is replaced by the variable definitio= n=20 >> of the base class. >> >> Por ejemplo: >> Code: >> --------------------------------------- >> #include >> >> class A { >> public: >> A(int z_) : z(z_) { } >> int z; >> }; >> >> class C: public A { >> public: >> C(int z_) : A(z_) { } >> }; >> >> class B { >> public: >> B() : a(1) { } >> void f2() { >> std::cout << "A:" << a.z << std::endl; >> } >> virtual A a; // Use A only from B >> }; >> >> class D : public B { >> public: >> D() : a(3) { } >> void f2(){ >> std::cout << "C+A:" << a.z << std::endl; >> B::f2(); >> } >> virtual C a; // Use C+A only from D >> }; >> >> int main() { >> B b; D d; >> b.f2(); d.f2(); >> return 0; >> } >> >> /* OUTPUT: >> A:1 >> C+A:3 >> A:3 >> */ >> >> --------------------------------------- >> >> well this might not be possible, since compiled the code, the binary cod= e=20 >> uses the data from an expected position in the variables within the=20 >> classes, so removing it from the list could alter the size and position = of=20 >> the variables. On the other hand it would be possible, since reserving= =20 >> memory from A or from C as C+A, generates a group of variables reserved = in=20 >> memories for each class even if it is C+A and calls functions of A which= =20 >> omits C of C+A Just to have A in view, this could be an advantage. >> I suggest some other option to simulate a replacement of variable A by C= .. >> If there is a better method that fulfills the same purpose without=20 >> breaking with the basic structure proposed here then much better. >> >> >> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D >> >> Tenemos 2 clases que contienen distintos conjuntos de datos, uno necesit= a=20 >> usar las funciones y variables de otra clase, por lo que se incluye como= =20 >> miembro apuntando a la otra clase: >> >> member >> A -------> B >> >> Code: >> --------------------------------------- >> >> class A { >> public: >> A(int z_) : z(z_) { } >> int z; >> }; >> >> class B { >> public: >> B() : a(1) { } >> void f2() { >> std::cout << "A:" << a.z << std::endl; >> } >> A a; >> }; >> >> --------------------------------------- >> >> Luego se necesita crear una herencia para cada clase, ya que necesita=20 >> cubrir una nueva capa de interfaz. Llamemos C y D, donde C usa la clase= =20 >> base A y D usa la clase base B. >> >> member >> A ---------> B >> | | >> |inheritance | inheritance >> | | >> v member v >> C ---------> D >> >> Code: >> --------------------------------------- >> >> class A { >> public: >> A(int z_) : z(z_) { } >> int z; >> }; >> class C: public A { >> public: >> C(int y_) : y(y_), A(y_) { } >> long y; >> }; >> >> class B { >> public: >> B() : a(1) { } >> void f2() { >> std::cout << "A:" << a.z << std::endl; >> } >> A a; >> }; >> class D: public B { >> public: >> B() : a(3) { } >> void f2() { >> std::cout << "A:" << a.z << std::endl; >> } >> C a; >> }; >> >> --------------------------------------- >> >> Aqui donde aparecen los problemas, si creas B puedes acceder a los=20 >> datos de A con las funciones de B. Pero si creas D, accedes a los datos = de=20 >> C+A con las funciones de D y C, pero tan pronto llamas a las funciones= =20 >> heredadas de B, B solo puede acceder a los datos de A que estan totalmen= te=20 >> en desuso, es decir, crear una instancia en D genera en la memoria una= =20 >> variable "a" con C+A, y tambien una variable "a" con A, siendo en la cla= se=20 >> D un C+A,A , donde la ultima A no se usa, solo C+A , en A se termina usa= ndo=20 >> cuando se llama a las funciones de B. >> Solo es posible crear una nueva funcion tipo virtual como unico caso= =20 >> para acceder solamente a C+A, pero esto genera codigo basura y en la=20 >> memoria se reserva A como basura: >> >> Code: >> --------------------------------------- >> #include >> >> class A { >> public: >> A(int z_) : z(z_) { } >> int z; >> }; >> >> class C: public A { >> public: >> C(int z_) : A(z_) { } >> }; >> >> class B { >> public: >> B() : a(1) { } >> void f2() { >> std::cout << "A:" << get_C_a().z << std::endl; >> } >> virtual A const& get_C_a() { return a; } >> A a; // Use only from B >> }; >> >> class D : public B { >> public: >> D() : a(3) { } >> void f2(){ >> std::cout << "C+A:" << a.z << std::endl; >> B::f2(); >> } >> C a; // Use C+A only from D >> A const& get_C_a() override { return a; } >> }; >> >> int main() { >> B b; D d; >> b.f2(); d.f2(); >> return 0; >> } >> >> /* OUTPUT: >> A:1 >> C+A:3 >> A:3 >> */ >> >> --------------------------------------- >> >> Esta seria la mejor opcion, pero no la ideal. Lo ideal es que "C a;" de = D=20 >> remplace a "A a;" de B, de esta forma D solo usa C+A de "C a;" y B solo = usa=20 >> A de "A a;" >> >> Esta es mi propuesta, una palabra clave como 'virtual' (o puede ser=20 >> cualquier otra) en la variable del mismo nombre que se remplace a la=20 >> definicion de variable de la clase base. >> >> Por ejemplo: >> Code: >> --------------------------------------- >> #include >> >> class A { >> public: >> A(int z_) : z(z_) { } >> int z; >> }; >> >> class C: public A { >> public: >> C(int z_) : A(z_) { } >> }; >> >> class B { >> public: >> B() : a(1) { } >> void f2() { >> std::cout << "A:" << a.z << std::endl; >> } >> virtual A a; // Use A only from B >> }; >> >> class D : public B { >> public: >> D() : a(3) { } >> void f2(){ >> std::cout << "C+A:" << a.z << std::endl; >> B::f2(); >> } >> virtual C a; // Use C+A only from D >> }; >> >> >> > [snip] > This problem sounds similar to the "dual inheritance > hierarchy" problem that this reference: > > > https://pdfs.semanticscholar.org/f403/361d9ef3dac96463787b852b605a918bc4f= e.pdf > > addresses. For example, the A and B classes above > correspond to the B1 and B2 classes in Figure 3 of the above > reference, and the C and D classes above correspond to the > D1 and D2 classes in Figure 3 of the above reference. > > Is that how it seems to you? > > -regards, > Larry > > --=20 You received this message because you are subscribed to the Google Groups "= ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an e= mail to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp= ..org/d/msgid/std-proposals/3a9e87f5-de59-4297-8096-f8f887b9f656%40isocpp.or= g. ------=_Part_9519_957408359.1532539003445 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Yes, that's exactly what I was looking for!
I did n= ot know that the keyword 'mutable' had to be used, although there i= s still the problem of having to use the virtual function to directly extra= ct the class already converted from a parent class...
The class structur= e that I use is similar, but directly uses a "D2 itsD2;", a direc= t statement within the class itself...

=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

si, = es justamente lo que estaba buscando!...
no sabia que habia que usar la = palabra clave 'mutable', aunque sigue estando el problema de tener = que usar la funcion virtual para extraer directamente la clase ya convertid= a desde una clase padre...
La estructura de clases que yo uso es similar= , pero usa directamente un "D2 itsD2;", una declaraci=C3=B3n dire= cta dentro de la propia clase...

El mi=C3=A9rcoles, 18 de julio de 2= 018, 7:04:33 (UTC-3), cpplj...@gmail.com escribi=C3=B3:


On Monday, June 11, 2018 = at 7:17:54 PM UTC-5, rmb...@gmail.com wrote:
We have 2 classes that contain different d= ata sets, one needs to use the functions and variables of another class, so= it is included as a member pointing to the other class:

=C2=A0=C2= =A0 member
A -------> B

Code:
-----------------------------= ----------

class A {
public:
=C2=A0=C2=A0 =C2=A0A(int z_)= : z(z_) {=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0int z;
};

cl= ass B {
public:
=C2=A0=C2=A0 =C2=A0B() : a(1) { }
=C2=A0=C2=A0 =C2= =A0void f2() {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::cout << = "A:" << a.z << std::endl;
=C2=A0=C2=A0 =C2=A0}
= =C2=A0=C2=A0 =C2=A0A a;
};

---------------------------------= ------

Then you need to create an inheritance for each class, since = you need to cover a new interface layer. Let's call C and D, where C us= es the base class A and D uses the base class B.

=C2=A0=C2=A0 member=
A ---------> B
|=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0 |
|inheritance | inheritance
|=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |
v=C2=A0 member=C2=A0= =C2=A0=C2=A0 v
C ---------> D

Code:
-----------------------= ----------------

class A {
public:
=C2=A0=C2=A0 =C2=A0A(i= nt z_) : z(z_) {=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0int z;
};
= class C: public A {
public:
=C2=A0=C2=A0 =C2=A0C(int y_) : y(y_), A(y= _) {=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0long y;
};

class B= {
public:
=C2=A0=C2=A0 =C2=A0B() : a(1) { }
=C2=A0=C2=A0 =C2=A0vo= id f2() {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::cout << "= ;A:" << a.z << std::endl;
=C2=A0=C2=A0 =C2=A0}
=C2= =A0=C2=A0 =C2=A0A a;
};
class D: public B {
public:
=C2=A0=C2= =A0 =C2=A0B() : a(3) { }
=C2=A0=C2=A0 =C2=A0void f2() {
=C2=A0=C2=A0 = =C2=A0=C2=A0=C2=A0 =C2=A0std::cout << "A:" << a.z <= ;< std::endl;
=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0C a;
};
---------------------------------------

Here where the pr= oblems appear, if you create B you can access the data of A with the functi= ons of B. But if you create D, you access the data of C+A with the function= s of D and C, but as soon as you call the functions inherited from B, B can= only access the data of A that are totally in disuse, that is, create an i= nstance in D generates in memory a variable "a" with C+A, and als= o a variable "a" with A , being in class D a C+A, A, where the la= st A is not used, only C+A, in A it ends up using when the functions of B. = are called.
=C2=A0 It is only possible to create a new virtual type func= tion as the only case to access only C+A, but this generates junk code and = in memory it reserves A as garbage:

Code:
-----------------------= ----------------
#include <iostream>

class A {
publ= ic:
=C2=A0=C2=A0 =C2=A0A(int z_) : z(z_) {=C2=A0=C2=A0 =C2=A0}
=C2=A0= =C2=A0 =C2=A0int z;
};

class C: public A {
public:
=C2=A0= =C2=A0 =C2=A0C(int z_) : A(z_) {=C2=A0=C2=A0 =C2=A0}
};

class B {=
public:
=C2=A0=C2=A0 =C2=A0B() : a(1) { }
=C2=A0=C2=A0 =C2=A0void= f2() {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::cout << "A= :" << get_C_a().z << std::endl;
=C2=A0=C2=A0 =C2=A0}=C2=A0=C2=A0 =C2=A0virtual A const& get_C_a() { return a; }
=C2=A0= =C2=A0 =C2=A0A a; // Use only from B
};

class D : public B {
p= ublic:
=C2=A0=C2=A0 =C2=A0D() : a(3) { }
=C2=A0=C2=A0 =C2=A0void f2()= {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::cout << "C+A:&qu= ot; << a.z << std::endl;
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2= =A0B::f2();
=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0C a; // Use C+A o= nly from D
=C2=A0=C2=A0 =C2=A0A const& get_C_a() override { return a= ; }
};

int main() {
=C2=A0=C2=A0 =C2=A0B b; D d;
=C2=A0=C2= =A0 =C2=A0b.f2(); d.f2();
=C2=A0=C2=A0 =C2=A0return 0;
}

/* OU= TPUT:
A:1
C+A:3
A:3
*/

---------------------------------------

This would be the best option, but not the ideal one. Id= eally, "C a;" of D replace "A a;" of B, in this way D o= nly uses C+A from "C a;" and B only uses A from "A a;"<= br>
This is my proposal, a keyword like 'virtual' (or can be any= other) in the variable of the same name that is replaced by the variable d= efinition of the base class.

Por ejemplo:
Code:
--------------= -------------------------
#include <iostream>

class A = {
public:
=C2=A0=C2=A0 =C2=A0A(int z_) : z(z_) {=C2=A0=C2=A0 =C2=A0}<= br>=C2=A0=C2=A0 =C2=A0int z;
};

class C: public A {
public:=C2=A0=C2=A0 =C2=A0C(int z_) : A(z_) {=C2=A0=C2=A0 =C2=A0}
};

cl= ass B {
public:
=C2=A0=C2=A0 =C2=A0B() : a(1) { }
=C2=A0=C2=A0 =C2= =A0void f2() {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::cout << = "A:" << a.z << std::endl;
=C2=A0=C2=A0 =C2=A0}
= =C2=A0=C2=A0 =C2=A0virtual A a; // Use A only from B
};

class D := public B {
public:
=C2=A0=C2=A0 =C2=A0D() : a(3) { }
=C2=A0=C2=A0= =C2=A0void f2(){
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::cout <&l= t; "C+A:" << a.z << std::endl;
=C2=A0=C2=A0 =C2=A0= =C2=A0=C2=A0 =C2=A0B::f2();
=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0v= irtual C a; // Use C+A only from D
};

int main() {
=C2=A0=C2= =A0 =C2=A0B b; D d;
=C2=A0=C2=A0 =C2=A0b.f2(); d.f2();
=C2=A0=C2=A0 = =C2=A0return 0;
}

/* OUTPUT:
A:1
C+A:3
A:3
*/

= ---------------------------------------

well this might not be = possible, since compiled the code, the binary code uses the data from an ex= pected position in the variables within the classes, so removing it from th= e list could alter the size and position of the variables. On the other han= d it would be possible, since reserving memory from A or from C as C+A, gen= erates a group of variables reserved in memories for each class even if it = is C+A and calls functions of A which omits C of C+A Just to have A in view= , this could be an advantage.
I suggest some other option to simulate a = replacement of variable A by C.
If there is a better method that fulfill= s the same purpose without breaking with the basic structure proposed here = then much better.

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

Tenemos 2 cla= ses que contienen distintos conjuntos de datos, uno necesita usar las funci= ones y variables de otra clase, por lo que se incluye como miembro apuntand= o a la otra clase:

=C2=A0=C2=A0 member
A -------> B

Cod= e:
---------------------------------------

class A {
publ= ic:
=C2=A0=C2=A0 =C2=A0A(int z_) : z(z_) {=C2=A0=C2=A0 =C2=A0}
=C2=A0= =C2=A0 =C2=A0int z;
};

class B {
public:
=C2=A0=C2=A0 =C2= =A0B() : a(1) { }
=C2=A0=C2=A0 =C2=A0void f2() {
=C2=A0=C2=A0 =C2=A0= =C2=A0=C2=A0 =C2=A0std::cout << "A:" << a.z << = std::endl;
=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0A a;
};

= ---------------------------------------

Luego se necesita crear= una herencia para cada clase, ya que necesita cubrir una nueva capa de int= erfaz. Llamemos C y D, donde C usa la clase base A y D usa la clase base B.=

=C2=A0=C2=A0 member
A ---------> B
|=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |
|inheritance | inheritan= ce
|=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 |=
v=C2=A0 member=C2=A0=C2=A0=C2=A0 v
C ---------> D

Code:---------------------------------------

class A {
public:=C2=A0=C2=A0 =C2=A0A(int z_) : z(z_) {=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2= =A0 =C2=A0int z;
};
class C: public A {
public:
=C2=A0=C2=A0 = =C2=A0C(int y_) : y(y_), A(y_) {=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0= long y;
};

class B {
public:
=C2=A0=C2=A0 =C2=A0B() : a(1) = { }
=C2=A0=C2=A0 =C2=A0void f2() {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 = =C2=A0std::cout << "A:" << a.z << std::endl;=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0A a;
};
class D: public B= {
public:
=C2=A0=C2=A0 =C2=A0B() : a(3) { }
=C2=A0=C2=A0 =C2=A0vo= id f2() {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::cout << "= ;A:" << a.z << std::endl;
=C2=A0=C2=A0 =C2=A0}
=C2= =A0=C2=A0 =C2=A0C a;
};

------------------------------------= ---

=C2=A0 Aqui donde aparecen los problemas, si creas B puedes acce= der a los datos de A con las funciones de B. Pero si creas D, accedes a los= datos de C+A con las funciones de D y C, pero tan pronto llamas a las func= iones heredadas de B, B solo puede acceder a los datos de A que estan total= mente en desuso, es decir, crear una instancia en D genera en la memoria un= a variable "a" con C+A, y tambien una variable "a" con = A, siendo en la clase D un C+A,A , donde la ultima A no se usa, solo C+A , = en A se termina usando cuando se llama a las funciones de B.
=C2=A0=C2= =A0 =C2=A0Solo es posible crear una nueva funcion tipo virtual como unico c= aso para acceder solamente a C+A, pero esto genera codigo basura y en la me= moria se reserva A como basura:

Code:
---------------------------= ------------
#include <iostream>

class A {
public:<= br>=C2=A0=C2=A0 =C2=A0A(int z_) : z(z_) {=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2= =A0 =C2=A0int z;
};

class C: public A {
public:
=C2=A0=C2= =A0 =C2=A0C(int z_) : A(z_) {=C2=A0=C2=A0 =C2=A0}
};

class B {public:
=C2=A0=C2=A0 =C2=A0B() : a(1) { }
=C2=A0=C2=A0 =C2=A0void f2= () {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::cout << "A:&q= uot; << get_C_a().z << std::endl;
=C2=A0=C2=A0 =C2=A0}
= =C2=A0=C2=A0 =C2=A0virtual A const& get_C_a() { return a; }
=C2=A0= =C2=A0 =C2=A0A a; // Use only from B
};

class D : public B {
p= ublic:
=C2=A0=C2=A0 =C2=A0D() : a(3) { }
=C2=A0=C2=A0 =C2=A0void f2()= {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::cout << "C+A:&qu= ot; << a.z << std::endl;
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2= =A0B::f2();
=C2=A0=C2=A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0C a; // Use C+A o= nly from D
=C2=A0=C2=A0 =C2=A0A const& get_C_a() override { return a= ; }
};

int main() {
=C2=A0=C2=A0 =C2=A0B b; D d;
=C2=A0=C2= =A0 =C2=A0b.f2(); d.f2();
=C2=A0=C2=A0 =C2=A0return 0;
}

/* OU= TPUT:
A:1
C+A:3
A:3
*/

---------------------------------------

Esta seria la mejor opcion, pero no la ideal. Lo ideal e= s que "C a;" de D remplace a "A a;" de B, de esta forma= D solo usa C+A de "C a;" y B solo usa A de "A a;"
<= br>Esta es mi propuesta, una palabra clave como 'virtual' (o puede = ser cualquier otra) en la variable del mismo nombre que se remplace a la de= finicion de variable de la clase base.

Por ejemplo:
Code:
----= -----------------------------------
#include <iostream>
class A {
public:
=C2=A0=C2=A0 =C2=A0A(int z_) : z(z_) {=C2=A0=C2= =A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0int z;
};

class C: public A {public:
=C2=A0=C2=A0 =C2=A0C(int z_) : A(z_) {=C2=A0=C2=A0 =C2=A0}
= };

class B {
public:
=C2=A0=C2=A0 =C2=A0B() : a(1) { }
=C2= =A0=C2=A0 =C2=A0void f2() {
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0std::c= out << "A:" << a.z << std::endl;
=C2=A0=C2= =A0 =C2=A0}
=C2=A0=C2=A0 =C2=A0virtual A a; // Use A only from B
};
class D : public B {
public:
=C2=A0=C2=A0 =C2=A0D() : a(3) { }<= br>=C2=A0=C2=A0 =C2=A0void f2(){
=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0s= td::cout << "C+A:" << a.z << std::endl;
=C2= =A0=C2=A0 =C2=A0=C2=A0=C2=A0 =C2=A0B::f2();
=C2=A0=C2=A0 =C2=A0}
=C2= =A0=C2=A0 =C2=A0virtual C a; // Use C+A only from D
};


=

[snip]
This problem sounds simi= lar to the "dual inheritance
hierarchy" problem that this refe= rence:

https://pdfs.semanticscholar.org/f403/361d9ef3d= ac96463787b852b605a918bc4fe.pdf

addresses.=C2=A0 For exampl= e, the A and B classes above
correspond to the B1 and B2 classes in Figu= re 3 of the above
reference, and the C and D classes above correspond to= the
D1 and D2 classes in Figure 3 of the above reference.

Is tha= t how it seems to you?

-regards,
Larry

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