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sdspam@front.ru
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 Posted: Sat Jul 17, 2004 10:10 am Post subject: Using declaration for base class constructors |
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Hi!
It is often happens that I need to derive class D from B, where B has a
set of non-trivial constructors. But, in many cases, the construction
order isn't changed. Then a bulk of stupid code is written:
class B {
public:
B( int );
B( int, int );
/*...*/
virtual void goo() = 0;
/*...*/
};
class D : public B {
public:
//>> stupid code begin
D( int a ) : B( a ) {}
D( int a, int b ) : B( a, b ) {}
/*...*/
///<< stupid code end
virtual void goo() {
/*...*/
}
/*...*/
};
Moreover, some advanced techniques are broken by this issue. For
example, we cannot use class like following:
template< typename T >
class DefaultGoo : public T {
public:
virtual void goo() {
/*default code*/
}
};
It will be nice, if C++ allows us to inherit all constructors from a
base class. The hypothetical code is:
class D : public B {
public:
using B;
virtual void goo() {
/*...*/
}
/*...*/
};
template< typename T >
class DefaultGoo : public T {
public:
using T;
virtual void goo() {
/*default code*/
}
};
Obviously, there are problems with a multiple inheritance. But I hope
that a consistent solution exists.
Does anybody know how to fix easily the problem?
Does anybody know the reasons why it cannot be added in future C++?
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]
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Francis Glassborow
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| Posted: Sat Jul 17, 2004 5:12 pm Post subject: Re: Using declaration for base class constructors |
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In article <cd92fm$l3j (AT) odak26 (DOT) prod.google.com>, "sdspam (AT) front (DOT) ru"
<sdspam (AT) front (DOT) ru> writes
| Quote: | It is often happens that I need to derive class D from B, where B has a
set of non-trivial constructors. But, in many cases, the construction
order isn't changed. Then a bulk of stupid code is written:
|
Yes, this is a problem (though not everyone agrees as to how big a
problem) Uniquely (AFAIK) ctors are the only members that are not
inherited in any usable sense.
| Quote: |
class B {
public:
B( int );
B( int, int );
/*...*/
virtual void goo() = 0;
/*...*/
};
class D : public B {
public:
//>> stupid code begin
D( int a ) : B( a ) {}
D( int a, int b ) : B( a, b ) {}
/*...*/
///<< stupid code end
virtual void goo() {
/*...*/
}
/*...*/
};
Moreover, some advanced techniques are broken by this issue. For
example, we cannot use class like following:
template< typename T
class DefaultGoo : public T {
public:
virtual void goo() {
/*default code*/
}
};
It will be nice, if C++ allows us to inherit all constructors from a
base class. The hypothetical code is:
class D : public B {
public:
using B;
virtual void goo() {
/*...*/
}
/*...*/
};
template< typename T
class DefaultGoo : public T {
public:
using T;
virtual void goo() {
/*default code*/
}
};
Obviously, there are problems with a multiple inheritance. But I hope
that a consistent solution exists.
Does anybody know how to fix easily the problem?
Does anybody know the reasons why it cannot be added in future C++?
|
We are looking at this, indeed one of my papers (one that only got a
lukewarm reception in Sydney) was an attempt to address the issue. There
are other proposals for the next C++ release that could also be used to
provide a simple way to inject base class ctors into a derived class. I
hope that we at least manage to come up with a way to fix this. It is a
maintenance nightmare because if the base class acquires an extra ctor
in a new release the derived classes have to fix themselves.
--
Francis Glassborow ACCU
Author of 'You Can Do It!' see http://www.spellen.org/youcandoit
For project ideas and contributions: http://www.spellen.org/youcandoit/projects
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]
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