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Ash
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 Posted: Tue Aug 17, 2004 10:04 am Post subject: Template compilation question |
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Hello all,
I am trying to write a templated class that stores pointers to
auto_ptr in its data structure and another method invokes certain
methods on the pointer. But I am running into some compilation errors
and I am wondering if what I am doing is legal or not and how should I
go about doing this. Here is the simplest code that does reproduce the
problem.
<CODE>
#include <iostream>
#include <memory>
#include <vector>
using namespace std;
class Item
{
public:
void Bar () { cout << "Item::Bar" << endl; };
void Foo () { cout << "Item::Foo" << endl; };
};
template
{
public:
Reply () {};
~Reply () {};
// insert an element into the list
void Foo ()
{
auto_ptr <T> *pD = new auto_ptr <T> (new T);
m_Items.push_back (pD);
};
// get the first element and call a method on it
void Bar ()
{
auto_ptr <T> *pT = m_Items [0];
pT->Bar ();
}
private:
vector < auto_ptr m_Items;
};
int main ()
{
Reply <Item> Obj;
Obj.Foo ();
Obj.Bar ();
}
</CODE>
I get the following error from Visual studio:
<COMPILATION>
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.00.9466 for
80x86
Copyright (C) Microsoft Corporation 1984-2001. All rights reserved.
tst.cpp
tst.cpp(30) : error C2039: 'Bar' : is not a member of
'std::auto_ptr<_Ty>'
with
[
_Ty=Item
]
tst.cpp(2 : while compiling class-template member function
'void Reply<T>::Bar(void)'
with
[
T=Item
]
tst.cpp(40) : see reference to class template instantiation
'Reply<T>' being compiled
with
[
T=Item
]
</COMPILATION>
I cannot seem to understand why this is not allowed by the standard.
Any comments/suggestions would be appreciated.
Thanks,
/ash
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Ulrich Eckhardt
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| Posted: Tue Aug 17, 2004 7:20 pm Post subject: Re: Template compilation question |
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Ash wrote:
| Quote: | I am trying to write a templated class that stores pointers to
auto_ptr in its data structure and another method invokes certain
methods on the pointer. But I am running into some compilation errors
and I am wondering if what I am doing is legal or not and how should I
go about doing this. Here is the simplest code that does reproduce the
problem.
auto_ptr <T> *pT = m_Items [0];
pT->Bar ();
[...]
tst.cpp(30) : error C2039: 'Bar' : is not a member of
'std::auto_ptr<_Ty>'
|
pT is a pointer to an auto_ptr<T>. In order to invoke a memberfunction of
class T, you need to get at the auto_ptr first and then you can get at the
object: (*pT)->Bar();
About the idea of storing pointers to smart pointers: Why? Why? Why?
The point is, why do people use smart pointers in the first place? And why
don't you use a smart pointer to the auto_ptr<>s?
If you want to use auto_ptr at the surface of your container, in order to
make clear who owns the objects, using hand-made handling of pointers to
autopointers(think of allocation overhead!) is neither secure nor easy. In
that case, you can as well handle raw pointers, converting to auto_ptr when
passing out and release()ing when passing in. Getting this right in the
presence of exceptions is an art though.
My advice: take a look at boost::shared_ptr<> instead. Several things are so
much easier that way.
Uli
--
FAQ: http://parashift.com/c++-faq-lite/
/* bittersweet C++ */
default: break;
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Motti Lanzkron
Guest
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| Posted: Tue Aug 17, 2004 7:34 pm Post subject: Re: Template compilation question |
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Ash wrote:
| Quote: | Hello all,
I am trying to write a templated class that stores pointers to
auto_ptr in its data structure and another method invokes certain
methods on the pointer.
|
Why would you want to dynamically allocate an auto_ptr?
The whole point of auto_ptrs is that you don't have to manage the
memory explicitly and now you have to delete the auto_ptr. Nothing is
solved, instead of dealing with raw pointers to Item you're dealing
with raw pointers to auto_ptr<Item>.
| Quote: | But I am running into some compilation errors
and I am wondering if what I am doing is legal or not and how should I
go about doing this. Here is the simplest code that does reproduce the
problem.
|
....
| Quote: | // get the first element and call a method on it
void Bar ()
{
auto_ptr <T> *pT = m_Items [0];
pT->Bar (); // line 30
}
tst.cpp(30) : error C2039: 'Bar' : is not a member of
'std::auto_ptr<_Ty>'
with
[
_Ty=Item
]
|
pT->Bar() (when dealing with raw pointers) is equivalent to
(*pT).Bar(), the type of *pT is a reference to a auto_ptr<Item>& and
auto_ptr doesn't have a method Bar.
What's probably confusing you is that auto_ptr defines operator->
which returns a T* and language automatically "adds" a -> to the
pointer and you get the same behaviour as if you called
(&*ptr)->Bar(); This chain of events (I'm guessing wildly here) stops
when the return type of operator-> is a raw pointer and not a class.
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Victor Bazarov
Guest
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| Posted: Tue Aug 17, 2004 7:47 pm Post subject: Re: Template compilation question |
|
|
Ash wrote:
| Quote: | Hello all,
I am trying to write a templated class that stores pointers to
auto_ptr in its data structure and another method invokes certain
methods on the pointer. But I am running into some compilation errors
and I am wondering if what I am doing is legal or not and how should I
go about doing this. Here is the simplest code that does reproduce the
problem.
CODE
#include <iostream
#include
#include
using namespace std;
class Item
{
public:
void Bar () { cout << "Item::Bar" << endl; };
void Foo () { cout << "Item::Foo" << endl; };
};
template
{
public:
Reply () {};
~Reply () {};
|
Semicolons are superfluous. BTW, what do you do to clean up the mess
that is your vector of pointers to auto_ptr?
| Quote: |
// insert an element into the list
void Foo ()
{
auto_ptr <T> *pD = new auto_ptr <T> (new T);
m_Items.push_back (pD);
};
// get the first element and call a method on it
void Bar ()
{
auto_ptr <T> *pT = m_Items [0];
pT->Bar ();
|
'pT' is a _pointer_ to auto_ptr. In order to call auto_ptr::operator->
(to get to 'Bar' member of 'T') you need to dereference it first:
(*pT)->Bar();
Now, (*pT) is an auto_ptr<T>, and you can access members of T by using
the operator-> of the auto_ptr<T> class.
| Quote: | }
private:
vector < auto_ptr m_Items;
};
int main ()
{
Reply <Item> Obj;
Obj.Foo ();
Obj.Bar ();
}
/CODE
I get the following error from Visual studio:
COMPILATION
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.00.9466 for
80x86
Copyright (C) Microsoft Corporation 1984-2001. All rights reserved.
tst.cpp
tst.cpp(30) : error C2039: 'Bar' : is not a member of
'std::auto_ptr<_Ty>'
with
[
_Ty=Item
]
tst.cpp(2 : while compiling class-template member function
'void Reply<T>::Bar(void)'
with
[
T=Item
]
tst.cpp(40) : see reference to class template instantiation
'Reply<T>' being compiled
with
[
T=Item
]
/COMPILATION
I cannot seem to understand why this is not allowed by the standard.
|
'auto_ptr' itself doesn't have a member 'Bar'. That's why it's not
allowed.
| Quote: | Any comments/suggestions would be appreciated.
|
Two comments: since you're managing dynamic memory, your implementation
lacks proper destructor, copy constructor, and the copy assignment op.
Read bout "the Rule of Three". And having _pointers_ to 'auto_ptr' just
defeats the purpose of using it, really. What you want is to use the
'shared_ptr' from Boost and store it directly instead of storing the
pointers to auto_ptr.
Victor
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Florian Preknya
Guest
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| Posted: Tue Aug 17, 2004 7:50 pm Post subject: Re: Template compilation question |
|
|
The problem is you do not dereferate the auto_ptr pointer.
In the line
pT->Bar ();
where the pT is a pointer to an auto_ptr object, the -> operator is aplied
to dereferate the auto_ptr pointer.
So instead of...
void Bar ()
{
auto_ptr <T> *pT = m_Items [0];
pT->Bar ();
}
you have to change it this way
void Bar ()
{
auto_ptr <T> *pT = m_Items [0];
(*pT)->Bar ();
}
--
Good luck,
Florian.
"Ash" <ash4code (AT) yahoo (DOT) com> wrote
| Quote: | Hello all,
I am trying to write a templated class that stores pointers to
auto_ptr in its data structure and another method invokes certain
methods on the pointer. But I am running into some compilation errors
and I am wondering if what I am doing is legal or not and how should I
go about doing this. Here is the simplest code that does reproduce the
problem.
CODE
#include <iostream
#include
#include
using namespace std;
class Item
{
public:
void Bar () { cout << "Item::Bar" << endl; };
void Foo () { cout << "Item::Foo" << endl; };
};
template
{
public:
Reply () {};
~Reply () {};
// insert an element into the list
void Foo ()
{
auto_ptr <T> *pD = new auto_ptr <T> (new T);
m_Items.push_back (pD);
};
// get the first element and call a method on it
void Bar ()
{
auto_ptr <T> *pT = m_Items [0];
pT->Bar ();
}
private:
vector < auto_ptr m_Items;
};
int main ()
{
Reply <Item> Obj;
Obj.Foo ();
Obj.Bar ();
}
/CODE
I get the following error from Visual studio:
COMPILATION
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.00.9466 for
80x86
Copyright (C) Microsoft Corporation 1984-2001. All rights reserved.
tst.cpp
tst.cpp(30) : error C2039: 'Bar' : is not a member of
'std::auto_ptr<_Ty>'
with
[
_Ty=Item
]
tst.cpp(2 : while compiling class-template member function
'void Reply<T>::Bar(void)'
with
[
T=Item
]
tst.cpp(40) : see reference to class template instantiation
'Reply<T>' being compiled
with
[
T=Item
]
/COMPILATION
I cannot seem to understand why this is not allowed by the standard.
Any comments/suggestions would be appreciated.
Thanks,
/ash
|
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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John Pazarzis
Guest
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| Posted: Tue Aug 17, 2004 8:02 pm Post subject: Re: Template compilation question |
|
|
This will compile fine
#include <iostream>
#include <memory>
#include <vector>
using namespace std;
class Item
{
public:
void Bar () { cout << "Item::Bar" << endl; };
void Foo () { cout << "Item::Foo" << endl; };
};
template
{
public:
Reply () {};
~Reply () {};
// insert an element into the list
void Foo ()
{
auto_ptr <T> *pD = new auto_ptr <T> (new T);
m_Items.push_back (pD);
};
// get the first element and call a method on it
void Bar ()
{
auto_ptr <T> *pT = m_Items [0];
(*pT)->Bar ();
}
private:
vector < auto_ptr m_Items;
};
int main ()
{
Reply <Item> Obj;
Obj.Foo ();
Obj.Bar ();
}
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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P G
Guest
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| Posted: Tue Aug 17, 2004 10:04 pm Post subject: Re: Template compilation question |
|
|
[email]ash4code (AT) yahoo (DOT) com[/email] (Ash) wrote in message news:<e2c8643e.0408161853.5150430 (AT) posting (DOT) google.com>...
| Quote: | Hello all,
I am trying to write a templated class that stores pointers to
auto_ptr in its data structure and another method invokes certain
methods on the pointer. But I am running into some compilation errors
and I am wondering if what I am doing is legal or not and how should I
go about doing this. Here is the simplest code that does reproduce the
problem.
|
I don't know if it's illegal but I'd be interested to know why you'd
want to allocate an auto_ptr on the free store. It would seem that in
doing so one discards most of it's benefits.
| Quote: | CODE
#include <iostream
#include
#include
using namespace std;
class Item
{
public:
void Bar () { cout << "Item::Bar" << endl; };
void Foo () { cout << "Item::Foo" << endl; };
};
template
{
public:
Reply () {};
~Reply () {};
// insert an element into the list
void Foo ()
{
auto_ptr <T> *pD = new auto_ptr <T> (new T);
m_Items.push_back (pD);
};
// get the first element and call a method on it
void Bar ()
{
auto_ptr <T> *pT = m_Items [0];
pT->Bar ();
}
|
pT->Bar() is equivalent to (*pT).Bar. The compiler determined
that(*pT) is of type auto_ptr<T> and complained that auto_ptr<T>
doesn't have a member Bar().
| Quote: | tst.cpp(30) : error C2039: 'Bar' : is not a member of
'std::auto_ptr<_Ty>'
with
[
_Ty=Item
]
tst.cpp(2 : while compiling class-template member function
'void Reply<T>::Bar(void)'
with
[
T=Item
]
tst.cpp(40) : see reference to class template instantiation
'Reply<T>' being compiled
with
[
T=Item
]
/COMPILATION
I cannot seem to understand why this is not allowed by the standard.
Any comments/suggestions would be appreciated.
Thanks,
/ash
|
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Arkadiy Vertleyb
Guest
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| Posted: Tue Aug 17, 2004 10:06 pm Post subject: Re: Template compilation question |
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[email]ash4code (AT) yahoo (DOT) com[/email] (Ash) wrote
| Quote: | auto_ptr <T> *pT = m_Items [0];
pT->Bar ();
tst.cpp(30) : error C2039: 'Bar' : is not a member of
'std::auto_ptr<_Ty>'
|
You call Bar() on the auto_ptr itself rather than on the object it
points to, and the compiler does tell you about this. Try something
like:
(*pT)->Bar ();
HTH,
Arkadiy
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