Re: compile error about auto_ptr

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
microsoft.public.vc.language
Date:
Sat, 23 Feb 2008 15:37:41 +0100
Message-ID:
<13s0btn9ejtif99@corp.supernews.com>
* George:

I am confused to read what the compiler says. Could anyone explain what it
means?

[Code]
#include <memory>
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main( )
{
    int i = 0;

    vector<auto_ptr<int>> vc;

In C++0x you will be able to write ">>", but as of the current standard
you must write "> >".

     for (i = 0; i < 5; i ++) // 0 1 2 3 4
    {
        auto_ptr<int> pi (new int(i));
        vc.push_back (pi);
    }

    return 0;
}
[/Code]

1>Compiling...
1>main.cpp
1>d:\program files\microsoft visual studio 9.0\vc\include\vector(1209) :
error C2558: class 'std::auto_ptr<_Ty>' : no copy constructor available or
copy constructor is declared 'explicit'


This means that no copy constructor is available or the copy constructor
is declared 'explicit', for class 'std::auto_ptr<_Ty>'.

Evidently a copy constructor must be available.

Now, that wasn't so difficult, was it?

1> with
1> [
1> _Ty=int
1> ]


This says the type '_Ty' referred to earlier is actually 'int'.

I.e., there's no available copy constructor for 'std::auto_ptr<int>'.

1> d:\program files\microsoft visual studio
9.0\vc\include\vector(1158) : while compiling class template member function
'void
std::vector<_Ty>::_Insert_n(std::_Vector_const_iterator<_Ty,_Alloc>,unsigned
int,const _Ty &)'
1> with
1> [
1> _Ty=std::auto_ptr<int>,
1> _Alloc=std::allocator<std::auto_ptr<int>>
1> ]


This tells you that it discovered the earlier problem while compiling
the std::vector::_Insert_n member function.

The underscore at the start of the name tells you that it is an internal
implementation detail, not a member function you have called directly.

1> d:\visual studio
2008\projects\test_autoptr1\test_autoptr1\main.cpp(11) : see reference to
class template instantiation 'std::vector<_Ty>' being compiled
1> with
1> [
1> _Ty=std::auto_ptr<int>
1> ]


This offers you even more information the problem: it seems you have
instantiated std::vector, a standard container class template, with type
'std::auto_ptr<int>'.

Or, in the compiler's words, see reference to the class template
instantiation 'std::vector< std::auto_ptr< int > >' being compiled.

Silly.

Element types for standard containers must be copy-constructible.

Considering that std::auto_ptr is all about transferring ownership when
you attempt to copy it, do you think it's copy-constructible?

1>d:\program files\microsoft visual studio 9.0\vc\include\vector(1233) :
error C2558: class 'std::auto_ptr<_Ty>' : no copy constructor available or
copy constructor is declared 'explicit'
1> with
1> [
1> _Ty=int
1> ]


For some reason the compiler now again informs you that there's no copy
constructor available for your chosen std::vector element type.

thanks in advance,


As you can see above, all that's required is to /read/ the error messages.

Take one little bit at a time.

Cheers, & hth.,

- Alf

--
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