What is wrong with this code?(returning an item in the vector)

[Sims]

Hi,

Here is the code that .NET does not seem to like, but as far as i can see it
is valid C++ code.
Am i wrong?

....
// vector headers
....

struct MYSTRUCT
{
int m_iSomething;
};

typedef std::vector< MYSTRUCT, std::allocator< MYSTRUCT > > MYSTRUCT_VECTOR ;

....
// Within class
....

MYSTRUCT_VECTOR g_MyVector;

int CMyClass::FindS omeThing( int pos )
{
// return item
}

MYSTRUCT * CMyClass::GetSt ructure( int iSomething)
{
// Find the item in the vector
int pos = FindSomeThing( iSomething );
// Anything found
if( pos <0 ) return NULL;
// return what we have
return (g_MyVector.beg in()+pos);
}

The code above should work i think but .NET 2002 gives me an error? What am
i missing?

Many thanks

Sims

[Sims]

>

MYSTRUCT * CMyClass::GetSt ructure( int iSomething)
{
// Find the item in the vector
int pos = FindSomeThing( iSomething );
// Anything found
if( pos <0 ) return NULL;
// return what we have
return (g_MyVector.beg in()+pos);
}

Sorry, the error .NET gives me is that there is a conversion error in my
return value.

Sims

[Hendrik Belitz]

Sims wrote:

MYSTRUCT * CMyClass::GetSt ructure( int iSomething)
{
// Find the item in the vector
int pos = FindSomeThing( iSomething );
// Anything found
if( pos <0 ) return NULL;
// return what we have
return (g_MyVector.beg in()+pos);
}

Sorry, the error .NET gives me is that there is a conversion error in my
return value.

Your return value is of type vector<...>::it erator not of MYSTRUCT*. Just
because vector iterators are most often implemented as ordinary pointers
doesn't mean iterators and pointers are interchangeable .

--
To get my real email adress, remove the two onkas
--
Dipl.-Inform. Hendrik Belitz
Central Institute of Electronics
Research Center Juelich

[Howard]

"Hendrik Belitz" <ho************ **@fz-juelich.de> wrote in message
news:bu******** ***@zam602.zam. kfa-juelich.de...

Sims wrote:

MYSTRUCT * CMyClass::GetSt ructure( int iSomething)
{
// Find the item in the vector
int pos = FindSomeThing( iSomething );
// Anything found
if( pos <0 ) return NULL;
// return what we have
return (g_MyVector.beg in()+pos);
}

Sorry, the error .NET gives me is that there is a conversion error in my
return value.

Your return value is of type vector<...>::it erator not of MYSTRUCT*. Just
because vector iterators are most often implemented as ordinary pointers
doesn't mean iterators and pointers are interchangeable .

To follow up on that...

if you want to return pointers to MYSTRUCT, then you can always actually
STORE pointers to MYSTRUCT, instead of the objects themselves. In that
case, you dereference the iterator to get your pointer. (And remember that
you'll need to delete the objects pointed to when you're done with the
vector.)

I'm not sure if you can return a pointer to an object in a vector when
you're storing the objects themselves, however. Someone else would have to
answer that. (I've only used vectors where I store the pointers,
personally.)

-Howard

[Sharad Kala]

"Sims" <si*********@ho tmail.com> wrote in message
news:bu******** ****@ID-162430.news.uni-berlin.de...

MYSTRUCT * CMyClass::GetSt ructure( int iSomething)
{
// Find the item in the vector
int pos = FindSomeThing( iSomething );
// Anything found
if( pos <0 ) return NULL;
// return what we have
return (g_MyVector.beg in()+pos);
}

This is probably because Iterators for vector class on your implementation are
not implemented as pointers.
The same code (after some changes) however compiles fine on Comeau online.

So, to be safe use MYSTRUCT_VECTOR ::iterator as the return type instead of
MYSTRUCT *.
Also use std::advance instead of '+' on iterators.

Best wishes,
Sharad

[Hendrik Belitz]

Sharad Kala wrote:

So, to be safe use MYSTRUCT_VECTOR ::iterator as the return type instead
of MYSTRUCT *.
Also use std::advance instead of '+' on iterators.

Using operator+() on random access iterators is defined in the C++ Standard,
so I don't think you should use std::advance() here.

--
To get my real email adress, remove the two onkas
--
Dipl.-Inform. Hendrik Belitz
Central Institute of Electronics
Research Center Juelich

[Hendrik Belitz]

Howard wrote:

I'm not sure if you can return a pointer to an object in a vector when
you're storing the objects themselves, however. Someone else would have
to
answer that. (I've only used vectors where I store the pointers,
personally.)

AFAIK you can do the following:

vector<object> v;
....
vector<object>: :iterator it = v.begin();
....
object* ptr = &(*it);

So this works, altough I think working with references or even copies
(Insteas of returning a pointer) may be a better way.

--
To get my real email adress, remove the two onkas
--
Dipl.-Inform. Hendrik Belitz
Central Institute of Electronics
Research Center Juelich

[Sharad Kala]

"Hendrik Belitz" <ho************ **@fz-juelich.de> wrote in message
news:bu******** ***@zam602.zam. kfa-juelich.de...

Sharad Kala wrote:

So, to be safe use MYSTRUCT_VECTOR ::iterator as the return type instead
of MYSTRUCT *.
Also use std::advance instead of '+' on iterators.

Using operator+() on random access iterators is defined in the C++ Standard,
so I don't think you should use std::advance() here.

True :-)
But to make the code generic for other containers one should be using advance.
Since here it's a vector using '+' is justified but otherwise if the container
itself is generic then it's preferable to use advance.

Best wishes,
Sharad

[Chris Theis]

"Sims" <si*********@ho tmail.com> wrote in message
news:bu******** ****@ID-162430.news.uni-berlin.de...

MYSTRUCT * CMyClass::GetSt ructure( int iSomething)
{
// Find the item in the vector
int pos = FindSomeThing( iSomething );
// Anything found
if( pos <0 ) return NULL;
// return what we have
return (g_MyVector.beg in()+pos);
}

Sorry, the error .NET gives me is that there is a conversion error in my
return value.

Sims

The reason for the error is that what you actually return is a
vector::iterato r and not (necessarily) a pointer to MYSTRUCT. Although
vector iterators are very often implemented in terms of ordinary pointers
they still have an implementation specific type, which can be totally
different from an ordinary pointer. The portable way to solve this problem
is the following for example:

return &(*g_MyVector.b egin());

However, I'm amazed that .NET 2002 complains as VC++ 6.0 has no problem with
your approach (although I wouldn't recommend it!).

Cheers
Chris

[Sims]

Hi,

The reason for the error is that what you actually return is a
vector::iterato r and not (necessarily) a pointer to MYSTRUCT. Although
vector iterators are very often implemented in terms of ordinary pointers
they still have an implementation specific type, which can be totally
different from an ordinary pointer. The portable way to solve this problem
is the following for example:

return &(*g_MyVector.b egin());

However, I'm amazed that .NET 2002 complains as VC++ 6.0 has no problem with your approach (although I wouldn't recommend it!).

Sorry, what do you not recommend? VC++6 or My approach?, if it is my
approach what would you do instead?

Sims