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

[Hendrik Belitz]

Sims wrote:

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!).

Generally your approach is not portable and is also no good programming
style. I would recommend to strictly distinct between iterators and
pointers. Or see pointers as iterators, but not the other way round.

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

[Chris Theis]

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

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?

My posting might be a little misleading regarding what I do not recommend.
If you prefer to leave the code as it is returning a pointer to MYSTRUCT
then you should do it as I showed above:

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

On the other hand you can also change the return type to
MYSTRUCT_VECTOR ::iterator and leave the return statement as it is. The way
you implemented it works under VC++ 6.0 and Comeau although it is not
required to do so and it is not portable. Hence, I'd recommend to change
either the return type or the return statement.

Regards
Chris

[News 2]

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

You must be very careful if you return pointers to objects stored in a
vector. The vector can and likely
will reassign the objects to other locations when you add data to the
vector. Storing pointers rather than
object is a better solution but doesn't utilize the capabilities of vectors
and STL in general. Obviously you must look at how you are using the array
and decide what is appropriate.

James