I try to use the namespace in a class definition as follows,

#include <iostream>

class raw_data{
using namespace std; // Here, the compiler gave an error mesage
"syntax error"
.....
};

If the 'using namespace std' is put before the class raw_data, it's
OK. But I don't know why the above code segment cannot work. Is it
forbiden for C++ to use embeded namespace?

Could somebody here give me some hint on that?

Thanks in advance.

Z.L.

--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

Z.L. wrote:
> class raw_data{
> using namespace std; // Here, the compiler gave an error mesage
> "syntax error"
> ....
> };
>
> If the 'using namespace std' is put before the class raw_data, it's
> OK. But I don't know why the above code segment cannot work. Is it
> forbiden for C++ to use embeded namespace?

Yes, this is simply not supported. However, what you can do is being a bit
more specific with what you want:

using std::istream;

Further, your terminology is a bit misleading, because embedding could be
interpreted as nesting or importing. A using directive imports the named
symbols into the current namespace. Further, there are nested namespaces
and classes, i.e. namespaces or classes within other namespaces and
classes. You can nest both of them inside each other and you can nest
classes inside namespaces, but you can not nest namespaces inside classes.
If you need that, you can use a nested class (or structure) with only
public and static members as workaround.

Uli

--
Sator Laser GmbH
Geschäftsführer: Thorsten Föcking, Amtsgericht Hamburg HR B62 932


[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

On 2008-08-19 11:58, Z.L. wrote:
> I try to use the namespace in a class definition as follows,
>
> #include <iostream>
>
> class raw_data{
> using namespace std; // Here, the compiler gave an error mesage
> "syntax error"
> ....
> };
>
> If the 'using namespace std' is put before the class raw_data, it's
> OK. But I don't know why the above code segment cannot work. Is it
> forbiden for C++ to use embeded namespace?
>
> Could somebody here give me some hint on that?

The C++ standard is quite clear on this (it is the first sentence in the
section): "A using-directive shall not appear in class scope, but may
appear in namespace scope or in block scope."

--
Erik Wikström


[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]

<snip everything>

it is btw pity that this isn't possible.

By specifing those 'using' statements on global level, especially in
header files, it will be picked up by every client of the header file.
So thats imo not an option.

So then you must specify the complete namespace which is a lot of
typing and makes it quite unreadable (real life example of the handy
Boost.MultiIndex, often used in classes as replacement of STL
containers):

class AcclReceivers
{
//...

typedef std::pair<CWnd*, int> Entry;

typedef boost::multi_index::multi_index_container
<
Entry,
boost::multi_index::indexed_by
<
boost::multi_index::ordered_unique<
boost::multi_index::identity<Entry> >,

boost::multi_index::ordered_non_unique<
boost::multi_index::member<Entry, CWnd*, &Entry::first> >,
boost::multi_index::ordered_non_unique<
boost::multi_index::member<Entry, int, &Entry::second> >
>
> EntrySet;

EntrySet m_Entries;
};

Of course it can be solved by a global namespace alias, or putting the
class in its own namespace (and then one has the 'using' options
back), but this alias is only needed within the class for readability
improvements, so ideally it would only be put there.

--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]