Changing namespaces

[Blue Ocean]

Forgive me for asking so many questions so quickly. I am simply
asking as they come up unanswered in my book.

Can you change namespaces half way through a file? For instance, if
you want to use std and then half way through switch to your own:
{
...
using namespace std;
cout << "doing polygons";
using namespace polygon;
cout << poly1;
...
}

I know that this is a ridiculous example. Why would anyone define an
outstream for a 3D object with the name cout. But, suppose they did,
would this code snippet work, or do you have to use just one namespace
per file and be explicit about the use of other namespaces?

[John Harrison]

On 7 Jul 2004 13:34:39 -0700, Blue Ocean <bl*********@ho tmail.com> wrote:

Forgive me for asking so many questions so quickly. I am simply
asking as they come up unanswered in my book.

Can you change namespaces half way through a file? For instance, if
you want to use std and then half way through switch to your own:
{
...
using namespace std;
cout << "doing polygons";
using namespace polygon;
cout << poly1;
...
}

I know that this is a ridiculous example. Why would anyone define an
outstream for a 3D object with the name cout. But, suppose they did,
would this code snippet work, or do you have to use just one namespace
per file and be explicit about the use of other namespaces?

You aren't changing namespaces, you are adding to the set of namespaces
that will be searched. You can have as many 'using namespace ...' as you
like and all of them will be searched.

In your example, if cout was defined in namespace std and polygon, then
you would get a compiler error 'ambiguous name cout' or some such.

john

[Russell Hanneken]

Blue Ocean wrote:

Forgive me for asking so many questions so quickly. I am simply
asking as they come up unanswered in my book.

Can you change namespaces half way through a file? For instance, if
you want to use std and then half way through switch to your own:
{
...
using namespace std;
cout << "doing polygons";
using namespace polygon;
cout << poly1;
...
}

I think you misunderstand using directives. A using directive makes
names from a namespace available as if they had been declared in the
global namespace. The second using directive in your example doesn't
nullify the effect of the first one. Assuming there is a polygon::cout,
the code would fail to compile, because there would be a name conflict.

Since you're a Java programmer, this example might help:

import java.awt.*; // The java equivalent of a using directive.
import java.util.*;

public class Foo
{
public static void main (String[] args)
{
List list;
}
}

This Java code suffers from pretty much the same problem as your C++
example. Try to compile it and you'll see what I mean.

--
Russell Hanneken
eu*******@cbobk .pbz
Use ROT13 to decode my email address.

[JustSomeGuy]

John Harrison wrote:

On 7 Jul 2004 13:34:39 -0700, Blue Ocean <bl*********@ho tmail.com> wrote:

Forgive me for asking so many questions so quickly. I am simply
asking as they come up unanswered in my book.

Can you change namespaces half way through a file? For instance, if
you want to use std and then half way through switch to your own:
{
...
using namespace std;
cout << "doing polygons";
using namespace polygon;
cout << poly1;
...
}

I know that this is a ridiculous example. Why would anyone define an
outstream for a 3D object with the name cout. But, suppose they did,
would this code snippet work, or do you have to use just one namespace
per file and be explicit about the use of other namespaces?

You aren't changing namespaces, you are adding to the set of namespaces
that will be searched. You can have as many 'using namespace ...' as you
like and all of them will be searched.

In your example, if cout was defined in namespace std and polygon, then
you would get a compiler error 'ambiguous name cout' or some such.

john

Wow.. So if you put:

using namespace std;
using namespace system;
using namespace mine;

at the top of your code then all three namespaces will be searched... and
assuming there is no conflict they will be resolved?

So if you have "std:ios:in " in your code, you could change it for
using namespace std:
using namespace ios:
and then just specify "in"?

[JKop]

JustSomeGuy posted:

using namespace std;
using namespace system;
using namespace mine;

This will dump everything into the global namespace.

Not only can you just write:
string pp;
You can write:

::string pp;
Ahh!!
-JKop

[Russell Hanneken]

JustSomeGuy wrote:

Wow.. So if you put:

using namespace std;
using namespace system;
using namespace mine;

at the top of your code then all three namespaces will be searched... and
assuming there is no conflict they will be resolved?
Sure.
So if you have "std:ios:in " in your code, you could change it for
using namespace std:
using namespace ios:
and then just specify "in"?

I think you meant to type semicolons rather than colons at the end of
your using directives. Anyway, that wouldn't work, because ios is a
class, not a namespace.

--
Russell Hanneken
eu*******@cbobk .pbz
Use ROT13 to decode my email address.

[John Harrison]

On Wed, 07 Jul 2004 15:34:37 -0600, JustSomeGuy <No***@ucalgary .ca> wrote:

John Harrison wrote:

On 7 Jul 2004 13:34:39 -0700, Blue Ocean <bl*********@ho tmail.com>
wrote:

> Forgive me for asking so many questions so quickly. I am simply
> asking as they come up unanswered in my book.
>
> Can you change namespaces half way through a file? For instance, if
> you want to use std and then half way through switch to your own:
> {
> ...
> using namespace std;
> cout << "doing polygons";
> using namespace polygon;
> cout << poly1;
> ...
> }
>
> I know that this is a ridiculous example. Why would anyone define an
> outstream for a 3D object with the name cout. But, suppose they did,
> would this code snippet work, or do you have to use just one namespace
> per file and be explicit about the use of other namespaces?
You aren't changing namespaces, you are adding to the set of namespaces
that will be searched. You can have as many 'using namespace ...' as you
like and all of them will be searched.

In your example, if cout was defined in namespace std and polygon, then
you would get a compiler error 'ambiguous name cout' or some such.

john

Wow.. So if you put:

using namespace std;
using namespace system;
using namespace mine;

at the top of your code then all three namespaces will be searched... and
assuming there is no conflict they will be resolved?

Right

So if you have "std:ios:in " in your code, you could change it for
using namespace std:
using namespace ios:
and then just specify "in"?

No because ios is not a namespace, its a typedef for a template. But yes,
in general you're right.

namespace outer
{
namespace inner
{
int xxx;
}
}

using namespace outer;
using namespace inner;

int main()
{
xxx = 1;
}

john

[John Harrison]

On Wed, 07 Jul 2004 21:41:41 GMT, JKop <NU**@NULL.NULL > wrote:

JustSomeGuy posted:

using namespace std;
using namespace system;
using namespace mine;

This will dump everything into the global namespace.

Not only can you just write:
string pp;
You can write:

::string pp;
Ahh!!

Interesting, but ugly. I suppose that demonstrates that it's not really
the case that namespaces are searched, but rather that the names from a
namespace are put into the global namespace.

john

[Andre Kostur]

JKop <NU**@NULL.NULL > wrote in news:p6******** *********@news. indigo.ie:

JustSomeGuy posted:

using namespace std;
using namespace system;
using namespace mine;

This will dump everything into the global namespace.

Not only can you just write:
string pp;
You can write:

::string pp;

Somebody correct me if I'm wrong, but this is bad info.

I think of the using namespace declarations as adding to a search path for
identifiers, not actually copying them into other namespaces.

The "::string pp" is explicitly looking for a "string" identifier in the
global namespace, and no other namespaces. Thus the "::string" would not
be the same as "std::strin g".

[Russell Hanneken]

Andre Kostur wrote:

JKop <NU**@NULL.NULL > wrote in news:p6******** *********@news. indigo.ie:

JustSomeGuy posted:

using namespace std;

This will dump everything into the global namespace.

Not only can you just write:

string pp;

You can write:

::string pp;

Somebody correct me if I'm wrong, but this is bad info.

I think you're right. ::string fails to compile on GCC 3.3.3 and the
Comeau Online Compiler.

--
Russell Hanneken
eu*******@cbobk .pbz
Use ROT13 to decode my email address.