Forward declaration of a namespaced C++ class in in Objective C

Question

Is it possible to do forward declaration of a C++ class, which is inside a namespace, in an Objective C header file?

C++ class to forward declare in Objective C:

namespace name
{
    class Clazz
    {

    };
}

I know I can if the C++ class is not in a namespace. Taken from http://www.zedkep.com/blog/index.php?/archives/247-Forward-declaring-C++-classes-in-Objective-C.html

Any thoughts?
Thanks guys!

What use is a forward declared C++ class? just use a #typedef void Clazz in that situation. — Richard J. Ross III
– Richard J. Ross III, Commented Apr 11, 2012 at 1:54

justin · Accepted Answer · 2012-04-11 17:16:39Z

5

You just declare it as usual, and wrap it in the C++ #ifdef check when the header is also included in C and/or ObjC translations:

#ifdef __cplusplus
namespace name {
class Clazz;
} // << name
#endif

For obvious reasons, you cannot use name::Clazz in a C or ObjC translation, so this works out perfectly. You either need the forward declaration in the C++ translation, or it does not need to be visible to C or ObjC.

edited Apr 11, 2012 at 17:16

answered Apr 11, 2012 at 4:58

justin

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1 Comment

Butaca Over a year ago

I didn't know about the __cplusplus definition. It is all clear now. Thanks.

N_A · Accepted Answer · 2012-04-11 04:51:21Z

2

You can forward declare your class as a struct in your objective-c++ headers and then include them in C or obj-c code. The same restrictions that normally apply also apply to this case. The only caveat is that you have to put ifdefs around the namespace so they don't show up when including your header in C code since the C compiler doesn't know about namespaces.

#ifdef __cplusplus
namespace name{
#endif
    struct classname;
#ifdef __cplusplus
}
using namespace name;
#endif

answered Apr 11, 2012 at 4:51

N_A

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Butaca Over a year ago

Your answer was useful too. Thanks!

NG_ · Accepted Answer · 2025-02-27 21:10:09Z

1

You can declare a forward class in a namespace like this:

namespace MyNamespace
{
    class MyClass;
};

It should work in Obj-C++, but if not you could also try obj-c's @class, or just do a typedef void* Clazz.

edited Feb 27 at 21:10

NG_

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answered Apr 11, 2012 at 2:00

wquist

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1 Comment

Butaca Over a year ago

Your answer was useful too. Thanks!

Jerry Coffin · Accepted Answer · 2012-04-11 02:00:00Z

0

At least in normal C++, you'd do something like:

namespace a {
    class b;
}

int main(){
    a::b *c;
}

I can't say whether the Objective C compiler will accept that or not though.

answered Apr 11, 2012 at 2:00

Jerry Coffin

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1 Comment

N_A Over a year ago

objective-c is a pure superset of C so it wouldn't recognize the namespace identifier

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