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yuliy@gmx.de
Guest
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 Posted: Tue Nov 29, 2005 8:50 am Post subject: How to forward declare a class in namespace? |
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Hello gurus,
I stuck in following: how can I do forward declaration if the forward
declared class is in some namespace?
something like
// header
class std::string; // approach#1
namespace std
{
class string; // approach#2
};
struct A
{
A();
~A();
std::string * p;
};
both approaches does not compiled.
So, How can I do it?
Thanks
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Dervish
Guest
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| Posted: Tue Nov 29, 2005 9:11 am Post subject: Re: How to forward declare a class in namespace? |
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Following compiles well:
namespace std_
{
class string; // approach#2
};
struct A
{
std_::string * p;
};
So the problem is not in forward decalration in namespace. Actually
std::string is not a class but a typedef to template basic_string
instantiated with type char. Therefore forward declaration fails. So I
think there is no legal way to forward declare std::string.
Only classes accessed through <iostream> can be forward declared in
STL. One can use <iosfwd> to do this.
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Gabriel
Guest
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| Posted: Tue Nov 29, 2005 9:24 am Post subject: Re: How to forward declare a class in namespace? |
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[email]yuliy (AT) gmx (DOT) de[/email] wrote:
| Quote: | Hello gurus,
I stuck in following: how can I do forward declaration if the forward
declared class is in some namespace?
something like
// header
class std::string; // approach#1
namespace std
{
class string; // approach#2
};
struct A
{
A();
~A();
std::string * p;
};
both approaches does not compiled.
So, How can I do it?
Thanks
|
Approach #2 works in general. See this example, which compiles just fine
(compiling using cygwin: cygming special):
namespace ns
{ struct A; }
struct B
{ ns::A* a; };
namespace ns
{ struct A {}; }
int main()
{
B b; // "unused variable", I know
return 0;
}
You problem is that you got the type of std::string wrong. So, you
create a conflict between you saying
namespace std { class string; }
and the <string> header saying something else.
Consider this example, which also works:
// header.hpp
namespace std
{
template<typename _CharT, typename _Traits, typename _Alloc> class
basic_string;
typedef basic_string<char> string;
}
struct B
{ std::string* s; };
// main.cpp
#include <string>
#include "header.hpp"
int main()
{
B b;
return 0;
}
In the forward declaration I use the declaration which is also used in
the <string> library header.
Sadly, the above is still implementation-dependent. I would appreciate
some enlightenment how to do this really non-implementation-dependent.
Gabriel
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Gabriel
Guest
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| Posted: Tue Nov 29, 2005 9:26 am Post subject: Re: How to forward declare a class in namespace? |
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Dervish wrote:
| Quote: | Following compiles well:
namespace std_
{
class string; // approach#2
};
struct A
{
std_::string * p;
};
So the problem is not in forward decalration in namespace. Actually
std::string is not a class but a typedef to template basic_string
instantiated with type char. Therefore forward declaration fails. So I
think there is no legal way to forward declare std::string.
Only classes accessed through <iostream> can be forward declared in
STL. One can use <iosfwd> to do this.
|
Why is that?
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Neil Cerutti
Guest
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| Posted: Tue Nov 29, 2005 12:31 pm Post subject: Re: How to forward declare a class in namespace? |
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On 2005-11-29, Gabriel <abuse (AT) 127 (DOT) 0.0.1> wrote:
| Quote: | // header.hpp
namespace std
{
template<typename _CharT, typename _Traits, typename _Alloc> class
basic_string;
typedef basic_string<char> string;
}
struct B
{ std::string* s; };
// main.cpp
#include <string
#include "header.hpp"
int main()
{
B b;
return 0;
}
In the forward declaration I use the declaration which is also
used in the
Sadly, the above is still implementation-dependent. I would
appreciate some enlightenment how to do this really
non-implementation-dependent.
|
It can't be done. The implementation is allowed to include extra
trailing template parameters in the basic_string template, so no
forward declaration of std::basic_string will be portable.
A proxy class that contains a std::string could be used instead.
class string_proxy;
struct b
{
string_proxy *s;
};
string_proxy could be defined in your .cpp file.
#include <string>
class string_proxy
{
std::string s_;
public:
string_proxy(const std::string& s): s_(s) { }
std::string& operator() { return s_; }
const std::string& operator() const { return s_; }
};
The above is a simple-minded example; you might want something
with an automatic conversion to std::string, or implement the
subset of the std::string interface that you need for class B.
--
Neil Cerutti
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