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digitalmars.D - Forward Declarations

reply "Miguel Ferreira Simões" <Kobold netcabo.pt> writes:
Is there any way to forward declarate a class?
When I try to do it... the compiler says that forward declaration conflicts 
with the real declaration.

I can work around the problem using cross imports between modules, is it a 
good practice?

Thanks,
Miguel Ferreira Simoes
Feb 02 2005
next sibling parent "Nick Sabalausky" <z a.a> writes:
AFAIK, you shouldn't need to in D.

"Miguel Ferreira Simões" <Kobold netcabo.pt> wrote in message 
news:ctrd0j$u3j$1 digitaldaemon.com...

 Is there any way to forward declarate a class?
 When I try to do it... the compiler says that forward declaration 
 conflicts with the real declaration.

 I can work around the problem using cross imports between modules, is it a 
 good practice?

 Thanks,
 Miguel Ferreira Simoes
Feb 02 2005
prev sibling parent reply zwang <nehzgnaw gmail.com> writes:
Miguel Ferreira Simões wrote:

 Is there any way to forward declarate a class?
 When I try to do it... the compiler says that forward declaration conflicts 
 with the real declaration.
 
 I can work around the problem using cross imports between modules, is it a 
 good practice?
 
 Thanks,
 Miguel Ferreira Simoes 
 
 


The doc says:
"The program is looked at as a whole, and so not only is it not 
necessary to code forward declarations, it is not even allowed! "

(http://www.digitalmars.com/d/ctod.html#forwardfunc)
Feb 02 2005
next sibling parent "Miguel Ferreira Simões" <Kobold netcabo.pt> writes:
Thanks!
I should have looked better in the documentation. :(

Miguel Ferreira Simões
Feb 02 2005
prev sibling parent reply =?ISO-8859-1?Q?Anders_F_Bj=F6rklund?= <afb algonet.se> writes:
zwang wrote:


 Is there any way to forward declarate a class?
 When I try to do it... the compiler says that forward declaration 
 conflicts with the real declaration.

 I can work around the problem using cross imports between modules, is 
 it a good practice?


 The doc says:
 "The program is looked at as a whole, and so not only is it not 
 necessary to code forward declarations, it is not even allowed! "
 
 (http://www.digitalmars.com/d/ctod.html#forwardfunc)


However, if you need to provide D source modules
for symbols that will be defined elsewhere - then
you can do so called "import modules" (a.k.a. stubs).

They are similar to the regular code, but without
the implementation body that regularly follows...

See this example, from object.d:


 class Object
 {
     void print();
     char[] toString();
     uint toHash();
     int opCmp(Object o);
     int opEquals(Object o);
 }


That compiles alright, and when you link it then
it uses the implementation from internal/object.d
(that is contained in the "phobos" object library)

They both have the declaration: module object;

Thus you can ship one public "import module",
while keeping the implementation in a library.
(if you for some reason dislike Open Source)


This is similar to how you declare C functions,
in "import modules" for D usage. The only caveat
there is that any variables in it will be defined.


 extern (C) int variable; // declares "variable"!


This differs greatly from C/C++ headers,
where any "extern" variable declarations
will not be defined. But again, D is not C...

So if you need to import data symbols, then
you put those statements in *another* module,
which you then import from the import module...


Here's an example, for the "whatever" library:

whatever.d:

 // import module for whatever
 module whatever;
 
 private import whateverextern;
 
 extern (C) void function();


whateverextern.d:

 // extern module for whatever
 module whateverextern;
 
 extern (C) int variable;


The data module is called an "extern module", and
should *not* be compiled into the program - instead
you link against the C or D library you want to import.

std.c.linux and std.c.linuxextern are good examples
of using this to import data symbols from a C library.
(it can also be used for D code, by omitting "extern")


Confusing? Normally it isn't needed, and you can just
dump all your source code on "dmd" to compile and link...
(and having public data symbols in libraries is illegal
in three states already, and should be avoided if possible)

But the import modules comes in handy once, in a while.
(and there should be a tool to *automatically* create
them from the implementation, to avoid duplication...)

--anders
Feb 03 2005
parent "Miguel Ferreira Simões" <Kobold netcabo.pt> writes:
Interesting, I had never thought about it.
Thanks, for increasing my D knowloedge!

Miguel
Feb 03 2005