Re: Using typedef to define a forward declaration

"Balog Pal" <>
Mon, 9 Nov 2009 14:47:11 +0100
"Victor Bazarov" <>

Christoph Bartoschek wrote:

the following code does not work:

class A;
class B {};
typdef B A;

Is there a deep reason why this should not work?

You mean, besides the fact that it makes no real sense to do that?

It would make perfect sense to do that, and the miss of such feature is
pretty damn painful.

You're not using 'A' between the forward-declaration and the typedef.

Come on, now ;). Look up the zillion old threads asking why one can't say

class std::string;
void foo(class std::string const & str);

not including <string> in the header and write all the declarations riding
the incomplete type -- then when definition or use is due include the

So, why would you want to forward-declare it? Use an example closer to
real life.

people struggling for separation of interfaces and cut dependencies could
quote a big deal of code matching the problem. It breaks transparency
badly, suppose you had the code working with the incomplete type, than at
some point of time, where your class had a separate implementation in a
separate file.

Then you observe that the class is generic enough, and can be made into a
template, the original turining to some Templ<T> that can be put back as a
typedef. And then notice, that the forward declaration is not compatible.

I guess the reason lies with some historic feature of the linkers and name
mangling. Too bad it got not covered ever since. C++0x fixes the related
problem for enums.

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