Already Discussed? Proposal of Simplified Class Forward Declarations using Namespaces.

[lucky.r...@gmail.com]

Hi

I would like to create a proposal to simplify class forward declarations using namespaces.

Before I invest time into the proposal, I did like to know if this kind of proposal was already discussed and/or rejected. My internet search did not got any result, but this may be just the case because I did not found the right keywords for my search.

My core proposal is actually quite simple. If one is using libraries and namespaces, a forward declaration of a class in a namespace is a common thing

namespace ex {
class Foo;
class Bar;
}

class Example {
  ex::Foo *_foo;
  ex::Bar *_bar;
};

With nested namespace, this code gets worse:

namespace ex {
namespace foo {
class Foo;
}
namespace bar {
class Bar;
}
}

class Example {
  ex::foo::Foo *_foo;
  ex::bar::Bar *_bar;
};

If namespaces in forward declarations would be allowed, it would simplify the code:

class ::ex::foo::Foo;
class ::ex::bar::Bar;

class Example {
  ex::foo::Foo *_foo;
  ex::bar::Bar *_bar;
};

Was this feature ever discussed? Was it rejected, and why?

All the best,

LR

[Dejan Milosavljevic]

In forward declaration it is not clear are foo or bar is namespace or class. 

namespace ex::foo; //!< foo is namespace an this imply that ex is namespace too

namespace ex::bar;

class ::ex::foo::Foo;

class ::ex::bar::Bar;

class Example {
  ex::foo::Foo *_foo;
  ex::bar::Bar *_bar;
};

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[tobias....@gmail.com]

You are absolutely right!

A solution would be a forward declaration of namespaces, as you shown in your code.

This would still remove the requirements for the brackets around the class definitions and allow a simpler syntax.

All the best,

LR

[Victor Dyachenko]

Forwarding namespaces is an orthogonal task. Why do we need it at all?

namespace ex::foo {}

And it's legal already.

About allowing namespaces in forward declarations I think that it's a good idea. Missing this feature. It would be very handy.

[Gašper Ažman]

Namespaces and class names are in different... Name spaces. Even if you forward declare a namespace it's not a given a prefix is in fact a namespace. What we have is as good as it gets.

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[Gašper Ažman]

I seem to have been wrong. Seems like you can't declare a type with the same name as a namespace. I was writing from a train, should have checked on godbolt first :).

On Fri, Mar 1, 2019, 09:51 <tobias....@gmail.com> wrote:

Even if you forward declare a namespace it's not a given a prefix is in fact a namespace.

Can you explain this in detail?

I would assume, if I declare a namespace:

namespace foo {}

In this scope, "foo" would be recognised as a namespace, so the following forward declaration would be clear for the compiler.

class foo::Example;

What did I miss? 

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