为什么我不能在C ++中的合格名称空间中向前声明类型？

Question

I am trying to avoid including an auto-generated header (outside of my control), from inside my own headers. I therefore need to forward-declare a type Type that lives inside a namespace nm , which itself lives in the global namespace. Here is an example of MyHeader.h :

// My project defines two levels of nested namespaces 
namespace foo { namespace bar {
    namespace nm { struct Type; }
    ...
} }

Unfortunately this defines the new namespace foo::bar::nm and forward-declares the type foo::bar::nm::Type , which is not what I want. Ideally I would be able to forward-declare a type in the qualified namespace ::nm like this:

namespace foo { namespace bar {
    namespace ::nm { struct Type; }
    ...
} }

My compiler complains I cannot use a qualified namespace here (using Intel ICC15 with C++11 settings). This forces me to put all such forward declarations at the beginning:

namespace nm { struct Type; }
namespace foo { namespace bar {
    ...
} }

In my case, this is inconvenient because I need to forward-declare many types and would prefer to do so alongside definitions of my own in various scattered places in my header. A workaround could be to constantly be "closing" and "re-opening" my nested namespace, which is not ideal.

Why can't I forward-declare a type in a qualified namespace ?

Answer 1

因为C ++标准是这样说的。

Answer 2

There is no real reason I know of. This is just not supported, probably, no one needed it and no one ever requested it. I see the benefit in your particular case, but since the forward declarations are expected to be provided in a separate file, you'd be better off by simply creating your own *_fwd.h with all the forward declarations inside it in the single namespace.

Answer 3

You can't because it's ambiguous. If you wrote:

extern int foo::bar;

Are you forward declaring an int named bar in namespace foo , or are you forward declaring an int named bar inside the class foo ? There's no way for the compiler to know.