Forward declarations in C++ modules (MSVC)

问题

I have been experimenting with modules implementation as provided by the MSVC lately and I've run into an interesting scenario. I have two classes that have a mutual dependency in their interfaces, which means that I'll have to use forward declarations to get it to compile. The following code shows an example:

Module interface

export module FooBar;

export namespace FooBar {
    class Bar;

    class Foo {
    public:
        Bar createBar();
    };

    class Bar {
    public:
        Foo createFoo();
    };
}

Module implementation

module FooBar;

namespace FooBar {
    Bar Foo::createBar() {
        return Bar();
    }

    Foo Bar::createFoo() {
        return Foo();
    }
}

Now I would like to split these two classes up into their own modules named Foo and Bar. However, each module needs to import the other as their interfaces depend on each other. And according to the modules proposal as it currently stands, circular interface imports are not allowed. This article suggests to use the proclaimed ownership declaration, but it seems that this is not yet implemented in the MSVC implementation of modules.

Therefore, am I correct in assuming that this situation currently cannot be resolved using the current implementation as provided by the MSVC? Or is there some alternative I am missing? In this scenario the situation is pretty trivial, however I encountered this problem while modularizing a library which has many classes that have such dependencies. I realize circular dependencies are often an indication of a bad design, however in certain instances they are unavoidable or difficult to refactor away.

回答1:

You can create a third module which exports only forward declarations to each of your classes (could be many classes).
Then you import this module into both (or all) of your modules, where it provides the forward declarations needed to implement each module.

Unfortunately, MSVC has still (today is version 16.7) issues with modules; although this approach works, you get often completely wild error messages; for example, "cannot convert MyClass* to MyClass* - no conversion provided (this example happens when you directly add forward decalarations to the same class into several modules; the compiler considers them different animals).
Another issue is if you forget to import all the modules needed, the error message is either grossly misleading ('no such method in that class'), or the compiler aborts with an internal error.

Don't expect too much until they have completed their work.

来源：https://stackoverflow.com/questions/51362591/forward-declarations-in-c-modules-msvc