You don't need anything like this for C++

Posted Jul 3, 2012 19:49 UTC (Tue) by cmccabe (guest, #60281)
In reply to: You don't need anything like this for C++ by joib
Parent article: Calling for a new openSUSE development model

One thing I'm curious about. Are you intended to duplicate all of your declarations in both namespaces, or are you intended to do something like this:

namespace version1 {
  void foo(int64_t a);
  void bar(int);
}

namespace version2 {
  void foo(float b);
  using namespace version1;
}

If you are intended to do something like what I've just described, there are obvious problems with implicit type conversions. For example, foo(0x123456789LL) will give you the version1 function, whereas foo(0x1234) will give you the version2 function, which is probably not what you want.

Ultimately, this seems like just another variant of "new version, new datatypes / functions," which you can easily do in C or older versions of C++. You can even hide the old version declarations in the header file by default using macros. I think FUSE does this-- if you don't set FUSE_VERSION to an "old" number, you can't see the old deprecated APIs in the header file.

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You don't need anything like this for C++

Posted Jul 4, 2012 18:16 UTC (Wed) by jwakely (subscriber, #60262) [Link] (3 responses)

foo(0x123456789LL) will give you the version1 function

Are you sure? I don't understand your example, as there isn't actually an inline namespace there, but if version2 is meant to be inline, then both calls to foo would get the float version. i.e. adding your version2 would have change the API in incompatible ways, as the original foo would no longer be used by anyone. You could do that, but it's probably not what you want.

If you use a using declaration instead of a using directive then both overloads are found and overload resolution picks the best one.

Using declarations work better with inline namespaces. Consider version 1 of the API:

namespace api {
  inline namespace v1 {
    void foo(int) {
      // implementation
    }
    void bar() {
      // implementation
    }
  }
}

Code that calls api::foo(1) links to api::v1::foo(int).

Later, version 2 is released:

namespace api {
  namespace v1 {  // not inline now
    // as before
  }
  inline namespace v2 {
    void foo(int i) {
      // improved v2 implementation
    }
    void foo(float) {
      // new impl for float
    }
    using v1::bar;
  }
}

Code that was linked to api::v1:foo still finds that symbol, with the same implementation. Code that is compiled against the new version and calls api::foo(1) links to api::v2::foo(int). Both symbols can coexist in the library (just as with ELF symbol versioning.) Code that calls api::bar() gets api::v1::bar() in both cases, so there's no duplication (except for adding a using declaration to the v2 namespace.)

You don't need anything like this for C++

Posted Jul 5, 2012 22:20 UTC (Thu) by cmccabe (guest, #60281) [Link] (2 responses)

> Are you sure?

Well, let's try it.

#include <stdint.h>
#include <stdio.h>
namespace version1 {
    void foo(int64_t a) {
        printf("version1 foo\n");
    }
}
inline namespace version2 {
    void foo(float b) {
        printf("version2 foo\n");
    }
    using namespace version1;
}
int main(void) {
    foo(0x123);
    return 0;
}

gcc 4.6.2 gives me "foo.cpp:22:14: error: call of overloaded ‘foo(int)’ is ambiguous". So it's not as bad as I thought-- just a compile-time failure, not silently doing the wrong thing.

[snip example]
Yes, you could add an explicit "using" line for each version1 symbol, to avoid this problem.

This all seems very similar to just adding a "using namespace version2" to your header file. I guess some people are worried about the potential for conflicts with whatever is already in your namespace, though.

Overall it I give it a "meh." It's not that useful, but it's not seriously harmful either. Which means it's doing better than a lot of language extensions. On the other hand, I do think the copy constructor stuff will be useful (so you see, I'm not just a complete curmudgeon) :)

You don't need anything like this for C++

Posted Jul 9, 2012 10:46 UTC (Mon) by jwakely (subscriber, #60262) [Link] (1 responses)

It's not really "very similar" to using namespace version2, an inline namespace is a lot more transparent so users never need know it exists (unless they look at the mangled symbols in their objects.)

A using-directive just makes names visible, but they are not treated as first-class members of the namespace containing the using-directive and will not be found by qualified name lookup if there are declarations of the same name in the namespace containing the using-directive. That's not the case for inline namespaces.

Templates defined in an inline namespace can be instantiated and specialized as though they are members of the enclosing namespace, not possible via a using-directive.

An inline namespace is an associated namespace of types in its enclosing namespace and vice versa, so it plays nicely with ADL.

Don't dismiss the feature because you don't understand it yet.

You don't need anything like this for C++

Posted Jul 10, 2012 21:31 UTC (Tue) by cmccabe (guest, #60281) [Link]

It seems like the best practice is to put all of your C++ library declarations into one namespace-- for the sake of argument, call it foolib.

It seems like what you're recommending then is that you have something like this:

namespace foolib {
   namespace version1 {
       ...
   }
   inline namespace version2 {
       ...
   }
}

Given that use-case, I don't think we need to care about the scenario where there are declarations of the same name in the namespace containing the using-directive-- neither the library user nor the library implementor should be doing that.

Templates defined in an inline namespace can be instantiated and specialized as though they are members of the enclosing namespace, not possible via a using-directive.

That's fair. I suppose template specialization is where the "using directive" approach really breaks down.

The cost of this feature is that debugging becomes (even more) difficult, since you search the symbol table of your application for foolib::doit and you don't find it. Instead you find N versions and have to decide which one you're really using. But of course, none of us here writes bugs, so that shouldn't be a problem :)