C++ 11 Feature Proposal: Inherited constructors

[Matthew Dempsky]

C++11 adds a way to explicitly inherit base class constructors; e.g.:

    class Base {

     public:

      Base(int, double) { ... }

      Base(char*) { ... }

      ...

    };

    class Derived : public Base {

     public:

      // Similar to explicitly defining:

      //     Derived(int x, double y) : Base(x, y) {}

      //     Derived(char* z) : Base(z) {}

      using Base::Base;

      ...

    };

I'd like to see them allowed by the Chromium C++ style guide, but they're currently not listed at all on chromium-cpp.appspot.com.

My use case is I have some typedefs for a class, but they're purely an implementation detail, so it seems unfortunate to need to define them in the header.  As an alternative, I've found this seems to work nicely:

    foo.h:

        class Foo {

          ...

         private:

          struct FooKey;

          FooKey MakeKey(...);

          ...

        };

    foo.cc:

        #include "base/tuple.h"

        struct Foo::FooKey : public Tuple4<X, Y, Z, W> {

          using Tuple4<X, Y, Z, W>::Tuple4;

        };

        ...

Then I don't need to include base/tuple.h or any headers necessary for declaring X/Y/Z/W in the header, and I don't need to add boilerplate passthrough constructors myself.

[Alex Vakulenko]

The feature is called "Delegated Constructors" and we have a discussion thread for it: https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/0zVA8Ctx3Xo

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[Alex Vakulenko]

Oh, sorry, I jumped the gun. You are talking about something completely different... Apologies.

[Jeffrey Yasskin]

Delegating and Inheriting constructors are different features.
However, http://msdn.microsoft.com/en-us/library/hh567368.aspx says
MSVC 2013 doesn't implement inheriting constructors, so they're out
for now.

On Wed, Oct 29, 2014 at 4:08 PM, Alex Vakulenko <avaku...@chromium.org> wrote:

[Nico Weber]

Matthew, can you send a patch to add a row for them (in the banned section for now, it seems)?

[Matthew Dempsky]

On Wed, Oct 29, 2014 at 4:13 PM, Jeffrey Yasskin <jyas...@chromium.org> wrote:

However, http://msdn.microsoft.com/en-us/library/hh567368.aspx says
MSVC 2013 doesn't implement inheriting constructors, so they're out
for now.

"Oh, hah, I'm sorry, I keep forgetting. You were sick the day they taught C++ at C++ school."

[Matthew Dempsky]

Inherited constructors are now listed as banned in chromium-cpp.appspot.com, but I'm interested if anyone can suggest better wording and/or a better documentation link for the entry.  In a recent CL, I added a "using Base::Member;" declaration, and my reviewer thought the style guide disallowed *all* "using Base::X;" declarations, not simply the special case of "using Base::Base;".

[Etienne Pierre-doray]

https://msdn.microsoft.com/en-us/library/hh567368.aspx states that MSVC2015 supports inherited constructor.

The following code compile and execute correctly using MSVS2015:

class Base {

 public:

  Base(int) {}

  Base(double) {}

};

class Derived : public Base {

 public:

  using Base::Base;

};

int main() {

  Derived d1(3);

  Derived d2(3.0);

}

It seems like the reason for their ban was that MSVC2013 didn't implement them. I'd like to see them allowed now that MSVC2015 supports them.

[Nico Weber]

Sounds fine to me. Please send a CL that adds a use of that (to show that it works) and that moves the feature to the "allowed" list.

[Jeremy Roman]

+c...@chromium.org

[Peter Kasting]

On Fri, Jul 29, 2016 at 8:46 AM, 'Etienne Pierre-doray' via Chromium-dev <chromi...@chromium.org> wrote:

It seems like the reason for their ban was that MSVC2013 didn't implement them. I'd like to see them allowed now that MSVC2015 supports them.

As of https://crrev.com/3e35c53fdc3e68aa70efbca7f49a8494df79db1b , this is now done.

PK