700910 - Implement macro-based support for C++11 = delete deleted function syntax, in at least some compilers

Status: RESOLVED FIXED

(Core :: MFBT, defect)

Description

[Jeff Walden [:Waldo]]

Attachment: Patch

This is the common idiom to prevent copy-construction and default-assignability in C++98:

  struct C
  {
    private:
      C(const C& other);
      void operator=(const C& other);
  };

In other words, declare private versions of these methods but never implement them.  This is a bit unclear, because nothing prevents someone from later defining these methods.  (They could only be used by C or friends of C, to be sure, but for sufficiently complex code it's possible someone might make a mistake.)  C++11 formalizes and clarifies the trick by introducing deleted function syntax:

  struct C
  {
    C(const C& other) = delete;
    void operator=(const C& other) = delete;
  };

Besides its clarity, a further advantage to this syntax is that defining a deleted method out-of-line, or accidentally attempting to use either method, produces a compile error -- not just a link error or, worse yet, no error at all.

I've wrapped up support for deleted functions under a small MOZ_DELETE macro.  It's not as nice as = delete, but it's searchable, and the adjacent comment will clarify matters.  Unfortunately compile-time error behavior only works in clang, but that's an improvement over status quo.  And I lack the knowledge and time to get gcc working now.  :-\

I would do the check for -Wno-c++0x-extensions support the same way support for -Wno-invalid-offsetof is divined, but it turns out that approach is buggy.  gcc (at least 4.6) compiles successfully with unrecognized -Wno-* command line flags, so the only way to target clang only is to check for clang specifically.

I'm sure I could spatter this macro many more places, but it's not easy to search for the pattern, so changing things incrementally seems simplest.  I've made a few changes here only to verify that the macro works in both Gecko and SpiderMonkey.

Comment 1

[Chris Jones [:cjones] inactive; ni?/f?/r? if you need me]

Comment on attachment 573062 [details] [diff] [review]
Patch

Good idea.

Could use __GXX_EXPERIMENTAL_CXX0X__ to enable this for gcc but it's probably not worth the effort.

Comment 2

[Jeff Walden [:Waldo]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/f2184dfdda31

I'll blog about this and probably note that it's only awesome on clang for the moment.  I'm sure somebody can make it work on gcc given the right incentives.  :-)

Comment 3

[Jeff Walden [:Waldo]]

I blogged about this:

http://whereswalden.com/2011/11/09/introducing-moz_delete-a-macro-improving-upon-the-deliberately-unimplemented-method-idiom/

And I posted to m.d.platform about it:

http://groups.google.com/group/mozilla.dev.platform/browse_thread/thread/c8319804e98df69f#

Comment 4

[Marco Bonardo [:mak]]

https://hg.mozilla.org/mozilla-central/rev/f2184dfdda31

Comment 5

[Jeff Walden [:Waldo]]

I consider the comments by these macros to be much better documentation than anything maintained externally, so I think this document roughly pointing people to it is reasonable documentation:

https://developer.mozilla.org/en/mfbt

Note that this macro no longer lives in Util.h, rather in Attributes.h now, so that people have a better chance guessing where it's defined in mfbt/ based solely on the functionality desired.