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Re: reasons for preferring C over C++

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On Sun, 19 Oct 2014 17:46:18 -0500, Tim Wescott wrote:

> gnu C has a way of defining a function as "link this only if you need it"
> (with attribute ((__weak__)), IIRC).  It basically makes a function act
> like a function in a library, even though it's been compiled into a .o
> file.  If they use that method with templatized methods, then only one
> will get included in the link, even if multiple copies are scattered
> around in the object files.
> 
> I don't KNOW that they use that, but I'd be surprised if they didn't.

The usual behaviour for g++ is to instantiate the template in each object
file, and leave it to the linker to remove duplicates.

If the linker doesn't support that, there are switches to either track
template instantiations so that they can be compiled and linked separately
(-frepo), or to suppress automatic instantiation and require the code to
explicitly instantiate any templates which it uses (-fno-implicit-templates).

https://gcc.gnu.org/onlinedocs/gcc/Template-Instantiation.html

However, that isn't what Paul was talking about. While exact duplicates
can be removed, if you instantiate a template several times with different
parameters, each distinct set of parameters will get its own customised
version of the template code.

This allows for maximum optimisation, as each individual instantiation has
all of its parameters fixed at compile time. But it does result in code
bloat relative to languages which generate one set of code which handles
polymorphism by accepting parameters at run time.

The designer of the template may be able to minimise bloat by identifying
large sections of code which are only weakly dependent upon the
parameters. E.g. there could be portions of code which only care about the
type's size, and generating different versions for different sizes would
have negligible benefit (particularly on a system with a fast multiply).
So the template code could delegate such tasks to a non-template helper
function which takes the size as a (run-time) parameter.

Unfortunately, this isn't particularly straightforward for container
templates, as core operations such as copy, move, assign, swap, etc may
have to use the relevant type-specific methods or overloads, and using
indirect calls for such primitive operations would hurt performance badly.

So if you want to go down that route, you have to manually "dissect" the
parameters using type traits such as is_trivial or is_trivially_copyable
etc in order to delegate to less-specialised templates where possible
while retaining full specialisation where necessary (or where the
alternatives are impractical).

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reasons for preferring C over C++ Wouter van Ooijen <wouter@voti.nl> - 2014-10-15 19:21 +0200
  Re: reasons for preferring C over C++ Paul Rubin <no.email@nospam.invalid> - 2014-10-15 10:36 -0700
    Re: reasons for preferring C over C++ Wouter van Ooijen <wouter@voti.nl> - 2014-10-15 19:48 +0200
      Re: reasons for preferring C over C++ Tom Gardner <spamjunk@blueyonder.co.uk> - 2014-10-15 21:25 +0100
        Re: reasons for preferring C over C++ Wouter van Ooijen <wouter@voti.nl> - 2014-10-15 23:28 +0200
          Re: reasons for preferring C over C++ Tom Gardner <spamjunk@blueyonder.co.uk> - 2014-10-15 23:40 +0100
  Re: reasons for preferring C over C++ Don Y <this@is.not.me.com> - 2014-10-15 10:58 -0700
  Re: reasons for preferring C over C++ upsidedown@downunder.com - 2014-10-15 21:03 +0300
    Re: reasons for preferring C over C++ Don Y <this@is.not.me.com> - 2014-10-15 11:11 -0700
    Re: reasons for preferring C over C++ Wouter van Ooijen <wouter@voti.nl> - 2014-10-15 20:26 +0200
  Re: reasons for preferring C over C++ Stefan Reuther <stefan.news@arcor.de> - 2014-10-15 20:44 +0200
    Re: reasons for preferring C over C++ Wouter van Ooijen <wouter@voti.nl> - 2014-10-15 23:30 +0200
      Re: reasons for preferring C over C++ Stefan Reuther <stefan.news@arcor.de> - 2014-10-16 18:19 +0200
        Re: reasons for preferring C over C++ Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2014-10-16 23:12 +0200
          Re: reasons for preferring C over C++ Reinhardt Behm <rbehm@hushmail.com> - 2014-10-17 13:25 +0800
          Re: reasons for preferring C over C++ Stefan Reuther <stefan.news@arcor.de> - 2014-10-17 18:43 +0200
    Re: reasons for preferring C over C++ "tim....." <tims_new_home@yahoo.co.uk> - 2014-10-18 12:29 +0100
      Re: reasons for preferring C over C++ Paul Rubin <no.email@nospam.invalid> - 2014-10-18 11:44 -0700
        Re: reasons for preferring C over C++ Tom Gardner <spamjunk@blueyonder.co.uk> - 2014-10-18 22:25 +0100
          Re: reasons for preferring C over C++ Paul Rubin <no.email@nospam.invalid> - 2014-10-18 15:42 -0700
            Re: reasons for preferring C over C++ Niklas Holsti <niklas.holsti@tidorum.invalid> - 2014-10-19 11:03 +0300
            Re: reasons for preferring C over C++ Tim Wescott <seemywebsite@myfooter.really> - 2014-10-19 17:46 -0500
              Re: reasons for preferring C over C++ Paul Rubin <no.email@nospam.invalid> - 2014-10-19 16:28 -0700
              Re: reasons for preferring C over C++ Nobody <nobody@nowhere.invalid> - 2014-10-20 02:47 +0100
        Re: reasons for preferring C over C++ "tim....." <tims_new_home@yahoo.co.uk> - 2014-10-19 09:34 +0100
          Re: reasons for preferring C over C++ Paul Rubin <no.email@nospam.invalid> - 2014-10-19 17:00 -0700
            Re: reasons for preferring C over C++ Tom Gardner <spamjunk@blueyonder.co.uk> - 2014-10-20 08:50 +0100
              Re: reasons for preferring C over C++ "tim....." <tims_new_home@yahoo.co.uk> - 2014-10-20 11:57 +0100
                Re: reasons for preferring C over C++ Tom Gardner <spamjunk@blueyonder.co.uk> - 2014-10-20 12:11 +0100
                Re: reasons for preferring C over C++ upsidedown@downunder.com - 2014-10-20 22:36 +0300
                Re: reasons for preferring C over C++ Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP> - 2014-10-21 00:36 +0000
            Re: reasons for preferring C over C++ Tom Gardner <spamjunk@blueyonder.co.uk> - 2014-10-20 08:52 +0100
            Re: reasons for preferring C over C++ "tim....." <tims_new_home@yahoo.co.uk> - 2014-10-20 11:56 +0100
        Re: reasons for preferring C over C++ Stefan Reuther <stefan.news@arcor.de> - 2014-10-19 10:52 +0200
          Re: reasons for preferring C over C++ Paul Rubin <no.email@nospam.invalid> - 2014-10-19 17:16 -0700
      Re: reasons for preferring C over C++ glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-10-18 23:02 +0000
        Re: reasons for preferring C over C++ "tim....." <tims_new_home@yahoo.co.uk> - 2014-10-19 09:45 +0100
  Re: reasons for preferring C over C++ David Brown <david.brown@hesbynett.no> - 2014-10-15 22:12 +0200
    Re: reasons for preferring C over C++ Wouter van Ooijen <wouter@voti.nl> - 2014-10-15 23:35 +0200
      Re: reasons for preferring C over C++ Paul Rubin <no.email@nospam.invalid> - 2014-10-15 19:10 -0700
      Re: reasons for preferring C over C++ Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2014-10-16 10:34 +0300
        Re: reasons for preferring C over C++ Tom Gardner <spamjunk@blueyonder.co.uk> - 2014-10-16 10:10 +0100
  Re: reasons for preferring C over C++ Tim Wescott <tim@seemywebsite.com> - 2014-10-16 01:06 -0500
  Re: reasons for preferring C over C++ Anders.Montonen@kapsi.spam.stop.fi.invalid - 2014-10-16 08:32 +0000
    Re: reasons for preferring C over C++ David Brown <david.brown@hesbynett.no> - 2014-10-16 20:54 +0200

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