Re: [Sheflug] c++ & mem

["Chris J/#6" <nospam [at] nccnet.co.uk>]

> No, I disagree completely. If you're planning on programming in C++, you
> need to test the compiler *with* C++, not C - that means the libraries, etc.
> If you compile that C hello world, it means you have C dev libraries
> installed. Woopee-doo. That's no use if you want to program C++ eventually.
> g++ is gcc anyway, so if you've compiled things on your system before,
> compiling that program tells you nothing.
> 

This follows from both Alex's post above and Will's comments. Basically I put 
it to a practical test along the lines of "let's actually have a look to see 
what the differences are" ... one basic "hello world" using printf, built 
with both gcc and g++, and one basic hello world using IOstream.

The results are ... :)

The printf with gcc is a little smaller than the same program built by g++ 
.. 3080 bytes for C, 4060 for C++. When built with gcc, it links against 
libc and libm. When built with g++, it *still* links against libc and libm, 
but it also links with libstc++.

The IOstream verseion of hello world is slightly larger again, being 4208 
bytes. Despite it being C++ it still links against libc and libm, as well as 
libstc++ (as expected).

Running: executing the program gives a better idea of whats going on. With 
the basic printf() form, doing ltrace gives the same results for both the gcc 
and g++ versions of the code (no point doing an strace ... it's the libraries 
we're interested in). The library trace goes simply:
	__libc_start_main()
	__register_frame_info()
	printf()
	__deregister_frame_info()
as can be seen, nothing fancy as far a C++ is concerned. But the compiler 
works as its producing a binary that runs :)

The iostreams one is different, as you'd expect. The ltrace goes:
	__libc_start_main()
	__register_frame_info()
	ostream::ls()
	__deregister_frame_info()
now we can see that there is a little bit of object orientation going on with 
the ostream::ls() :) This shows that the object orientation part of the 
compiler probably works. What you'd ideally need to do now is write a simple 
hello world that uses an object, so you aren't relying on library functions :)

Conclusions: both compilers work :) But how you interpret the results, and 
what you are looking for may vary. If writing standard C, then the only 
difference I can tell in this far-from-exhaustive study is the the inclusion 
of the libstc++ library in the linkage. And it looks like, from the ltrace, 
that this isn't used at all if you stick to standard libc calls...it's just 
part of the linkage that makes it a C++ program :)

So...

I'll leave these results for analysis, criticism and ridicule :-)

Chris...


-- 
Chris Johnson            \  "If not for me then, do it for yourself. If not
sixie@nccnet.co.uk        \  for then do it for the world." -- Stevie Nicks
www.nccnet.co.uk/~sixie/   ~---------------------------------------+
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