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Subject: Re: [boost] [swap] please update documentation...
From: adam.butcher_at_[hidden]
Date: 2009-06-30 03:19:11

Hi Niels,

I hope you don't mind me interjecting. We use the stlport std library
with gcc 4.4.0 so I thought I'd add info about that to the thread.

Niels Dekker wrote on 29/06/2009 18:33:18:
> Re: [boost] [swap] please update documentation...
> >> I'm very surprised it doesn't pick std::swap, as found by
> >> argument-dependent lookup. What version of GCC are you using exactly?
> Peter Simons wrote:
> > The test program was compiled with g++ 4.4.0 20090526 (prerelease) on
> > ArchLinux/x86.
> Thanks Peter! It looks like a brandnew GCC bug to me... Can you please

> confirm that the following simplified version also compiles and runs on
> your installation of GCC 4.4.0? And if so, does it indeed throw
> "foo::swap is called"?
I tried the above with gcc 4.4.0 and stlport. It chose std::swap
(stlport's) via ADL and failed to compile due to 'invalid array
assignment' as expected. It seems that the issue is with gcc 4.4.0's
latest standard library rather than a broken ADL or other such detail.
Looks like the latest gcc std::swap supports c-style array swapping.

#---------- GCC 4.4.0 + STLport 5.1.3 ------------

g++ swap.cpp
swap.cpp:19: instantiated from here
.../STLport-5.1.3-native/stlport/stl/_algobase.h:93: error: invalid array

#---------- GCC 4.4.0 + libstdc++ 4.4.0 -----------

CPLUS_INCLUDE_PATH= g++ swap.cpp
# no exception is thrown -- it's picking std::swap

#---------- GCC 4.2.3 + libstdc++ 4.2.3 ------------

CPLUS_INCLUDE_PATH= g++-4.2.3 swap.cpp
swap.cpp:19: instantiated from here
../include/c++/4.2.3/bits/stl_algobase.h:100: error: invalid array

#---------- GCC 4.4.0 + libstdc++ 4.4.0 -----------

The following compiles fine with 4.4.0 std library. Switch to stlport
headers and it fails to compile as expected.

CPLUS_INCLUDE_PATH= g++ -xc++ - << EOF
#include <string>
#include <algorithm>

int main()
   std::string a[42], b[42];
   std::swap(a, b);

Does the C++0x library allow for std::swap with c-style arrays? If so,
maybe the issue is that g++ is implementing it without needing you to
specify --std=c++0x. I guess it is not breaking any existing use of
std::swap -- just making a missing case work (?)

Just to check that it actually is swapping I changed the body of the last
snippet to:

   std::string a[42] = { "a", }, b[42] = { "b", };
   std::swap(a, b);
   return a[0][0] - 'a';

and it did indeed swap. The return code was 1, not 0 -- so the first
string in 'a' after the swap began with char 'b'.

Hope this info's useful.

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