Re: [boost] [swap] please update documentation...

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 
assignment

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

CPLUS_INCLUDE_PATH= g++ swap.cpp
./a.out
# 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 
assignment

#---------- 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);
}
EOF
#----------------------------------------

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.
Regards
Adam

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