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

Subject: Re: [boost] [swap] please update documentation...
From: Dmitry Goncharov (dgoncharov_at_[hidden])
Date: 2009-06-30 03:36:35

adam.butcher_at_[hidden] wrote:
> Hi Niels,
>
>   
> 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.
>
>   
You are right. bits/algorithmfwd.h contains
template <typename _Tp, size_t _Nm>
void
swap(_Tp (&)[_Nm], _Tp (&)[_Nm]);
> #---------- 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
>
>
>   
BR, Dmitry