Re: Sth. wrong with std::valarray slices?

"Alf P. Steinbach" <>
Fri, 06 Mar 2009 04:55:12 +0100
* Mirco Wahab:

The following code goes through gcc 4.3.x in
'-pedantic' mode, Visual C++ 2008 (VC9 SP1)
bails on the '= operator' with linker error
(as noted in the source).


 #include <valarray>
 using namespace std;

 int main(int argc, char*argv[])
 double values[10] = {1,2,3,4,5,6,7,8,9,0};
 valarray<double> va(values, 10);
 valarray<double> vb(3.141, 10000);

 va[ slice(2,4,1) ] = vb[ slice(100,4,1) ];
// MS-VS2008 unresolved external symbol
// "public: class std::slice_array<double> & __thiscall
// (class std::slice_array<double> const &)"
// referenced in function _main

 return 0;

The code compiles and links fine with MinGW g++ 3.4.5 and with MSVC 7.1.

It fails with Comeau Online,

Comeau C/C++ (Oct 6 2008 11:28:09) for ONLINE_EVALUATION_BETA2
Copyright 1988-2008 Comeau Computing. All rights reserved.
MODE:strict errors C++ C++0x_extensions

"valarray", line 1073: error: implicitly generated assignment operator
           cannot copy:
             reference member
                       "std::slice_array<_Tp>::_M_array [with _Tp=double]"
   class slice_array {
           detected during:
             implicit generation of "std::slice_array<double>
                       std::slice_array<double> &) [with _Tp=double]" at line
                       10 of "ComeauTest.c"
             instantiation of class "std::slice_array<_Tp> [with _Tp=double]"
                       at line 10 of "ComeauTest.c"

1 error detected in the compilation of "ComeauTest.c".

The error message is intesting because the standard requires slice_array to have
  a 'private' -- inacessible -- declaration of the copy assignment operator.
It is not required to be defined. So the compiler shouldn't implicitly generate
that copy assignment operator.

Although I'm not familiar with 'valarray', it seems to me that MSVC 9 is the
only one of those three compilers to get it right.

Namely, that you can't directly assign a slice_array to another slice_array.

You should however be able to do (I'm not sure what that means for efficiency!)

   va[ slice(2,4,1) ] = valarray<double>( vb[ slice(100,4,1) ] );

And regarding that, note that original C++98 standard inconsistently declares
that assignment operator as having argument type T, but then defines its
semantics with argument type valarray<T>. As far as I know this inconsistency
was not corrected in C++03. Instead they chose to remove a comment in the class
definition. Even more amazingly, in C++0x, at least the first committee draft
(C++0x CD1, the n2800 document), the assignment you used has been added (as
allowed), but the original C++98 inconsistency has still not been fixed! Argh.

I therefore suggest you post this question to [comp.std.c++].

Perhaps first checking how it is regarding Active Issues, but responders in
[comp.std.c++] will do that anyway.

Cheers & hth.,

- Alf

Due to hosting requirements I need visits to [].
No ads, and there is some C++ stuff! :-) Just going there is good. Linking
to it is even better! Thanks in advance!

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