Re: valarray problem with gcc-2.95.1

[Gabriel Dos Reis <dosreis at cmla dot ens-cachan dot fr>]

Serge Barral <sbarral@ippt.gov.pl> writes:

| Hi,
| 
| I'm using libsdc++-2.90.6 with gcc-2.95.1, and I have problems when
| slicing a valarray. For instance this code:
| 
| #include <valarray>
| 
| using namespace std;
| 
| int main()
| {
|    slice range(1,2,1);
|    double l[]={1.0 , 2.0 , 3.0 , 4.0};
|    valarray<double> c(l,4);
|    valarray<double> b(2);
| 
|    b=c[range]+c[range];  // error here
| 
|    return 0;
| }
| 
| 
| gives the following compile error:
| 
| >rien.cpp:In function `int main()':
| >rien.cpp:12: no match for `slice_array<double> + slice_array<double>'
| 
| 
| It looks like gcc does not choose the right signature for 'operator[]'
| of class valarray, which I think in this case should be:
|     _Expr<_SClos<_ValArray,_Tp>, _Tp> operator[](slice) const;
| and not:
|     slice_array<_Tp>    operator[](slice);

Welcome to C++.

First some C++ rules recollection: when a member function is
overloaded using 'const', the const version is selected only when the
expression used to invoke it is 'const'.  Thus 'c[range]' in 

	c[range] + c[range]

is of type slice_array<double> and not valarray<double>.   
Yes, C++ is not intuitive.

| Surprisingly, if I comment out the second signature in the file
| 'std_valarray.h', the code compiles. However, an expression like
| 'b=exp(c[slice(..,..,..])' will still not compile.
| 
| I'm quite new to c++, so maybe I'm missing something?
| Is it really a problem  with gcc? In this case, should I try gcc-2.95.0
| or 2.95.2?

It isn't a porblem in GCC.  It is the way C++ rules goes.

I agree with you there ought to be ways to add slice_array<T>s but the
Standard says otherwise.

-- Gaby