[C++] Derive a class from a tamplate

[stefano luceri]

Hello to all .... excuse for my bad english

I've the following problem writing a simple program. I would use the 
metods of std::valarray class adding some self writed method for elemnts 
of double type.

I've thought something like this:

/****************************/
using namespace std;

class mvector : public valarray<double>
{
mvector();
mvector(int size);
}

mvector :: mvector() : valarray<double>() {}
mvector :: mvector(int size) : valarray<double>(size) {}

int main()
{
   mvector v(3);
  
   v = (v+2.0);
}

/*****************************/

Compiling this code I obtain the following error:

geom.cpp: In function `int main()':
geom.cpp:38: error: no match for 'operator=' in 'v = std::operator+(const
  std::valarray<_Tp>&, const _Tp&) [with _Tp = double]((&1.0e+0))'
geom.cpp:22: error: candidates are: mvector& mvector::operator=(const 
mvector&)
make: *** [geom.o] Error 1

Why can't I use the operator = like I do with valarray?