Surcharger l'opérateur << pour la classe valarray

template <class ostream,typename _T>ostream& operator<<(ostream& os,valarray<_T>>;

#include <iostream>
#include <valarray>
using namespace std;

template <class ostream,typename _T>  
ostream& operator<<(ostream& os,valarray<_T> array)
{
  for ( int i(0); i < array.size(); i++)
    {
      os << array[i] << " ";
    }
  return os;
}
	
int main ()
{
  
  int init[]= {10,20,30,40};
  valarray<int> foo (init, 4);  //  10 20 30 40    
  cout << foo.size() << endl;
  cout << foo << endl;
  return 0;
}

#include <ostream> 

template <typename T> 
std::ostream & operator << (
  std::ostream & oss,
  const valarray<T> > & x
) { 
  for(std::size_t i = 0; i < x.size(); ++i) { 
    oss << x[i] << ' '; 
  } 
  return oss; 
}

#include <iostream> 
#include <cmath> 
#include <valarray> 
#include <ostream> 

template <typename T> 
std::ostream & operator << ( 
    std::ostream & out, 
    const std::valarray<T> & x 
) { 
    out << "[ "; 
    for (std::size_t i = 0; i < x.size(); ++i) { 
        out << x[i] << ' '; 
    } 
    out << ']'; 
    return out; 
} 
                                     
int main () { 
    double val[] = {1.1, 2.2, 4.4, 8.8}; 
    std::valarray<double> foo(val, 4); 
    std::valarray<double> bar = exp(foo); 

    std::cout << "foo: " << foo << std::endl 
              << "bar: " << bar << std::endl; 

    return 0; 
}

foo: [ 1.1 2.2 4.4 8.8 ] 
bar: [ 3.00417 9.02501 81.4509 6634.24 ]