std::mask_array

template <class T> mask_array;

Valarray mask selection

This class is used as an intermediate type returned by valarray 's subscript operator (operator[]) when used with masks.

It references the elements in the valarray object that are selected by a mask (a valarray<bool>), and overloads the assignment and compound assignment operators, allowing direct access to the elements in the selection.

The type is convertible to a valarray (see valarray constructor ), producing a new object with copies of the referred elements.

Objects of this type are obtained as the return value of a call to valarray::operator[] . They cannot be directly constructed nor copied. It is declared as:

template <class T> class mask_array {
public:
 typedef T value_type;
 void operator= (const valarray<T>&) const;
 void operator*= (const valarray<T>&) const;
 void operator/= (const valarray<T>&) const;
 void operator%= (const valarray<T>&) const;
 void operator+= (const valarray<T>&) const;
 void operator-= (const valarray<T>&) const;
 void operator^= (const valarray<T>&) const;
 void operator&= (const valarray<T>&) const;
 void operator|= (const valarray<T>&) const;
 void operator<<= (const valarray<T>&) const;
 void operator>>= (const valarray<T>&) const;
 void operator=(const T&);
 ~mask_array();
private:
 mask_array();
 mask_array(const mask_array&);
 mask_array& operator= (const mask_array&);
};

Objects of this type are obtained as the return value of a call to valarray::operator[] . They cannot be default-constructed, but can be copied. It is declared as:

template <class T> class mask_array {
public:
 typedef T value_type;
 void operator= (const valarray<T>&) const;
 void operator*= (const valarray<T>&) const;
 void operator/= (const valarray<T>&) const;
 void operator%= (const valarray<T>&) const;
 void operator+= (const valarray<T>&) const;
 void operator-= (const valarray<T>&) const;
 void operator^= (const valarray<T>&) const;
 void operator&= (const valarray<T>&) const;
 void operator|= (const valarray<T>&) const;
 void operator<<= (const valarray<T>&) const;
 void operator>>= (const valarray<T>&) const;
 void operator=(const T&) const;
 mask_array() = delete;
 mask_array(const mask_array&);
 const mask_array& operator= (const mask_array&) const;
 ~mask_array();
};

Example

// mask_array example
#include <iostream> // std::cout
#include <cstddef> // std::size_t
#include <valarray> // std::valarray
int main ()
{
 std::valarray<int> foo (10);
 for (int i=0; i<10; ++i) foo[i]=i; // 0 1 2 3 4 5 6 7 8 9
 std::valarray<bool> mymask (10);
 for (int i=0; i<10; ++i)
 mymask[i]= ((i%2)==1); // f t f t f t f t f t
 foo[mymask] *= std::valarray<int>(10,5); // 0 10 2 30 4 50 6 70 8 90
 foo[!mymask] = 0; // 0 10 0 30 0 50 0 70 0 90
 std::cout << "foo:";
 for (std::size_t i=0; i<foo.size(); ++i)
 std::cout << foo[i] << ' ';
 std::cout << '\n';
 return 0;
}

Output:


foo: 0 10 0 30 0 50 0 70 0 90

classes

functions

std::mask_array

Example

See also