26 Numerics library [numerics]

26.6 Numeric arrays [numarray]

26.6.8 Class template mask_array[template.mask.array]

26.6.8.1 Class template mask_array overview [template.mask.array.overview]

```namespace std {
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;

mask_array(const mask_array&);
~mask_array();
const mask_array& operator=(const mask_array&) const;
void operator=(const T&) const;

mask_array() = delete;        // as implied by declaring copy constructor above
};
}
```

This template is a helper template used by the mask subscript operator:

``` mask_array<T> valarray<T>::operator[](const valarray<bool>&). ```

It has reference semantics to a subset of an array specified by a boolean mask. Thus, the expression a[mask] = b; has the effect of assigning the elements of b to the masked elements in a (those for which the corresponding element in mask is true.)

26.6.8.2mask_array assignment [mask.array.assign]

``` void operator=(const valarray<T>&) const; const mask_array& operator=(const mask_array&) const; ```

These assignment operators have reference semantics, assigning the values of the argument array elements to selected elements of the valarray<T> object to which it refers.

26.6.8.3mask_array computed assignment [mask.array.comp.assign]

``` 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; ```

These computed assignments have reference semantics, applying the indicated operation to the elements of the argument array and selected elements of the valarray<T> object to which the mask object refers.

26.6.8.4mask_array fill function [mask.array.fill]

``` void operator=(const T&) const; ```

This function has reference semantics, assigning the value of its argument to the elements of the valarray<T> object to which the mask_array object refers.