# 28 Numerics library [numerics]

## 28.6 Numeric arrays [numarray]

#### 28.6.8.1 Overview [template.mask.array.overview]

namespace std { template<class T> class mask_array { public: using value_type = T; 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).

#### 28.6.8.2 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 the mask_array object refers.

#### 28.6.8.3 Compound 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 compound 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_array object refers.

#### 28.6.8.4 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.