# 24 Numerics library [numerics]

## 24.8 Numeric arrays [numarray]

```namespace std {
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;

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.)

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