[template.mask.array]

# 29 Numerics library [[numerics]](./#numerics)

## 29.6 Numeric arrays [[numarray]](numarray#template.mask.array)

### 29.6.8 Class template mask_array [template.mask.array]

#### [29.6.8.1](#overview) Overview [[template.mask.array.overview]](template.mask.array.overview)

[🔗](#lib:mask_array)

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};}

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L8913)

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

[🔗](#lib:operator%5b%5d,mask_array)

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

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L8920)

It has reference semantics to a subset of an array specified by a boolean mask[.](#overview-2.sentence-1)

Thus, the expression a[mask] = b; has the effect of assigning the elements ofb to the masked elements in a (those for which the corresponding element inmask is true)[.](#overview-2.sentence-2)

#### [29.6.8.2](#mask.array.assign) Assignment [[mask.array.assign]](mask.array.assign)

[🔗](#lib:operator=,mask_array)

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

[1](#mask.array.assign-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L8937)

These assignment operators have reference semantics, assigning the values
of the argument array elements to selected elements of thevalarray<T> object to which the mask_array object refers[.](#mask.array.assign-1.sentence-1)

#### [29.6.8.3](#mask.array.comp.assign) Compound assignment [[mask.array.comp.assign]](mask.array.comp.assign)

[🔗](#lib:operator*=,mask_array)

`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;
`

[1](#mask.array.comp.assign-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L8970)

These compound assignments have reference semantics, applying the
indicated operation to the elements of the argument array and selected elements
of thevalarray<T> object to which the mask_array object refers[.](#mask.array.comp.assign-1.sentence-1)

#### [29.6.8.4](#mask.array.fill) Fill function [[mask.array.fill]](mask.array.fill)

[🔗](#lib:operator=,mask_array_)

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

[1](#mask.array.fill-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L8986)

This function has reference semantics, assigning the value of its
argument to the elements of thevalarray<T> object to which themask_array object refers[.](#mask.array.fill-1.sentence-1)