[template.indirect.array]

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

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

### 29.6.9 Class template indirect_array [template.indirect.array]

#### [29.6.9.1](#overview) Overview [[template.indirect.array.overview]](template.indirect.array.overview)

[🔗](#lib:indirect_array)

namespace std {template<class T> class indirect_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;

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

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

[1](#overview-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L9029)

This template is a helper template used by the indirect subscript operator

[🔗](#lib:operator%5b%5d,indirect_array)

`indirect_array<T> valarray<T>::operator[](const valarray<size_t>&);
`

[2](#overview-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L9036)

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

Thus, the expressiona[​indirect] = b; has the effect of assigning the elements ofb to the elements ina whose indices appear inindirect[.](#overview-2.sentence-2)

#### [29.6.9.2](#indirect.array.assign) Assignment [[indirect.array.assign]](indirect.array.assign)

[🔗](#lib:operator=,indirect_array)

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

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

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L9057)

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

[2](#indirect.array.assign-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L9063)

If theindirect_array specifies an element in thevalarray<T> object to which it refers more than once, the behavior is undefined[.](#indirect.array.assign-2.sentence-1)

[3](#indirect.array.assign-3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L9071)

[*Example [1](#indirect.array.assign-example-1)*:

int addr[] = {2, 3, 1, 4, 4};
valarray<size_t> indirect(addr, 5);
valarray<double> a(0., 10), b(1., 5);
a[indirect] = b; results in undefined behavior since element 4 is specified twice in the
indirection[.](#indirect.array.assign-3.sentence-1)

— *end example*]

#### [29.6.9.3](#indirect.array.comp.assign) Compound assignment [[indirect.array.comp.assign]](indirect.array.comp.assign)

[🔗](#lib:operator*=,indirect_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](#indirect.array.comp.assign-1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L9110)

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 theindirect_array object refers[.](#indirect.array.comp.assign-1.sentence-1)

[2](#indirect.array.comp.assign-2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L9118)

If theindirect_array specifies an element in thevalarray<T> object to which it refers more than once,
the behavior is undefined[.](#indirect.array.comp.assign-2.sentence-1)

#### [29.6.9.4](#indirect.array.fill) Fill function [[indirect.array.fill]](indirect.array.fill)

[🔗](#lib:operator=,indirect_array_)

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

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

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/numerics.tex#L9136)

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