# 26 Numerics library [numerics]

## 26.7 Numeric arrays [numarray]

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

#### 26.7.9.1 Overview [template.indirect.array.overview]

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 }; }
This template is a helper template used by the indirect subscript operator
```indirect_array<T> valarray<T>::operator[](const valarray<size_t>&). ```
It has reference semantics to a subset of an array specified by an indirect_­array.
Thus, the expression a[​indirect] = b; has the effect of assigning the elements of b to the elements in a whose indices appear in indirect.

#### 26.7.9.2 Assignment [indirect.array.assign]

```void operator=(const valarray<T>&) const; const indirect_array& operator=(const indirect_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.
If the indirect_­array specifies an element in the valarray<T> object to which it refers more than once, the behavior is undefined.
[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.
â€” end example]

#### 26.7.9.3 Compound assignment [indirect.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 indirect_­array object refers.
If the indirect_­array specifies an element in the valarray<T> object to which it refers more than once, the behavior is undefined.

#### 26.7.9.4 Fill function [indirect.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 indirect_­array object refers.