28 Numerics library [numerics]

28.6 Numeric arrays [numarray]

28.6.9 Class template indirect_array [template.indirect.array] 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.
— end example]