const T& operator[](size_t) const;
T& operator[](size_t);
The subscript operator returns a reference to the corresponding element of the array.
Thus, the expression (a[i] = q, a[i]) == q evaluates as true for any non-constant valarray<T> a, any T q, and for any size_t i such that the value of i is less than the length of a.
The expression &a[i+j] == &a[i] + j evaluates as true for all size_t i and size_t j such that i+j is less than the length of the array a.
Likewise, the expression &a[i] != &b[j] evaluates as true for any two arrays a and b and for any size_t i and size_t j such that i is less than the length of a and j is less than the length of b. This property indicates an absence of aliasing and may be used to advantage by optimizing compilers.287
The reference returned by the subscript operator for an array is guaranteed to be valid until the member function resize(size_t, T) ([valarray.members]) is called for that array or until the lifetime of that array ends, whichever happens first.
Compilers may take advantage of inlining, constant propagation, loop fusion, tracking of pointers obtained from operator new, and other techniques to generate efficient valarrays.