Suppose I have some array-based code that is enabled to be used by expression templates. E.g., I have operator[] for these arrays overloaded and also have overloaded the arithmetic operator + etc.
Now I would like to let the STL algorithm any_of run on such arrays. The simple way is to do
ExprArray<double, N> b, c; // init etc.
auto a = b + c; // for (auto i = 0; i < N; ++i) { a[i] = b[i] + c[i]; }
auto res = std::any_of(begin(a), end(a), SomePred{});
Of course, I would like to be able to short-circuit the computation and have a modified (range-based) lib::any_of that does
// only compute b[i] + c[i] until SomePred is satisified
auto res = lib::any_of(b + c, SomePred{}); // write as explicit loop over b[i] + c[i]
Writing lib::any_of in terms of operator[] on its input will do that job, the same as it was done for the overloaded operator+. However, this would require similar reimplementations of all STL algorithms that I could possibly run on such arrays.
Question: So suppose I want to re-use existing range-based algorithms (Boost.Range, range-v3) by only modifying the ExprArray iterators. Is it possible to modify the ExprArray iterator operator* and operator++ in such a way that this is transparent to range-based algorithms?
// only compute b[i] + c[i] until SomePred is satisified
// needs to eventually dispatch to
// for (auto i = 0; i < N; ++i)
// if (SomePred(b[i] + c[i])) return true;
// return false;
auto res = ranges::any_of(b + c, SomePred{});
So if the algorithm version actually is implemented in terms of iterators, the loop for (auto it = first; it != last; ++it) needs *it to be aware of the fact that it needs to compute b[i] + c[i], and ++it has to know that it needs to do ++i.
This question seems to reduce to "Can I implement iterators for my expression templates?" which I think is fairly straightforward. Assuming that "expression templates" know their size and have overloaded operator[] an iterator simply needs to hold a reference to the expression object and an offset into the range it represents:
template <class Expr>
class iterator {
public:
using iterator_category = ranges::random_access_iterator_tag;
using difference_type = std::ptrdiff_t;
using value_type = typename Expr::value_type;
iterator() = default;
constexpr iterator(Expr& e, difference_type i) :
expr_{&e}, i_{i} {}
constexpr bool operator==(const iterator& that) const {
return assert(expr_ == that.expr_), i_ == that.i_;
}
constexpr bool operator!=(const iterator& that) const {
return !(*this == that);
}
// Similarly for operators <, >, <=, >=
value_type operator*() const {
return (*expr_)[i_];
}
value_type operator[](difference_type n) const {
return (*expr_)[i_ + n];
iterator& operator++() & { ++i_; }
iterator operator++(int) & { auto tmp = *this; ++*this; return tmp; }
// Similarly for operator--
iterator operator+(difference_type n) const {
return iterator{expr_, i_ + n};
}
// Similarly for operators -, +=, and -=
friend iterator operator+(difference_type n, const iterator& i) {
return i + n;
}
private:
Expr* expr_;
difference_type i_;
};
Now you simply have to arrange for "Expression templates" to have begin and end members that return iterator{*this, 0} and iterator{*this, size()}.