std::views::as_const provides a constant view over a range. Using it removes the possibility of inadvertently modifying the underlying
data of a range. This function is especially useful because in C++ const only applies to the top-level element, it is not deep-const.
One common misunderstanding of how const works is when iterating on ranges whose element type is a pair/tuple of references. Even if the iteration
variable is const-qualified, it is possible to modify the range’s underlying data through this variable:
void example() {
std::vector<int> v1 = createVector1();
std::vector<int> v2 = createVector2();
for (auto const&t : std::views::zip(v1, v2)) {
get<0>(t) = get<1>(t); // Modifies data in v1
}
In this example, zip returns a view of std::tuple<int&, int&>, and even if p is
const-qualified, it is still possible to change the underlying data via p, because the references inside the tuple never
become const.
On the other hand, if the example is rewritten this way:
void example() {
std::vector<int> v1 = createVector1();
std::vector<int> v2 = createVector2();
for (auto const &t : std::views::zip(v1, v2) | std::views::as_const) {
get<0>(t) = get<1>(t); // Does not compile
}
The view is now over elements of type std::tuple<int const&, int const&> and can no longer be used to modify the data in
v1 or v2.
This rule raises an issue when iterating with a range-based for loop over a range whose elements are tuples or pairs containing non-constant
references, and the iteration variable is a reference to a constant.
This situation can easily happen when using std::views::zip, std::views::enumerate, std::flat_map…
