Use a `span<T>` or a `span_p<T>` to designate a half-open sequence

Reason

Informal/non-explicit ranges are a source of errors.

Example

X* find(span<X> r, const X& v);    // find v in r

vector<X> vec;
// ...
auto p = find({vec.begin(), vec.end()}, X{});  // find X{} in vec

Note

Ranges are extremely common in C++ code. Typically, they are implicit and their correct use is very hard to ensure. In particular, given a pair of arguments (p, n) designating an array [p:p+n), it is in general impossible to know if there really are n elements to access following *p. span<T> and span_p<T> are simple helper classes designating a [p:q) range and a range starting with p and ending with the first element for which a predicate is true, respectively.

Example

A span represents a range of elements, but how do we manipulate elements of that range?

void f(span<int> s)
{
    // range traversal (guaranteed correct)
    for (int x : s) cout << x << '\n';

    // C-style traversal (potentially checked)
    for (gsl::index i = 0; i < s.size(); ++i) cout << s[i] << '\n';

    // random access (potentially checked)
    s[7] = 9;

    // extract pointers (potentially checked)
    std::sort(&s[0], &s[s.size() / 2]);
}

Note

A span<T> object does not own its elements and is so small that it can be passed by value.

Passing a span object as an argument is exactly as efficient as passing a pair of pointer arguments or passing a pointer and an integer count.

See also: Support library

Enforcement

(Complex) Warn where accesses to pointer parameters are bounded by other parameters that are integral types and suggest they could use span instead.