Why can't I construct a gsl::span with a brace-enclosed initializer list

Question

According to the C++ Core Guidelines, I should use a gsl::span to pass a half-open sequence.

I think that means that instead of writing a function like:

void func(const std::vector<int>& data) {
    for (auto v : data) std::cout << v << " ";
}

I should prefer:

void func(gsl::span<const int> data) {
    for (auto v : data) std::cout << v << " ";
}

Which has the advantage that it does not assume the caller has their data in a vector, or force them to construct a temporary vector. They could pass a std::array for example.

But a common use-case is to pass a brace-enclosed initializer list:

func({0,1,2,3})

This works for a function taking a std::vector but for a function taking a gsl::span I get the error message:

error C2664: 'void func(gsl::span)' : cannot convert argument 1 from 'initializer-list' to 'gsl::span'

It looks like gsl::span has a templated constructor designed to take any container.

Is this just something missing from the Microsoft GSL implementation or is there a good reason to prevent this practice?

Answer 1

This now works with absl::Span. The following example is copied from https://abseil.io/tips/93:

void TakesSpan(absl::Span<const int> ints);

void PassALiteral() {
  // Span does not need a temporary allocation and copy, so it is faster.
  TakesSpan({1, 2, 3});
}