scnlib  0.2.0
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Rationale

Why take just views? Why not every possible range?

First off, taking it's not possible to take every range; operator-- is required for error recovery, so at least bidirectional_range is needed.

views have clearer lifetime semantics, and make it more difficult to write less performant code.

std::string str = "verylongstring";
auto ret = scn::scan(str, ...);
// str would have to be reallocated and its contents moved

Why take arguments by reference?

Relevant GitHub issue

Another frequent complaint is how the library requires default-constructing your arguments, and then passing them by reference. A proposed alternative is returning the arguments as a tuple, and then unpacking them at call site.

This is covered pretty well by the above GitHub issue, but to summarize:

  • std::tuple has measurable overhead (~5% slowdown)
  • it still would require your arguments to be default-constructible

To elaborate on the second bullet point, consider this example:

auto [result, i, str] =
scn::scan_tuple<int, non_default_constructible_string>(
range, scn::default_tag);

Now, consider what would happen if an error occurs during scanning the integer. The function would need to return, but what to do with the string? It must be default-constructed (std::tuple doesn't allow unconstructed members).

Would it be more convenient, especially with C++17 structured bindings? One could argue that, and that's why an alternative API, returning a tuple, is available, in the header <scn/tuple_return.h>. The rationale of putting it in a separate header is to avoid pulling in the entirety of very heavy standard headers <tuple> and <functional>.

What's with all the vscan, basic_args and arg_store stuff?

This approach is borrowed (cough stolen cough) from fmtlib, for the same reason it's in there as well. Consider this peace of code:

int i;
std::string str;
scn::scan(range, scn::default_tag, i, str);
scn::scan(range, scn::default_tag, str, i);

If the arguments were not type-erased, almost all of the internals would have to be instantiated for every given combination of argument types.