scn::scan/prompt from stdin with literal text does not work

[melak47]

When trying to scan a format string with a literal at the start, a debug assertions fires:

File: C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.26.28801\include\xstring
Line: 1853

Expression: cannot seek string iterator before begin

Code I used, adapted from the examples (using scnlib 0.3):

#include <scn/scn.h>
#include <iostream>

int main() {
    int i = 0;
    if (scn::prompt("Hi there! What's your favorite number? ", "uhh, {}", i)) {
        std::cout << "Ooh, " << i << ", interesting!\n";
    }
    else {
        std::cout << "That doesn't look like any number I've ever seen.\n";
    }
}

[melak47]

It seems to be related specifically to the file putback, it works when you scan from e.g. a string_view.

[melak47]

In range_wrapper::advance:

scnlib/include/scn/detail/range.h

Lines 142 to 149 in cf9497e

	iterator advance(difference_type n = 1) noexcept
	{
	m_read += n;
	if (!is_caching_range<Range>::value) {
	ranges::advance(m_begin, n);
	}
	return m_begin;
	}

the begin iterator of the range is not incremented for the caching cfile iterator.
As far as I can tell, this leads to operator* always yielding the same character, thus matching the literal fails.

Also, in range_wrapper::reset_to_rollback_point:

scnlib/include/scn/detail/range.h

Lines 193 to 196 in cf9497e

	error reset_to_rollback_point()
	{
	for (; m_read != 0; --m_read) {
	--m_begin;

the iterator is decremented m_read times, even though it may have been incremented less than that (outside of range_wrapper).

[melak47]

This seems to be resolved on master.

[eliaskosunen]

It is, sorry for not responding and fixing this earlier.