To add to the problem, how to extend this formatting to process extra spaces between words of the string.
Actually, this is a simpler case than accounting for multiple leading and trailing white-space characters. All you need to do is remove duplicate adjacent white-space characters from the entire string.
The predicate for adjacent white space would simply be:
auto by_space = [](unsigned char a, unsigned char b) {
return std::isspace(a) and std::isspace(b);
};
and then you can get rid of those duplicate adjacent white-space characters with std::unique, and the erase-remove idiom:
// s = " This is a sample string "
s.erase(std::unique(std::begin(s), std::end(s), by_space),
std::end(s));
// s = " This is a sample string "
This does potentially leave an extra white-space character at the front and/or the back. This can be removed quite easily:
if (std::size(s) && std::isspace(s.back()))
s.pop_back();
if (std::size(s) && std::isspace(s.front()))
s.erase(0, 1);
Here's a demo.