How to find a substring in only a portion of a std::string?

Question

I have a std::string and i want to be able to find text only in a section of it : so i need to specify a start and an end position.

In std::string::find() one can only specify the start position.

I am trying to find the best way to search only in a portion of haystack. That would consist of giving an end_pos to stop the search even though it doesn't reach the end of haystack.

For example, search only from its 100th character to its 200th character.

Does anybody know how I could achieve this (elegantly)?

• only by searching in haystack itself, not in a substr() of it
• without creating a copy of haystack
• in C++98

Answer 1

std::string does not have a method that suits your requirement to search a sub-range of the haystack string. Have a look at std::search() for that instead, eg:

std::string needle = ...;

std::string::iterator end_iter = haystack.begin() + end_pos;
std::string::iterator found_iter = std::search(haystack.begin() + start_pos, end_iter, needle.begin(), needle.end());
if (found_iter != end_iter) {
    // needle found...
}
else {
    // needle not found...
}

If your compiler does not have std::search() available, then just write your own search() function, such as with the implementation provided on cppreference.com, eg:

namespace my {

template<class ForwardIt1, class ForwardIt2>
ForwardIt1 search(ForwardIt1 first, ForwardIt1 last,
                  ForwardIt2 s_first, ForwardIt2 s_last)
{
    while (1) {
        ForwardIt1 it = first;
        for (ForwardIt2 s_it = s_first; ; ++it, ++s_it) {
            if (s_it == s_last) return first;
            if (it == last) return last;
            if (!(*it == *s_it)) break;
        }
        ++first;
    }
}

}

...

std::string needle = ...;

std::string::iterator end_iter = haystack.begin() + end_pos;
std::string::iterator found_iter = my::search(haystack.begin() + start_pos, end_iter, needle.begin(), needle.end());
if (found_iter != end_iter) {
    // needle found...
}
else {
    // needle not found...
}