C++ Find last ocurrence of a string inside a substring

Question

I need a method that helps me to find a string inside another substring, or in other words, find a string inside a subrange of other string. Besides, I need to find it in reverse order because I know that the string I'm looking for is closed to the end of the substring used as "haystack".

Let's suppose the following piece of code, where rfind_in_substr is the method I'm asking for:

std::string sample("An example with the example word example trice");

// substring "ample with the example wo"
std::size_t substr_beg = 5;
std::size_t substr_size = 24;

// (1)
std::size_t pos = rfind_in_substr(sample, substr_beg,
                                  substr_size, "example");

// pos == 20, because its the index of the start of the second
// "example" word inside the main string. 

Of course, the line (1) could be replaced by:

std::size_t pos = substr_beg + sample.substr
            (substr_beg, substr_size).rfind("example");

But that implies an unnecesary copy of the substring. Is there any method or C++/boost method that could help me doing that?

I was looking at boost::algorithm::string library but I've found nothing (that I had understood). I know that C++17 has the std::string_view class, that would be perfect, but I'm using C++14.

Answer 1

From Boost.StringAlgo:

#include <boost/algorithm/string/find.hpp>

auto haystack = boost::make_iterator_range(str.begin() + from, str.begin() + from + len);
auto found = boost::algorithm::find_last(haystack, needle);

Now, if you need to use this with other member functions from std::string, you need to do extra steps in converting a resulting range into an index like this answer does, but if you aren't, then simply use the range interface and avoid the std::string's "helpful" methods.

Another option is to use boost::string_ref which is what std::string_view is basically based on:

#include <iostream>
#include <boost/utility/string_ref.hpp>


std::size_t rfind_in_substr(std::string const& str, std::size_t from,
                            std::size_t len, std::string const& s)
{

    return from + boost::string_ref(str).substr(from, len).rfind(s);
}

int main()
{
    std::string sample("An example with the example word example trice");

    // substring "ample with the example wo"
    std::size_t substr_beg = 5;
    std::size_t substr_size = 24;

    // (1)
    std::size_t pos = rfind_in_substr(sample, substr_beg,
                                      substr_size, "example");

    // pos == 20, because its the index of the start of the second
    // "example" word inside the main string. 
    std::cout << pos << "\n";
}