Question

given a std::vector< std::string >, the vector is ordered by string length, how can I find a range of equal length strength?

I am looking forward an idiomatic solution in C++.

I have found this solution:

// any idea for a better name? (English is not my mother tongue)
bool less_length( const std::string& lhs, const std::string& rhs )
{
    return lhs.length() < rhs.length();
}

std::vector< std::string > words;
words.push_back("ape");
words.push_back("cat");
words.push_back("dog");
words.push_back("camel");
size_t length = 3;
// this will give a range from "ape" to "dog" (included):
std::equal_range( words.begin(), words.end(), std::string( length, 'a' ), less_length );

Is there a standard way of doing this (beautifully)?

Answer 1

I expect that you could write a comparator as follows:

struct LengthComparator {
    bool operator()(const std::string &lhs, std::string::size_type rhs) {
        return lhs.size() < rhs;
    }
    bool operator()(std::string::size_type lhs, const std::string &rhs) {
        return lhs < rhs.size();
    }
    bool operator()(const std::string &lhs, const std::string &rhs) {
        return lhs.size() < rhs.size();
    }
};

Then use it:

std::equal_range(words.begin(), words.end(), length, LengthComparator());

I expect the third overload of operator() is never used, because the information it provides is redundant. The range has to be pre-sorted, so there’s no point the algorithm comparing two items from the range, it should be comparing items from the range against the target you supply. But the standard doesn’t guarantee that. [Edit: and defining all three means you can use the same comparator class to put the vector in order in the first place, which might be convenient].

This works for me (gcc 4.3.4), and while I think this will work on your implementation too, I’m less sure that it is actually valid. It implements the comparisons that the description of equal_range says will be true of the result, and 25.3.3/1 doesn’t require that the template parameter T must be exactly the type of the objects referred to by the iterators. But there might be some text I’ve missed which adds more restrictions, so I’d do more standards-trawling before using it in anything important.