Convert vector of string between ranges to a delimited string(boost allowed)

Question

I have a vector of strings with a size of 10. I want to join it to a string from index 3 to 6 with space as delimiter. I know boost::algorithm::join could do it with the whole vector, but I want something that does it on a specific range with minimal copy and optimal efficiency. i know lot of stringstream based solution but i want something that doesn't have overhead of stringstream to string.

Answer 1

You can create a range from something other than a complete container, and use boost::algorithm::join just like in your original example.

#include <boost/algorithm/string/join.hpp>
#include <vector>
#include <iostream>

int main(int, char **)
{
    std::vector<std::string> list;
    list.push_back("Something");
    list.push_back("that's");
    list.push_back("not");
    list.push_back("Hello");
    list.push_back("World!");
    list.push_back("really");
    list.push_back("it's");
    list.push_back("not");

    boost::iterator_range<std::vector<std::string>::const_iterator> 
            rng (list.begin() + 3, list.begin() + 5);
    std::string joined = boost::algorithm::join(rng, ", ");
    std::cout << joined << std::endl;
}

still prints "Hello, World!"

Answer 2

As you need to reduce overhead I suggest to implement a custom version of std::back_insert_iterator for std::string. So you might do this:

class my_back_insert_iterator :
    public std::iterator<std::output_iterator_tag,void,void,void,void>
{
protected:
  std::string* container;

public:
  typedef std::string container_type;
  explicit my_back_insert_iterator(std::string& x) : container(std::addressof(x)) {}

  my_back_insert_iterator& operator=(const std::string& value)
    { container->append(value + " "); return *this; }

  my_back_insert_iterator& operator=(std::string&& value)
    { container->append(std::move(value + " ")); return *this; }

  my_back_insert_iterator& operator*()
    { return *this; }

  my_back_insert_iterator& operator++()
    { return *this; }

  my_back_insert_iterator& operator++(int)
    { return *this; }
};

After that you could easily do the following:

std::vector<std::string> strings = {"0", "1", "2", "3", "4",
                                    "5", "6", "7", "8", "9"};
std::ptrdiff_t from = 3ULL;
std::ptrdiff_t to = 6ULL;

std::string joinedStrings;
std::move(strings.begin()+from, strings.begin()+to,
          my_back_insert_iterator(joinedStrings));
joinedStrings += *(strings.begin()+to);

Try this online, too!

Old Answer

You could do the following:

std::vector<std::string> strings = {"0", "1", "2", "3", "4",
                                    "5", "6", "7", "8", "9"};
std::ptrdiff_t from = 3ULL;
std::ptrdiff_t to = 6ULL;

std::stringstream ss;
std::copy(strings.begin()+from, strings.begin()+to,
          std::ostream_iterator<std::string>(ss, " "));
ss << *(strings.begin()+to);

std::string joinedStrings = ss.str();

See it working online!