c++ - Implementing a split() function -

accelerated c++, exercise 14.5 involves reimplementing split function (which turns text input vector of strings). 1 must use store input in std::string - class (str) & use split function return vec<str>, vec being std::vector - container. str class manages custom pointer (ptr) underlying vec<char> data.

the str class provides constructor str(i,j), in str.h below, constructs ptr underlying vec problem arises when try create substrings calling str(i,j) i've detailed in code the issues arise. here whittled-down version of str class (can post more code if needed):

str.h

#include "ptr.h" #include "vec.h"   class str {     friend std::istream& operator>>(std::istream&, str&); public:      // define iterators      typedef char* iterator;             typedef char* const_iterator;        iterator begin() { return data->begin(); }     const_iterator begin() const { return data->begin(); }      iterator end() { return data->end(); }     const_iterator end() const { return data->end(); }      //** define constructor `ptr`s substrings **      template<class in> str(in i, in j): data(new vec<char>) {          std::copy(i, j, std::back_inserter(*data));     }  private:     // store ptr vec     ptr< vec<char> > data; }; 

split.h

vec<str> split(const str& str) {     typedef str::const_iterator iter;     vec<str> ret;      iter = str.begin();     while (i != str.end()) {          // ignore leading blanks         = find_if(i, str.end(), not_space);          // find end of next word         iter j = find_if(i, str.end(), space);          // copy characters in `[i,' `j)'         if (i != str.end())              ret.push_back(**substring**);        // need create substrings here                                                  // call str(i,j) gives error, detailed below         = j;     }      return ret; } 

my first thought use constructor create (pointers to) required substrings. calling str(i,j) here gives error message

type 'const str' not provide call operator 

1. it appears if 1 cannot call str(i,j) here. why not?
2. could solution write str member function similar substr?