C++: vector< set<string> > initialization by function

Question

In C++, I would like to initialize a vector of sets of strings by:

string border(getBorder());
vector<string> W(_h_,border+"_"+border);
vector< set<string> > W_f(_h_,getNgrams(&W[0]));

where:

set<string> getNgrams(string word){ ... }

and:

string getBorder(){ ... }

The error message reads:

conversion from 'std::basic_string<char, std::char_traits<char>, std::allocator<char> >*' to non-scalar type 'std::string' requested

Answer 1

W[0] Results in a std::string, hence &W[0] is a pointer to string and you have getNgrams(std::string) not getNgrams(std::string*), hence the compiler complains about the conversion from std::string* to std::string

Answer 2

You are trying to assign an std::string* to an std::string here:

vector< set<string> > W_f(_h_,getNgrams(&W[0]));

&W[0] is an std::string*, but getNgrams expects an std::string.

Answer 3

set<string> getNgrams(string word){ ... }

your getNgrams function takes a string but you're passing in a string* here:

vector<string> W(_h_,border+"_"+border);
vector< set<string> > W_f(_h_,getNgrams(&W[0])); // look at the argument being passed.

w is a vector<string>, w[0] therefore is a string, and you're passing &w[0] which is a string*

Answer 4

On this line:

vector< set<string> > W_f(_h_,getNgrams(&W[0]));

you get the address of W[0] using the & operator. The type of W[0] is std::string and the type of the address is std::string* (or more specifically, what you see in the error message). You then pass that pointer to the function getNgrams which expects a string instead of a pointer.

Simply remove the & operator to fix. You may wish to use a const reference instead though, to avoid copying the string:

set<string> getNgrams(const string& word)