C++ Programming : Error when assigning the iterator to vector - c++

Question

vector<vector<string> > vvs;
vector<string> vs;
vs.push_back("r1-c1");
vs.push_back("r1-c2");
vs.push_back("r1-c3");
vvs.push_back(vs);
for (vector<vector<string> >::iterator vvsi = vvs.begin(); vvsi != vvs.end(); vvsi++) {
vector<string> vec_str = *vvsi;
for (vector<string>::iterator vsi = vec_str.begin(); vsi != vec_str.end(); vsi++) {
cout << *vsi << ", ";
}
cout << "\n";
}
In the above C++ code, to avoid the copy of vector(vector vec_str = *vvsi) i tried the below code
vector<string> *vec_str = vvsi.base(); //Working. which (returns a const pointer&)
vector<string> *vec_str = &(*vvsi); //Working. Assigning the address
But
vector<string> *vec_str = vvsi; //Error. Not able to assign
Error
(build error : cannot convert 'std::vector<std::vector<std::basic_string<char> > >::iterator to 'std::vector<std::basic_string<char> >*' in initialization)
In a case of integer
int a=10;
int *b = &a; //working. Assigning address
int *c = &(*b); //working. Assigning address
int *d = b; //working. Assigning address
*c=11;
std::cout << a<<"\n";
*d=12;
std::cout << a<<"\n";
In the case of vector, why the build error when assigning (can't able to understand from the C++ iterator documentation)?

Answer 1

An iterator is not a pointer. It's interface was made to be used as a pointer, but it certainly is not a pointer.
However, if you simply want to iterate through the elements of each vector in the parent vector, there is no need to assign it to a temporary vector or to a pointer, the iterator itself is good enough:
for (vector<vector<string> >::iterator vvsi = vvs.begin(); vvsi != vvs.end(); ++vvsi) {
for (vector<string>::iterator vsi = vvsi->begin(); vsi != vvsi->end(); ++vsi) {
cout << *vsi << ", ";
}
cout << "\n";
}
Also, since you put c++11 as a tag in your question, I assume you might be interested in a more modern expression:
for(auto& vst : vvs){
for(auto& st : vst){
cout << st << ", ";
}
cout << endl;
}

Answer 2

You're trying to assign the value of an iterator to a pointer. That the iterator points to the same type as the pointer is irrelevant, since RandomAccessIterators, which are the iterator type of std::vector, are not implicitly convertible to pointers.
If you're using c++11, there are simpler ways to go about iterating over a vector of a vector even than #A Hernandez's solution (or any container of containers at that), using the range-for syntax available in c++14:
for(auto &&stringVec : vvs){
for(auto &&str : stringVec){
cout << str << ", ";
}
}
cout << '\n';
The && here allows binding to temporary containers as well as existing ones, so you can use this syntax with both existing container containers and container containers returned from a function without storing them.