Taking reference of std::vector elements

How can we create a vector of a particular data type and get it's elements' references later ?

I am new to C++ and have some understanding of pointers and references, but I am having a hard time understanding a particular pointer and reference behaviour.

In some_file.hxx:

class some_class {
    protected:
        std::vector<float>* _parentVector;
    //rest of the body
};

In some_file.cxx under init function for class (works just like contructor):

init() {
    int num = 10;
    _parentVector = new std::vector<float>[num];
    for(int i=0;i<num;i++)
        _parentVector[i].resize(num,0);
}

In some_file.cxx inside member function:

some_class::some_func(i) {
    std::vector<float>& some_var = _parentVector[i]
}

ABOVE CODE works totally fine as expected, giving me reference of a float vector from an array of vectors.

But, when I do the same with vector of strings, everything breaks inside function saying, "invalid initialization of std::basic_string& with std::basic_string

Please comment, if you want me to paste "string" usage code.

I would like to know what is the internal mechanism ? How is float working but string isn't and what pointer magic is happening ??

Thanks.