Question

For the following two lines of C++ code

map<string, vector<size_t> >::iterator beg = mapper.begin();
vector<size_t>& indics = (*beg).second;

How to understand what do they want to achieve, and in specific, what do the & and * in the second line of code mean?

Answer 1

map<string, vector<size_t> >::iterator beg = mapper.begin();

We have a map that maps from string to vector<size_t>. We get an iterator to the first element in the map, that would be the one with the lesser key value, according to std::less<string>.

vector<size_t>& indics = (*beg).second;

or what is the same

vector<size_t>& indics = beg->second;

We get the second value in the key-value pair, that is we get the vector<size_t> for the first element in the map. We keep a non-const reference to it, so we can modify its values.