C++11 - Range-based for loop on a two-dimensional map

Question

I have a two-dimensional map that I declared like that :

typedef std::map<std::string, std::map<std::string, Objective>> objectives_t;

I want to save the content of this 2d map into a file.

So I tried something like that, inspired by some code I found on the web :

for (auto const &subject : m_objectives) {
    for (auto const &objective : m_objectives[subject.first]) {
        //Print the objective
    }
}

But of course, this doesn't work. How should I do that ? I'm not really sure what are subject and objective (are they some iterators ?).

On the second line, I get :

error: passing 'const objectives_t {aka const std::map<std::basic_string<char>, std::map<std::basic_string<char>, Objective> >}' as 'this' argument of 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = std::basic_string<char>; _Tp = std::map<std::basic_string<char>, Objective>; _Compare = std::less<std::basic_string<char> >; _Alloc = std::allocator<std::pair<const std::basic_string<char>, std::map<std::basic_string<char>, Obj|

Answer 1

Your outter loop is correct. But the inner should be iterating over subject.second

for (auto const &subject : m_objectives) {
    for (auto const &objective : subject.second) {
        // print out objective.second
    }
}

This is because after the first range-based for loop, subject has type

std::pair<const std::string, std::map<std::string, Objective>> const&

So each item is

subject.first    // std::string
subject.second   // std::map<std::string, Objective>

Then when you iterate over subject.second, your objective is now a

std::pair<const std::string, Objective> const&

So again to pull apart the elements

objective.first    // std::string
objective.second   // Objective