Get value in a stacked map - c++

Question

I have a stacked map that looks like this:
std::multimap <double, std::map<int,TypeA> > m
How can I point to the value of TypeA? I have tried:
TypeA t = m.second.second
or pointer:
for (std::multimap <double, std::map<int,StopID>>::iterator p = m.begin(); p != m.end(); p ++ ){
....
TypeA t = p->second->second
}
However, I got this error
no member named second in std::map...
Any idea?

Answer 1

for (std::multimap <double, std::map<int,StopID>>::iterator p = m.begin(); p != m.end(); p ++ ){
In the above you are iterating over the multimap and p->second points at the whole inner map, not a single entry in it. You need to iterate over the map too to access all entries in that.
You can use range based for-loops and structured bindings to make life easier.
for(auto& [the_double, the_map] : m) {
for(auto& [the_int, the_type_a] : the_map) {
// do what you want with the TypeA reference "the_type_a"
}
}
Edit: If I understand comments correctly, the map always contains exactly one int, TypeA pair - and in that case, just replace the map with a std::pair<int, TypeA> and you can have one loop only:
#include <utility> // std::pair
int main() {
std::multimap <double, std::pair<int, TypeA>> m;
for(auto& [the_double, the_pair] : m) {
auto& [the_int, the_type_a] = the_pair;
// do what you want with the int and TypeA references
}
}

Answer 2

m->second is a std::map object. You must use a loop for the next map:
using multimap_t = std::multimap <double, std::map<int, TypeA>>;
for (multimap_t::iterator p = m.begin(); p != m.end(); p++) {
std::map<int, TypeA>>& map = p->second;
for (decltype(p->second)::iterator miter = map.begin(); miter != map.end(); ++miter) {
TypeA t = miter->second;
// ...
}
}
A simpler version with auto (C++11):
for (auto p = m.begin(); p != m.end(); p++) {
auto& map = p->second;
for (auto miter = map.begin(); miter != map.end(); ++miter) {
TypeA t = miter->second;
// ...
}
}