C++ inserting into a map<double, multimap<int, Object*>> - c++

Question

How does one insert into a map<double, multimap<int, Object*> >?
I tried mymap.insert(make_pair(double, make_pair(int, Object*)));. Furthermore how do you access the Object* when iterating through it?
I tried it->second->second.

Answer 1

You seem to be wanting:
using MyMap = std::map<double, std::multimap<int, Object*> >;
MyMap mymap;
mymap.insert(std::make_pair(dbl, std::multimap<int, Object*>());
To enumerate:
#include <iostream>
#include <map>
struct Object {};
int main()
{
using MyMap = std::map<double, std::multimap<int, Object*> >;
MyMap mymap;
auto ins = mymap.insert(std::make_pair(42.0, std::multimap<int, Object*>()));
Object obj1, obj2;
ins.first->second.insert(std::make_pair(7, &obj1));
ins.first->second.insert(std::make_pair(7, &obj2));
for (auto const& pr : mymap)
{
std::cout << "dblkey: " << pr.first << '\n';
for (auto const& pr2 : pr.second)
{
std::cout << "\tintkey: "<< pr2.first
<< ", value: " << static_cast<const void*>(pr2.second) << '\n';
}
}
}
Output (pointer values vary, obviously)
dblkey: 42
intkey: 7, value: 0x7fff5fbff160
intkey: 7, value: 0x7fff5fbff158
In the end, your problem was a type issue more than anything else. You were attempting to insert a std::pair<int, Object*> where a std::multimap<int, Object*> instance belonged.

Answer 2

In a std::map<double, std::multimap<int, Object*> >, the second value is a single std::multimap object, but you are trying to set it to a std::pair object instead. std::multimap does not have a constructor that accepts a std::pair as input, which is why your code fails.
You need to construct a std::multimap object to insert into the std::map, eg:
double d = ...;
int i = ...;
Object *obj = ...;
std::multimap mm;
mm.insert(std::make_pair(i, obj));
mymap.insert(std::make_pair(d, mm));
Or:
mymap[d].insert(std::make_pair(i, obj));
When iterating the std::map, it will return an iterator to a std::pair where ->first is double and ->second is std::multimap<int, Object*>.
When iterating the std::multimap, it will return an iterator to a std::pair where ->first is int and ->second is Object*.
auto iter1 = mymap.begin;
while (iter1 != mymap.end())
{
auto &mm = iter1->second; // mm = std::multimap<int, Object*>
auto iter2 = mm.begin();
while (iter2 != mm.end())
{
auto obj = iter2->second; // obj = Object*
// use obj as needed...
++iter2;
}
++iter1;
}
This would be simpler if you used a ranged-based for loop instead:
for (auto &v1: mymap) // v1 = std::pair<double, std::multimap<int, Object*> >
{
auto &mm = v1.second; // mm = std::multimap<int, Object*>
for (auto &v2: mm) // v2 = std::pair<int, Object*>
{
auto obj = v2.second; // obj = Object*
// use obj as needed...
}
}