C++ Insert to map in map

How can I insert another map into the map?

In the code I try to copy the map from another.

multimap<string, map<size_t, size_t>> sorted;

for (auto itr=m_Items.begin(); itr !=m_Items.end(); ++itr)
     sorted.emplace(itr->first, make_pair(itr->second.m_Date.m_Time, itr->second.m_Cnt));

>Solution :

Assuming that you have m_Items as

struct Date {
    std::size_t m_Time;
};

struct MyStruct
{
    Date m_Date;
    std::size_t m_Cnt;
};

std::multimap<std::string, MyStruct> m_Items;

then you need to

for (auto itr = m_Items.begin(); itr != m_Items.end(); ++itr)
sorted.emplace(itr->first, std::map<size_t, size_t>{ {itr->second.m_Date.m_Time, itr->second.m_Cnt} });
//                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Your map’s value is again a map (i.e. std::map<size_t, size_t>) not a std::par. Therefore, you need to insert as above.

In c++17 with the help of structured binding declaration, and with a range based for loop(since c++11), you could write much intuitive:

for (const auto& [key, value] : m_Items)
  sorted.emplace(key, std::map<size_t, size_t>{ {value.m_Date.m_Time, value.m_Cnt} });
//                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 

Here is a (short demo.)

However, if you only need an unsorted key-value as pair as entry in sorted, I would suggest a

std::multimap<std::string, std::pair<size_t, size_t>> sorted;
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^

instead there.