What does freq_table[field_pm_value] = map<string, int>(); means?

Question

I am going through some code and not able to figure out below syntax:

freq_table[field_pm_value] = map<string, int>();

I am confused what is happening here "map string, int()". freq_table is of type

map<string, map<string, int> >& freq_table 

The full code is https://github.com/vishalsingh8989/Star-Cubing-Algorithm/blob/master/src/csvreader.cpp

thanks

Answer 1

 freq_table[field_pm_value] = map<string, int>();

This creates a new empty map from string to int , and then copies it to the freq_table entry for field_pm_value

If freq_table has no entry for field_pm_value , a new entry is created for it.

Answer 2

Map

Maps are associative containers that store elements formed by a combination of a key value and a mapped value, following a specific order.

This structure is similar to: In a house you have different rooms, representing your "outter map container" map<string, map<string, int> >& freq_table and in each room you have unique furniture and the amount of them. These furniture are defined in the "inner map".

As std::map have unique keys, then you can't have 2 bedrooms in your house and can't have 2 beds in the same room.

In your case you are checking whether you "room" already have furniture in it, otherwise you construct an empty "room".

I am sorry about the awkward analogy, although I hope it may help.