Map changes without having to change in Dart

Question

I have this code:

Map<String, dynamic> dataUser = {
  'name': group.name,
  'simplify_group_debts': group.simplifyGroupDebts,
  'admin_user': group.adminUser,
  'timestamp': ServerValue.timestamp,
};

Map<String, dynamic> data = dataUser;

print(dataUser);

data.putIfAbsent('members', () => {user.uid: true});

print(dataUser);

I only need to put "members" in the data map. But after use "putIfAbsent", dataUser changes too.

This is the output:

Why is this happening? Thanks in advance.

Answer 1

This works, like you've shown, because of pointers to this data structures. If we will go 'under the hood' of the language (not only Dart, it could be Java also).
In your example you assign to 'data' variable a pointer to 'dataUser', not a new instance or copy of this data.

What is pointer? With pointer you show compiler the address of the memory, where data is stored, and give a way of modifying them.

So, let's return to your example:

Map<String, dynamic> dataUser = {
      'name': group.name,
      'simplify_group_debts': group.simplifyGroupDebts,
      'admin_user': group.adminUser,
      'timestamp': ServerValue.timestamp,
    };

Map<String, dynamic> data = dataUser;

data.putIfAbsent('members', () => {user.uid: true});

In line 2 you assign to 'data' variable the pointer to 'dataUser', so in 'data' we have exactly the same data address, as 'dataUser' has.

Map<String, dynamic> data = dataUser;

Instead, to avoid such behavior, you need to create new instance (give it own memory address/room to store data) of 'dataUser's Map, using .from method:

var data = Map.from(dataUser);

Which will create you new, independent instance from 'dataUser' map, with its unique memory address pointer. So, when you will mutate 'dataUser' or 'data' map, they will not mutate each other, each map will change only itself.

Answer 2

I solve this issue ussing

final data = Map.from(dataUser);

instead of

Map<String, dynamic> data = dataUser;

I also don't understand why.

Answer 3

This is working on the right way :

data.putIfAbsent('members', () => {user.uid: true});

If you want only members in a map then you will take a new map() or clear this map and insert members in that map().

Please read official document for Map().putIfAbsent method.