Pass class object to two objects

Question

I have a class Database. In my class A I create a new object from the database and two objects of class B. Now I want to access the database object from both class B objects. I tried passing the object as pointer and reference but when I print the adress it's always different.

Class A.h:

Database database; 
B b1; 
B b2;

Class A.cpp:

b1.setDatabase(database); 
b2.setDatabase(database);

b1.insert("A"); 
b2.insert("B");
b1.insert("C");
b2.insert("D");

Class B.h

Database database;

Class B.cpp

void setDatabase(Database& database) { 
  this->database = database; 
}

void insert(std::string name) { 
  database.dataMap.insert({ name, 10 }); 

  std::cout << database.dataMap.size() << std::endl; 
}

Class Database.h

std::map<std::string, int> dataMap;

The output should be 1 2 3 4

But it is

1 1 2 2

So I think it's not the same object

B needs to store a pointer or reference to Database in order to do this. What does you B look like? — NathanOliver
– NathanOliver, Commented Aug 28, 2019 at 12:59
Looking at the code you show your B stores Database as a copy. So clearly when you modify one of the databases you do nothing to the other or the one you started with. — Qubit
– Qubit, Commented Aug 28, 2019 at 13:00
I changed the code to store a pointer object. The code compiles but if I run the program it closes immediately. — Martin M.
– Martin M., Commented Aug 28, 2019 at 13:04
@MartinM. We'd need a minimal reproducible example in order to help with that. That said, since you already have an answer here, you would need to ask a new question about that. — NathanOliver
– NathanOliver, Commented Aug 28, 2019 at 13:05

Guillaume Racicot · Accepted Answer · 2019-08-28 13:40:34Z

4

You are passing a reference. What you are doing is copying yout Database. You need to copy the address (pointer):

Class B.h

class B
{
private:
  Database* database; //Pointer to Database instead of actual Database object

public:
  void setDatabase(Database* database) //Pass pointer(*) instead of reference(&)
  { 
    this->database = database; // Assign pointer instead of calling the implicit copy constructor
  }

  void insert(string name) { 
    database->dataMap.insert({ name, 10 }); //Dereferencing and member access(->) instead of member access only(.)

    cout << database->dataMap.size() << endl;
  }

}

Class A.h:

  Database database; 
  B b1; 
  B b2;

Class a.cpp:

b1.setDatabase(&database); 
b2.setDatabase(&database);

b1.insert("A"); 
b2.insert("B");
b1.insert("C");
b2.insert("D");

Live example

edited Aug 28, 2019 at 13:40

Guillaume Racicot

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answered Aug 28, 2019 at 13:03

Detonar

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10 Comments

Martin M. Over a year ago

If I change the code to yours I can't insert the values to the map.

Detonar Over a year ago

You need to use the address operator(&) like this: setDatabase(&database) in your main.

Martin M. Over a year ago

Yes. But I mean the database->dataMap.insert() isn't working

Detonar Over a year ago

My fault. it's called db now, so 'db->dataMap.insert()' of course.

Martin M. Over a year ago

The db was not the problem. It isn't working anyway.

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