How is a const class object different from const data member? - c++

Question

I have 2 sample codes below:
#include<iostream>
#include<vector>
using namespace std;
class emp{
int emp_count;
public:
emp(){emp_count=2;}
int get_employee_count() const {return emp_count;}
};
int main(){
const emp e1;
cout<<e1.get_employee_count();
return 0;
}
Above code works fine, however, below code gives an error for trying to change a read-only value:
#include<iostream>
#include<vector>
using namespace std;
class emp{
const int emp_count;
public:
emp(){emp_count=2;}
int get_employee_count() const {return emp_count;}
};
int main(){
emp e1;
cout<<e1.get_employee_count();
return 0;
}
A const class object doesn't let me change the value of the data member, but based on the above example, how is a const class object different from a const data member as I am able to assign the value in the constructor body in the 1st code, while the 2nd code restricts me to do the same?

Answer 1

A const class object doesn't allow the value of any of its data members to be modified outside the constructor. For example:
#include<iostream>
#include<vector>
class emp {
public:
int emp_count; // Made public for demonstration
emp() { emp_count = 2; }
int get_employee_count() const { return emp_count; }
};
int main()
{
const emp e1;
emp e2;
e1.emp_count = 10; // Compiler error
std::cout << e1.get_employee_count();
return 0;
}
But emp_count can still be modified by the constructor:
emp()
{
emp_count = 2; // This is assignment, not initialization
}
A const data member is a data member that cannot be modified after initialization, neither inside the class nor outside the class.
To initialize a data member, just assign a value to it as such:
class emp
{
const int emp_count = 2;
public:
Or initialize it using the constructor as such:
class emp
{
const int emp_count;
public:
emp() : emp_count(2) { } // Initialization of emp_count
int get_employee_count() const { return emp_count; }
};
Also, consider not using the following line in your code:
using namespace std;
..as it's considered as bad practice.

Answer 2

emp_count=2; is not initialization, it is an assignment. Even if it happens in a constructor. And one cannot assign to const objects.
Use the initializer list to initialize const members:
emp(): emp_count(2) {}
Even better is to use in-class initialization for constants:
class emp{
int emp_count=2;
public:
int get_employee_count() const {return emp_count;}
};
This has the advantage of rule-of-zero and it propagates the value to all constructors if any are defined explicitly.