Mixing (non-const) public and private data members is a bad practice because it confuses the intention of the class:
- If the class is a collection of loosely related values, all the data members should be
public.
- On the other hand, if the class is trying to maintain an invariant, all the data members should be
private.
If we mix data members with different levels of accessibility, we lose clarity as to the purpose of the class.
Noncompliant code example
class MyClass { // Noncompliant
public:
int firstNumber1() const { return firstNumber; }
void setFirstNumber(int firstNumber) { this->firstNumber = firstNumber; }
int secondNumber = 2;
private:
int firstNumber = 1;
};
Compliant solution
class MyClass { // Depending on the case, the solution might be different. Here, since this class does not enforce any invariant, we make all the data members public
public:
int firstNumber;
int secondNumber;
};
Exceptions
Since const data members cannot be modified, it’s not breaking encapsulation to make a const value public, even in a class that
enforces an invariant.
class MyClass { // Compliant by exception
public:
const int constNumber = 0; // const data members are fine
private:
int number = 1;
};
