Question

What is wrong with the following code? class classA { protected: void setX(int a); //Line 1 //Postcondition: x = a; //Line 2 private: //Line 3 int x; //Line 4 }; . . . int main() { classA aObject; //Line 5 aObject.setX(4); //Line 6 return 0; //Line 7 }

Short answer

`setX` is protected; change it to public to call from `main`. Direct access is not allowed.

Step-by-step solution

Understanding the Code Structure

The code defines a class named `classA` with a protected method `setX` and a private integer member `x`. The method `setX` presumably is intended to set the value of `x`. In the `main` function, an object `aObject` of type `classA` is created, and `setX` is called to set `x` to 4.

Identify Access Modifiers

Access modifiers in C++ (public, protected, and private) determine the accessibility of class members outside the class. The method `setX` is declared as `protected`, which means it cannot be accessed directly by the object `aObject` in the `main` function.

Spot the Error in Method Call

The call `aObject.setX(4);` in the `main` function causes an error because `setX` is a `protected` member function. `Protected` members can only be accessed by the class itself and its derived classes, not by outside functions or objects.

Propose a Solution

To fix the error, change `setX` from `protected` to `public`. This will allow `main` and any other external functions to access it. Modify the class definition as follows: ```cpp class classA { public: void setX(int a); //Line 1 protected: int x; //Line 4 }; ```

Key concepts

protected members

In C++, the `protected` access modifier is used to define members of a class that can be accessed by the class itself and by its derived classes. This is useful when designing class hierarchies where you want to allow derived classes to use or modify some parts of the base class.

However, `protected` members cannot be accessed directly through objects of that class, as seen in the provided exercise. Consider the `classA` example where the method `setX` was declared as `protected`. This prevented it from being called in the `main` function via `aObject.setX(4)`, resulting in a compilation error.

To contrast, if you had a `classB` derived from `classA`, `classB` could potentially access `setX` and use it to modify its inherited state. This visibility restriction is intended to maintain a level of encapsulation and control over how data and behaviors are exposed. When to use `protected` depends largely on whether you need that level of access by derived classes while keeping access restricted from external functions or objects.

private members

The `private` access modifier is the most restrictive access level in C++. Members declared as `private` can only be accessed from within the same class, that is, through members of the class itself. They are not visible to derived classes or objects created from the class.

In the initial `classA` design, the integer variable `x` was declared as `private`. This means that it can't be directly modified or accessed outside of its class. Instead, class methods (like a potential `setX`) would have to manage these operations. This principle is fundamental in keeping the data secure and ensuring changes to private variables are controlled and occur in expected ways.

With `private` members, any attempt to directly access `x` from `main` would result in an error. The intended purpose here is to secure the data encapsulated within the class and provide interfaces (often public methods) through which the data can be accessed or modified in a controlled manner.

public access

The `public` access modifier in C++ allows members to be accessed from anywhere in the program. This means that once a member is declared as `public`, it can be called or modified directly using an object of that class without any restriction.

Switching the `setX` method in `classA` from `protected` to `public` resolves the access issue in the exercise. By doing so, the method can be successfully called from the `main` function using the `aObject` instance. Thus, `aObject.setX(4);` would legally compile and execute.

Public access is generally used for functions and data that define an interface for interacting with the class. This is common for methods intended to manipulate object state externally (via constructors, setters, getters) and safely. When something needs to be accessed broadly without breaking encapsulation or the intended class design, `public` is the go-to access level.