Copy Constructors

There are two specialized constructors: the Copy Constructor and the Move Constructor. Understanding these constructors is crucial for effective resource management, memory allocation, and object manipulation

Copy Constructor

When copying two variables of a primitive type, the process is straightforward and doesn't usually require special consideration. However, copying objects in is a more complex and tricky task. The distinction between shallow copy and deep copy will become clear when you will examine the following code:

main


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#include <iostream>

class Example {
public:
    // Constructor that initializes p_data with a new integer value
    Example(int data)
        : p_data(new int(data)) { }
        
    // Destructor that prints the value of p_data and deallocates memory
    ~Example() {
        std::cout << *p_data << std::endl;
        delete p_data;
    }
    
    int* p_data; // Pointer to an integer
};

int main()
{
    Example obj1(25); // Creating an object with an initial value of 25
    Example obj2(obj1); // Creating a second object using the obj1
    *obj2.p_data = 1000; // Modifying the value of by p_data in the obj2
}

Note

It generates error: free(): double free detected

Note

That is precisely why a copy constructor is essential. It enables us to perform a deep copy, ensuring safety in the process.

Syntax of Copy Constructor

Creating a copy constructor shares some similarities with creating a constructor, yet it has its own distinctive features. The general approach to creating a copy constructor as usual is outlined below:

Name: the copy constructor has the same name as the class. It does not have any unique prefix or symbol to differentiate it from the regular constructor.
Parameter Type: it takes a single parameter, typically a reference to a constant object of the same class.
No Return Type: similar to other constructors and the destructor, the copy constructor does not have a return type, not even void.
Special Usage: it is called automatically by the compiler in specific scenarios, such as when an object is passed by value to a function, returned from a function, or initialized using another object of the same class.

main


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#include <iostream>

class Example {
public:
    // Constructor that initializes p_data with a new integer value
    Example(int data)
        : p_data(new int(data)) { }
        
    // Copy constructor that performs deep copy of the Example object
    Example(const Example& other)
        : p_data(new int(*other.p_data)) { }
        
    // Destructor that prints the value pointed to by p_data and deallocates memory
    ~Example() {
        std::cout << *p_data << std::endl;
        delete p_data;
    }
    
    int* p_data; // Pointer to an integer
};

int main()
{
    Example first_obj(25); // Creating an Example object with an initial value of 25
    Example second_obj(first_obj); // Creating a obj2 using obj1
    *second_obj.p_data = 1000; // Modifying the value pointed
}

The Rule of Three

There is a guideline for classes that manage dynamically allocated memory or other resources. It states that if it needs to provide a custom implementation for any of the following three methods:

Destructor (~Example()).
Copy constructor (Example(const Example&)).
Copy assignment operator (Example& operator=(const Example&)).

Then it often needs to provide implementations for all three of them. This is because if a class manages resources that require custom cleanup or copying behavior, the default implementations provided by the compiler may not be suitable.

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Seção 2. Capítulo 8