Question

Explain why a class's copy constructor is called when an object of that class is passed by value into a function.

Short answer

A copy constructor is a special constructor that creates a new object based on an existing object's values, ensuring a deep copy rather than a shallow copy. It is called when an object is passed by value to a function to create a temporary object, which maintains the integrity of the original object and prevents unintended changes. The copy constructor's primary function is to allow the program to work with a copied object within the function without modifying the original object's data and resources.

Step-by-step solution

What is a copy constructor?

A copy constructor is a special constructor in a class that is called when an object is initialized with the same class's previously-created object. Its purpose is to create a new object that is a copy of an existing object. The syntax of a copy constructor is: ``` ClassName( const ClassName &obj ); ``` Where `ClassName` is the name of the class, and `obj` is the object being passed to the copy constructor as a reference.

Passing objects by value

When an object is passed by value to a function, that means a new local object is created with the same values as the original object. In order for this new object to be a true copy of the original, the class needs a copy constructor to ensure that the new object is constructed correctly, and to avoid the shallow copy of the original object.

Why is the copy constructor called?

The copy constructor is called in the following situations: 1. When an object is initialized with another object's values (explicitly or implicitly). 2. When an object is passed by value to a function. 3. When an object is returned by value from a function. In this case, we are focusing on point 2, when an object is passed by value into a function. The copy constructor is called to ensure that all data members and resources of the original object are properly duplicated for the new, temporary object being created within the function.

Benefits of a copy constructor

The copy constructor helps maintain the integrity of the original object in the following ways: 1. It ensures a deep copy is made, copying the values of the original object, and not just referencing the same memory locations. 2. It prevents unintended changes to the original object or its resources within the function because the function works with a copy, not the original object.

Example

Here's a simple example to demonstrate the concept: ```cpp #include using namespace std; class MyClass { int num; public: MyClass() {} MyClass(int a) { num = a; } MyClass(const MyClass &obj) { num = obj.num; } int getNum() { return num; } }; void someFunction(MyClass obj) { obj = 25; } int main() { MyClass originalObj(10); someFunction(originalObj); cout << "Value of originalObj is: " << originalObj.getNum() << endl; } ``` Output: ``` Value of originalObj is: 10 ``` If the copy constructor were not called when passing `originalObj` to `someFunction`, then the value of `originalObj` would have been modified inside `someFunction`. However, since the copy constructor is called, the value of `originalObj` remains unchanged.