Question

Why is a class's copy constructor called when an object of that class is passed by value into a function?

Short answer

Answer: A class's copy constructor is called when an object is passed by value into a function because the function needs to create a new local object and initialize it with the same data as the original object. The copy constructor is called to perform the correct data copying process from the original object to the new local object in the function. This ensures that the function operates on a separate copy of the data without modifying the original object outside the function's scope.

Step-by-step solution

Understand passing by value

When a class object is passed by value into a function, a new object is created as a local variable in that function. This new object must be initialized with the same data as the original object; hence, a copy constructor is required to perform this initialization process.

Clarify the role of copy constructor

A copy constructor is a special type of constructor that takes as input an object of the same class and initializes a new object with the same values and state as the input object. When an object is passed by value, the compiler uses the class's copy constructor to create a new local object within the function's scope.

Explain why copy constructor is called

When the function is called and the original object is passed by value, the function must create a new object to hold the copied data. To do this, the copy constructor is called to perform the correct data copying process from the original object to the new local object in the function. This ensures that the function operates on a separate copy of the data, without modifying the original object outside the function's scope.

Example

Consider a simple class with a copy constructor: class MyClass { public: int x; // Default constructor MyClass() { x = 0; } // Copy constructor MyClass(const MyClass& other) { x = other.x; } }; When a function takes an object of MyClass as a parameter: void myFunction(MyClass obj) { obj.x = 10; } And we call the function with an object of MyClass: MyClass myObj; myFunction(myObj); The copy constructor of MyClass will be called within myFunction to create a new local object with the same x value as myObj.