When and How a Copy Constructor is Called in C with Examples

Introduction to Copy Constructor in C

In C , when you want to create a new object that is a copy of an existing one, the copy constructor is automatically invoked. This mechanism ensures that the new object is initialized with a deep copy of the data from the original object, which is crucial for managing complex data structures and ensuring object integrity. This article will explore the scenarios in which a copy constructor is called and provide practical examples to illustrate the concept.

Scenarios for a Copy Constructor

1. Function Parameter Passing

When you pass an object as a function parameter, the copy constructor is used to create a temporary copy of the object on the call stack. This allows the function to work with a copy of the original object without affecting it.

Consider the following function that takes an object and prints its data members:

void printDetails(const Foo obj) {
    // Access and print data members
}

Here, the Foo object is passed by const reference, but a copy constructor is still called in the background. For example, to call this function:

Foo myObj;
printDetails(myObj);

In this case, the copy constructor is called to create a temporary copy of myObj that is passed to the function.

2. Object Assignment

When you assign one object to another of the same type, a copy constructor is also used to create a deep copy of the original object. This is in contrast to a memberwise assignment, which simply copies pointers or handles without copying the underlying objects.

For instance, consider the following code snippet:

Foo bar2  bar;

In this case, the copy constructor is called to create a deep copy of the bar object and assign it to bar2.

3. Object Instantiation

The copy constructor is also invoked when you create a new object and initialize it with an existing object of the same type. This is done using the form NewObj OldObj or NewObj(OldObj).

Here is an example demonstrating object instantiation with a copy constructor:

Foo bar; // Create an object
Foo bar2(bar); // Call copy constructor to create a deep copy of bar

In both cases, the copy constructor is called to create a new object that is a deep copy of the original object.

When a Copy Constructor is Not Called

There are some scenarios where a copy constructor is not used:

When Passing by Value: If an object is passed to a function by value, the move constructor is preferred over the copy constructor. This process is known as a move. The move constructor is called instead of the copy constructor to avoid unnecessary deep copying if the data is lightweight (e.g., primitive types).

When Passing by Pointer: If an object is passed by pointer, a copy constructor is not called. Instead, the pointer is simply copied.

When Returning by Value: If an object is returned by value from a function, the move constructor is again preferred over the copy constructor. This process is known as value return. The move constructor is called to efficiently transfer the resources from the temporary object to the returned object.

Conclusion

Copy constructors play a vital role in managing complex objects and ensuring correct resource management in C . Understanding when and how they are called can help you write better, more efficient C code. By leveraging these concepts, you can handle object duplication and assignment with confidence and avoid common pitfalls.

Keywords

Copy constructor, C , object instantiation, assignment operator, deep copy