Why copy constructor use pass by reference only not pointer?

In C++, when designing a copy constructor, it is common to pass the argument by reference rather than by pointer. This is because passing by reference allows for cleaner syntax and avoids unnecessary null pointer checks. Additionally, passing by reference ensures that the object being copied is valid and accessible.

Let’s consider an example to illustrate this. Suppose we have a class called MyClass with an integer member variable data.

class MyClass {
public:
    int data;

    MyClass(int value) : data(value) {}

    // Copy constructor
    MyClass(const MyClass& other) : data(other.data) {}
};

In this example, the copy constructor takes a constant reference to MyClass (const MyClass&) as the parameter. Now, let’s see how we can use this copy constructor in different scenarios.

Using a reference parameter:

void someFunction(const MyClass& obj) {
    MyClass copy(obj);  // Use the copy constructor
    // ...
}

In this case, we pass the object obj by reference to the function someFunction. When we create the copy object copy, we can simply invoke the copy constructor by passing obj as the argument. This provides a clear and concise way of creating a copy of the object.

Using a pointer parameter:

void someFunction(const MyClass* ptr) {
    if (ptr != nullptr) {
        MyClass copy(*ptr);  // Use the copy constructor
        // ...
    }
}

Here, we pass the object ptr as a pointer to the function someFunction. To create a copy of the object, we first need to check if the pointer is valid (i.e., not null) before dereferencing it. Once we confirm that the pointer is not null, we can invoke the copy constructor by dereferencing the pointer (*ptr) and passing it as the argument.

By passing the object by reference, as in the first example, we eliminate the need for null pointer checks and simplify the code. Additionally, using a reference parameter allows us to pass objects directly without explicitly referencing them with the dereference operator (*), resulting in cleaner syntax.

However, it’s worth mentioning that there might be situations where passing by pointer could be appropriate, such as when dealing with optional objects that can be null. But in general, when designing copy constructors, passing by reference is the recommended approach for simplicity and readability.

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