Скачать или смотреть Understanding the this Pointer in C+ + : Fixing Object Value Changes in Functions

Learn how to effectively use the `this` pointer in C+ + to ensure changes in object values are correctly captured in non-member functions.
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Understanding the this Pointer in C+ + : Fixing Object Value Changes in Functions

In C+ + , managing object data especially while working with pointers and references can sometimes lead to confusion, especially for those who are new to the language. One common issue arises when trying to manipulate an object's state through non-member functions. This post explores a specific problem related to the this pointer and how to properly modify object data in a non-member function.

The Problem at Hand

Imagine you have two objects, num1 and num2, both initialized to the integer value of 5. You want to add these two values together using a non-member function and expect the result to be 10. However, upon execution, the output shows 5. This makes us question our understanding of the this pointer and class member functions.

Code Example

Here's a look at the problematic C+ + code that illustrates the issue:

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The class definition for Person includes methods to capture and retrieve values, but the addition operation is implemented in such a way that it does not reflect the expected outcome.

Understanding the this Pointer and Class Methods

Before we dive into the solution, let's clarify how the this pointer works in C+ + . The this pointer refers to the object for which a member function is invoked. It can be used within class methods to access member variables and functions.

Dissecting the Current Approach

The function to add Person objects is defined as follows:

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In this code:

We create a copy of Ldata named DataSum.

We then call the combine method on DataSum, which updates the values of the copy but not the original Ldata.

Why the Output is Incorrect

At this point, the output shows 5 because:

We operate on a copy of Ldata, so the changes made to DataSum do not affect num1.

The changes are made to a local instance, leaving the original objects unchanged.

The Solution: Passing by Reference

To address the problem, we should modify the original Ldata object instead of working with a temporary copy. To do so, we can change the signature of the add function to take a non-const reference. Here’s how to do it:

Revised Code for Addition

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What This Change Accomplishes

By passing Ldata as a non-const reference:

We ensure that we are modifying the original num1 object directly.

Any changes made will reflect immediately in the num1 object, giving you the expected output.

Updated Output Validation

When you now run the modified add function, your output will correctly reflect the sum of 10 after adding num1 and num2.

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

The confusion addressed in this post highlights an important aspect of using pointers and references in C+ + . To ensure modifications are applied correctly, it's crucial to understand how to properly reference objects in your functions. By using direct references instead of copies where necessary, you can achieve the expected results and effectively leverage C+ + ’s features.

Next time you find yourself in a similar situation, remember this approach to handle object modifications. Happy coding!