Скачать или смотреть How to Properly Get the Length of a Variable Size Array of Structs in C+ + using pointers

Learn how to resolve the common issue of finding the length of a dynamically allocated array of structs in C+ + . This blog provides an effective solution and important context.
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Understanding the Challenge: Getting the Length of a Variable Size Array of Structs

If you've ever worked with arrays in C+ + , particularly arrays of structs, you may have faced the challenge of determining the length of a dynamically created array. This is especially true when you're working with data that comes from an external source, like a file. For instance, consider you have a struct for storing personal details, and you need to read from a file to populate an array of such structs. Here's a simplified breakdown of the challenge you're likely facing:

Struct Definition: You have a struct named person, which holds the fields for a person's name, city, and age.

Dynamic Array Creation: A function readFromFile creates a variable size array of these structs.

Length Retrieval Issues: You need to figure out how to retrieve the length of this dynamically allocated array, but attempts to use the sizeof operator yield incorrect results.

The Core Issue: Why sizeof Doesn't Work with Pointers

When dealing with dynamic memory allocation in C+ + , especially when using pointers, the sizeof operator yields unexpected results. This is because:

The sizeof operator returns the size of the pointer itself (e.g., 4 or 8 bytes depending on your architecture), not the size of the actual memory block it points to.

Since your array is dynamically allocated, the C+ + compiler has no way of determining how many elements are in that array after it has been allocated.

Solution: Changing Your Struct and Reading Logic

To effectively get the length of your array of structs, and ensure that your program runs smoothly, consider the following steps.

Use Fixed-Size Character Arrays for Strings

One common problem arises from using String objects in your struct definition. Instead, opt for fixed-size character arrays. This change allows for easier manipulation and management of your data without the overhead of dynamic string management. Here’s how you can redefine your struct:

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

Modify the Function for Reading from File

When implementing the readFromFile function, make sure it keeps track of the number of person structs it populates. An updated version of this function might look something like this:

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

Correctly Assign and Retrieve the Number of Elements

When you invoke readFromFile, make sure to capture the number of elements correctly by passing an integer reference:

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

Summary of Key Points

Struct Redefinition: Use fixed-size char arrays for string representation.

Function Adjustment: Pass an integer pointer to track the number of elements populated in the array.

Safe Looping: Ensure your loops use the actual number of elements read from the file.

By following these modifications, you should be able to effectively determine the size of your dynamically allocated array of structs and avoid common pitfalls associated with pointer management in C+ + .

Now you can focus on the logic of your program, confident that you'll retrieve and utilize your array of structs without running into errors related to length determination!