Understanding the size of a pointer to an empty class in C++

Explore the implications of using the [[no_unique_address]] attribute in C++20 and its impact on pointers to empty classes. Learn about size guarantees and practical examples in modern C++!
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Understanding the size of a pointer to an empty class in C++

In C++, the concept of pointers combines with intricate behaviors of classes, especially when dealing with empty classes and unique attributes introduced in C++20. This guide addresses a specific question many developers encounter when dealing with pointers to empty classes.

The Central Question

What is the size of a pointer to an empty class empty_t?

Does the C++20 attribute [[no_unique_address]] have any impact on that pointer size?

Understanding these questions is essential, especially for developers using modern C++ who are implementing custom data structures and need to optimize memory usage.

Exploring the Problem

Size of Pointers

First, let’s clarify one fundamental aspect. The size of a pointer, regardless of what it points to (including empty classes), remains consistent across modern platforms. Generally, all pointers will have the same size, which is typically dependent on the architecture (like 32-bit or 64-bit).

The Role of [[no_unique_address]]

The C++20 attribute [[no_unique_address]] was designed to optimize memory usage by allowing certain empty classes not to contribute a size increase to containing types. However, whether this directly affects the size of pointers to these classes is less straightforward.

Answering the Questions

1. What is the size of a pointer to an empty class empty_t?

The answer is straightforward: the size of the pointer itself. On modern platforms, all pointers have a standard size regardless of what they point to, which means a pointer to empty_t will have the same size as any other pointer.

2. Does [[no_unique_address]] impact pointer size?

The usage of [[no_unique_address]] does not have any specified impact on the size of pointers. Thus, a pointer declared with this attribute will still have the regular pointer size.

3. Conditional Size Changes

If [[no_unique_address]] param_t* conditionally becomes [[no_unique_address]] empty_t*, it is not guaranteed that it will have a size of zero. You must check that it meets your requirements for size optimizations.

4. Using Conditional Types for Size Optimization

You will indeed need to use std::conditional_t<use_parameters, some_parameters_t*, empty_t> alongside [[no_unique_address]] if you want to ensure that the pointer does not contribute to the size of the containing class when no parameters are used.

Practical Example

To check if the size changes with member addition or removal, you can use a simple line of code:

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

By adding or removing the member, you can observe the changes in sizes effectively.

Conclusion

In conclusion, when working with pointers to empty classes and utilizing the C++20 feature [[no_unique_address]], it’s important to recognize that:

The size of the pointer remains standard.

The attribute does not guarantee a zero size for pointers.

You must employ a conditional mechanism to achieve that objective.

By understanding these intricacies, developers can make more informed decisions when optimizing memory in their C++ applications. Don't forget to test and validate your assumptions regarding sizes using compiler features and runtime checks!