Скачать или смотреть How to Create a Templated Function that Requires Arguments to be Passed by Rvalue Reference

Learn how to define a C++ function template that specifically require its argument to be passed by `rvalue reference`, using `static_assert` and the `requires` clause for type checking.
---
This video is based on the question https://stackoverflow.com/q/71174313/ asked by the user 'Ben' ( https://stackoverflow.com/u/874660/ ) and on the answer https://stackoverflow.com/a/71174348/ provided by the user 'bipll' ( https://stackoverflow.com/u/2749717/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to have a templated function require its argument be passed by rvalue reference?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Rvalue References in C++

When you're working with C++, you may encounter scenarios where you want to have a function accept parameters by rvalue reference. This can be particularly useful for optimizing performance by allowing resources to be moved rather than copied. However, the syntax around rvalue references and forwarding references can be quite confusing. In this post, we will explore how to create a templated function that requires its argument to be passed as an rvalue reference.

The Confusion with Forwarding References

Let's clarify what forwarding references are. In C++, when you define a template like this:

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

Here, T&& is a forwarding reference, which means it can bind to both lvalues and rvalues based on what is passed to it at the point of the function call. However, this can lead to unintended behavior if you specifically want to accept only rvalue references.

The Goal: Accepting Only Rvalue References

To ensure that your function only accepts arguments passed by rvalue reference, we can leverage template metaprogramming. The idea is to make use of the static_assert alongside a condition to enforce this rule. Let’s break down how to achieve that effectively.

Step-by-Step Solution

To implement this, you will want to check that the type T is indeed an rvalue reference. Here's how you can do it:

Declare the Templated Function: Start by declaring your function template with T&& as the parameter.

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

Use static_assert: You can use static_assert to enforce that the passed type is indeed an rvalue reference. This will raise a compile-time error if it’s not.

An Alternative Approach: Instead of using static_assert, you can directly restrict the template itself. Here’s a clean way using requires:

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

Explanation of the Final Code

In the final code:

template<class T>: This declares a template that can take any type T.

void foo(T&&): This says the function takes an rvalue reference to T.

requires (!std::is_lvalue_reference_v<T>): This is the key condition. It checks that T is not an lvalue reference, thereby enforcing that the argument must be passed as an rvalue reference.

Putting It All Together

Here is the complete implementation of the templated function requiring an rvalue reference:

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

This construct ensures that when you call foo, the argument must be an rvalue, effectively enhancing the performance of your code by preventing unnecessary copies.

Conclusion

Creating a templated function that requires its argument to be passed by rvalue reference may seem complex due to the nuances in C++’s type system. However, by using a requires clause in conjunction with type traits, we can enforce this restriction clearly and concisely. This approach not only increases clarity in your code but also optimizes performance by favoring move semantics over copy semantics.

Now you have a robust understanding of how to specify that a function template must accept arguments as rvalue references. It’s a powerful technique that can vastly improve the efficiency of your C++ programs.