Скачать или смотреть Does C+ + 11 Allow Branching Based on Compile-Time Constants? Here's How!

Discover how to determine if a function parameter in C+ + 11 is a compile-time constant, and learn to branch function behavior accordingly.
---
This video is based on the question https://stackoverflow.com/q/76672976/ asked by the user 'lighthouse' ( https://stackoverflow.com/u/16687283/ ) and on the answer https://stackoverflow.com/a/76673819/ provided by the user 'ecatmur' ( https://stackoverflow.com/u/567292/ ) 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: In C+ + 11 Is there any way to branch by if a function parameter is compile-time constant or not?

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.
---
Does C+ + 11 Allow Branching Based on Compile-Time Constants? Here's How!

When programming in C+ + , particularly with C+ + 11, developers sometimes face the challenge of determining if a function parameter is a compile-time constant. This knowledge can significantly influence how we design our functions, allowing us to optimize performance and resource management based on the values passed to them. In this guide, we’ll explore how you can branch a function's body based on whether a function parameter is a compile-time constant or not.

The Problem Statement

Suppose we have a function designed to calculate the size of a data structure based on its type. For instance, if we pass an std::array, we want to evaluate its size at compile time, while for other data structures like std::vector, we may need to calculate the size at runtime. The ultimate challenge is to create a function that can handle both these scenarios specifically designed for C+ + 11 and possibly later standards.

Here's a sketch of what we want to achieve:

Example Code

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

The complex part comes when deciding whether length can be treated as a compile-time constant, allowing us to instantiate arrays at compile time.

The Proposed Solution

To determine if a parameter can be evaluated at compile time, we can leverage C+ + templates and integral constants. Here’s how we can accomplish this effectively:

Step 1: Modify get_length Function

The first step involves updating the get_length function to return an integral_constant rather than a simple size. This will allow the function to reflect compile-time information.

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

Step 2: Implement the foo Function

Next, we define the foo function that will utilize get_length. Here we can take advantage of if constexpr available in C+ + 17, but we’ll ensure that our solution is clear enough to be backported to C+ + 11:

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

Explanation of the Code

Using integral_constant: The get_length function returns an std::integral_constant, which allows us to use the size of arrays known at compile time.

Branching with if constexpr: The foo function checks if length can be instantiated as an std::array. If length is a compile-time constant, it creates an std::array, otherwise it falls back to std::vector which requires runtime evaluation.

Conclusion

By using C+ + 's template and type traits capabilities, we can effectively differentiate between compile-time constants and runtime values, optimizing our code accordingly. Although this example utilizes features from C+ + 14 and beyond, the core ideas can be adapted for C+ + 11, allowing developers to harness the power of compile-time evaluations.

This rigorous approach to function branching ensures not only code clarity but also runtime efficiency. If you ever find yourself grappling with similar questions in your C+ + endeavors, refer back to this strategy for guidance!