How to Determine if a Parameter is a constexpr in Compile-Time скачать в хорошем качестве

How to Determine if a Parameter is a constexpr in Compile-Time

A detailed guide on determining if a parameter is a `constexpr` in compile-time within your C+ + programs. Learn efficient methods to handle compile-time constants in your string classes. --- This video is based on the question https://stackoverflow.com/q/73523486/ asked by the user 'steve02081504' ( https://stackoverflow.com/u/19379355/ ) and on the answer https://stackoverflow.com/a/73524033/ provided by the user 'user17732522' ( https://stackoverflow.com/u/17732522/ ) 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 determine if a parameter is a constexpr in compile-time 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 Compile-Time Constants in C+ + When working with C+ + , particularly with templates and compile-time computations, the ability to determine whether a parameter is a constexpr becomes critical. For those building custom string classes or utility types that leverage compile-time capabilities, this can often lead to confusion. In this guide, we’ll explore how to efficiently determine if a given parameter is a constexpr, and demonstrate an approach to improve your string class design. The Problem Imagine you have a compile-time string class that allows you to compute results such as hashing and fast lookup tables right at compile time. Your string class may look something like this: [[See Video to Reveal this Text or Code Snippet]] This design supports creation from constexpr strings for performance advantages, as shown below: [[See Video to Reveal this Text or Code Snippet]] However, the challenge lies in wanting to allow the string class to seamlessly handle both constant strings and non-constant strings. Specifically, you want the constructor string("hello world!") to automatically treat the string as a constexpr when it is indeed a compile-time constant. Proposed Solution To address this requirement, we can delve into how C+ + allows us to discern between compile-time constants and regular run-time values. Function Overloads One method to handle this situation is through function overloading. You can define two versions of your constructor: One that takes constexpr_str_t Another that accepts const char* However, there is a critical limitation in C+ + : while it’s possible to overload based on types, you cannot overload based on consteval. Thus, making the decision inside the constructor itself is not feasible in the usual ways. Utilizing if consteval In modern C+ + , starting from C+ + 20, we have features like if consteval that will allow code to conditionally execute based on whether we are in a context that requires a constant expression. Here’s a refinement of how you might structure your string constructor: [[See Video to Reveal this Text or Code Snippet]] Important Considerations Handle Non-Constant Expressions Gracefully: If the const char* string argument is used in a context that isn’t a constant expression, you can simply proceed with a runtime evaluation. Simplification: You can simplify calculations for string properties when using if consteval: [[See Video to Reveal this Text or Code Snippet]] This way, if used in a context that requires a constant expression, the calculations take place at compile time. If not, they are executed at runtime. Limitations and Alternatives It’s crucial to accept the limitations of function calls in C+ + . You cannot write a check that distinguishes constant expressions at a function level. Thus, while you can enforce compile-time evaluations, accomplishing such checks outside function-level constraints may necessitate other approaches, including macros or templates. Conclusion Determining if a parameter is a constexpr in C+ + can significantly enhance the functionality and efficiency of your classes, especially for compile-time utilities like string processing. While the language imposes certain restrictions, the features introduced in C+ + 20 provide powerful tools to navigate around them effectively. By using compile-time evaluations intelligently, you can design a robust string class that seamlessly integrates compile-time constants with regular string literals. Embracing modern C+ + practices will lead to better designs and improved performance. Feel free to share your experiences or further questions rel