Скачать или смотреть Transforming constexpr Arrays in C+ + : A Guide to Compile Time Array Initialization

Learn how to effectively define a new `constexpr` array by transforming an existing one using C+ + 20 features and lambda functions.
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Transforming constexpr Arrays in C+ + : A Guide to Compile Time Array Initialization

When programming in C+ + , especially using the modern features introduced in C+ + 20, you might find yourself wanting to transform an array at compile time. One common problem developers encounter is how to define a new constexpr array based on transformations from an existing constexpr array. In this post, we'll explore how to effectively achieve this goal, dissecting the solution step by step.

The Problem Explained

Let’s assume you have a constexpr array of string views, like this:

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You want to create a new constexpr array that reflects the sizes of the strings contained in arr. However, when using std::transform in conjunction with arr, you may encounter compilation errors that can be frustrating, especially since C+ + 20 introduced a constexpr version of std::transform.

The error message you receive might look something like this:

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This happens because std::transform tries to modify elements in an array that is marked as constexpr, meaning the data is read-only at compile time. So how can we work around this limitation?

The Solution: Using a Lambda Function

The key to resolving this issue is to write a function or lambda that constructs the required array. Instead of trying to transform the original array directly, we can create a temporary array within a constexpr lambda. Here’s how you can do it:

Step-by-Step Implementation

Define the Original Array: Start with your existing constexpr array as planned.

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Create a Lambda to Build the New Array: Create a lambda function that will perform the transformation.

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Initialize the New Array: Call the lambda to initialize your new constexpr array.

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Verify Using static_assert: You can now use arr2 as needed, and check values using static assertions.

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Summary of the Code Snippet

Here’s the complete approach brought together into a single snippet:

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Conclusion

By utilizing a constexpr lambda function, you can successfully create a new constexpr array through transformations of an existing one. This approach avoids the pitfalls of direct assignment to read-only locations and makes your code not only more robust but also cleaner and more maintainable. If you're diving into advanced C+ + programming, mastering techniques like this can greatly expand your toolkit! Happy coding!