Скачать или смотреть Non-trivial designated initializers in C++: Understanding and Implementing Workarounds

This guide explores how to handle non-trivial designated initializers in C++ while working on OS utilities, providing effective solutions using modern C++ features.
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Tackling Non-Trivial Designated Initializers in C++

When developing an operating system, especially when working on utility sections like the temporary directory, you'll often find yourself needing to initialize data structures in certain ways. A common scenario arises when you are dealing with non-trivial designated initializers in C++, which are not directly supported. In this guide, we will dive into this issue and provide an effective workaround using modern C++ features.

The Problem

You may find yourself working on a segment of code similar to the following:

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Here, you'd want to initialize the tmp_pgdir array with specific values at designated indices. However, standard C++ does not support the designated initializer syntax ([...]=) as you would see in C. This limitation raises the question: What is the best workaround for this issue in C++?

The Solution

Overview of the Approach

To handle designated initializers in C++, we can utilize modern features such as std::array, lambda expressions, and constexpr. This will allow us to initialize the array in a way that is both readable and compliant with C++ standards.

Step-by-Step Implementation

Define Aligned Array:
To maintain the alignment as in your original C code, you can use alignas:

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Using std::array:
The std::array type in C++ offers advantages such as proper function passing behavior and compile-time bounds checking, making it a suitable choice for our case.

Compile-Time Initialization:
Use constinit (available in C20) to ensure that the array is initialized at compile-time. This assertion helps catch any errors during the compilation phase. If you’re not using C20, you may rely on a constexpr function:

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Here's the example using a named constexpr function:

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Accessing Array Elements:
To obtain pointers to the beginning or the end of the std::array, you can utilize:

tmp_pgdir.data() for the starting point,

std::to_address(std::end(tmp_pgdir)) for one-past-the-end.

Avoid using direct pointers like &tmp_pgdir[0] as they may cause compatibility issues.

Summary

Non-trivial designated initializers are not supported in C++, but with modern features like std::array and constexpr, we can effectively mimic their behavior. This approach not only keeps your code compliant with C++ standards but also encourages better programming practices by avoiding preprocessor directives like #define.

By employing the above strategies, you can ensure that your code remains clean, maintainable, and aligned with industry standards for C++ development.