Скачать или смотреть Understanding unordered_map with Custom Allocators in C+ + 17 and C+ + 20

Learn how to resolve issues related to `unordered_map` and custom memory allocators combining C+ + 17 and C+ + 20 features, specifically focusing on type matching for allocators.
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Understanding unordered_map with Custom Allocators in C+ + 17 and C+ + 20

When working with unordered_map in C+ + 17 and C+ + 20, you might encounter issues related to custom allocators. This guide aims to break down the problem and provide clear solutions, focusing on a commonly faced compiler error regarding type matching for allocators. Let’s explore this together and ensure you can successfully implement your custom allocator with unordered_map.

Identifying the Problem

In your code, you created a custom allocator derived from std::allocator, and defined an unordered_map using this allocator. However, you faced the following compiler error:

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

What This Error Means

This error indicates a mismatch between the expected value_type of the allocator and the type being stored in the unordered_map. More specifically, the allocator's value_type must match the type of the elements you're inserting into the container.

Analyzing the Code Structure

Let’s take a closer look at the critical components of your code:

Custom Allocator Declaration:

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

Using the Custom Allocator with unordered_map:

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

Highlighting the Cause of the Error

The key detail to note here is in the usage of std::pair. The allocator expects the pair’s first type to be const SomeObject* const, not just const SomeObject*. The original code refers to a pointer to a constant object rather than a constant pointer to an object that could be constant or mutable.

Solution Breakdown

To resolve the issue, you need to adjust the unordered_map definition so that the value_type as required by the allocator matches the expected type.

Modified Code Example

Here’s the corrected version of your class definition for unordered_map:

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

Key Changes:

Change in Type Declaration:

From: const SomeObject*

To: const SomeObject* const

By using a const qualifier on the pointer type, you ensure that the value_type used in the unordered_map and the allocator are consistent. This avoids the mismatch error and aligns with the requirements specified in the standard library.

Conclusion

Using custom allocators with unordered_map in C+ + 17 and C+ + 20 can seem challenging, especially when dealing with template types. However, adhering to the expectations set by the standard library helps in preventing common pitfalls like the type mismatch error.

Always remember, matching the value_type of your allocator to the types stored in your containers is crucial for smooth compilation and functionality.

If you find yourself facing similar issues or have further questions, feel free to share your experiences in the comments!