Скачать или смотреть Understanding Why std::multimap Elements Change After Loop Exit in C+ +

Learn why elements in `std::multimap` can change after exiting a loop and discover solutions to prevent dangling pointers in your C+ + code.
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This video is based on the question https://stackoverflow.com/q/76431849/ asked by the user 'Roman Leshchuk' ( https://stackoverflow.com/u/19271847/ ) and on the answer https://stackoverflow.com/a/76432028/ provided by the user 'Ted Lyngmo' ( https://stackoverflow.com/u/7582247/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Unraveling the Mystery of Changing std::multimap Elements

In programming with C+ + , particularly when dealing with the Standard Template Library (STL), developers often encounter unexpected behaviors. One common issue arises with the std::multimap, where the elements appear to change after exiting a loop. This can lead to confusion and bugs in your application. Today, we will explore a specific example related to this problem, understand what’s going wrong, and see how to fix it.

The Problem: Unexpected Output in std::multimap

Consider the following code snippet:

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Upon executing this program, the output is 1 1 0. The first two numbers are 1, indicating there are that many meshes in the first sprite, but the last number is 0. This discrepancy seems puzzling, especially after examining the intended logic of the code.

The Cause: Dangling Pointers

What is happening here? The key issue arises from how we are handling the pointer to the Sprite objects within the loop. Here's what goes wrong:

Out of Scope: The variable spritePair is defined as a local variable in the loop. After each iteration, it goes out of scope, and consequently, any pointer that references its internal data becomes a dangling pointer — a pointer pointing to memory that is no longer valid.

Accessing Invalid Data: When translucentSprites.begin()->second is being accessed outside of the loop, it's attempting to reference a Sprite object that isn’t guaranteed to exist anymore. This results in unexpected behavior and the output of 0 for the third number.

The Solution: Keeping References Valid

To resolve this issue, you have a couple of solid options that ensure your pointers remain valid while iterating through the std::unordered_map.

Option 1: Use References Correctly

Instead of using a regular pair, modify the loop to use a reference to the std::pair like this:

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This adjustment ensures that the spritePair refers to the actual pair in the map, preventing scope issues that lead to dangling pointers.

Option 2: Structured Bindings (C+ + 17 and later)

If you're using C+ + 17 or newer, a modern approach involves structured bindings, simplifying the syntax while maintaining proper references:

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Option 3: Emplace Instead of Insert

You can also use emplace instead of insert, which directly constructs the new element in place and may provide slight performance benefits:

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Conclusion

In summary, the confusing behavior of changing elements in a std::multimap loop arises from the misuse of pointers to local objects that go out of scope. By making small adjustments either to how you handle references or by using modern C+ + features, you can avoid dangling pointers and ensure your code behaves as expected.

Now that you understand the reasoning behind the output and its solutions, you can implement these changes to enhance your C+ + code's stability and correctness!