Inserting in std::map without a Default Constructor for Your Class Foo

Learn how to efficiently insert and fill data in `std::map` when your class lacks a default constructor. Explore alternatives like `emplace`, `insert_or_assign`, and `try_emplace`.
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This video is based on the question https://stackoverflow.com/q/76005097/ asked by the user 'Caduchon' ( https://stackoverflow.com/u/3378179/ ) and on the answer https://stackoverflow.com/a/76005837/ provided by the user 'Daniel Langr' ( https://stackoverflow.com/u/580083/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Insert into std::map Without a Default Constructor for Your Class

When working with std::map in C++, you may encounter a situation where the elements you want to store do not have a default constructor. This can be particularly tricky when you need to insert instances of a class, like Foo, which you want to fill with data but has lost its default constructor during refactoring. If you've found yourself in this position and are wondering about your options, this guide is here to help you navigate through the alternatives.

Understanding the Problem

In the original code example, the author effectively used a default constructor for Foo, which allowed them to easily insert new instances into the map and fill them afterward. Here's a simplified version of that logic:

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However, after changes to the Foo class, the lack of a default constructor means that we need a different approach. But fear not! There are several methods to insert and fill data without default construction. Let’s explore those options.

Alternative Methods to Insert and Fill Data in std::map

The modern C++ standards (C++11 and C++17) introduce several methods that can help you achieve your goal. Here's a detailed look at each method:

1. Using emplace

The emplace function allows you to construct the object in-place. You can directly provide the parameters required for Foo:

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Pros: This method is efficient because it constructs the Foo object directly within the map.

Cons: It may not check if a key already exists before insertion.

2. Using try_emplace (C++17)

For even better handling, especially when you want to avoid overwriting existing entries, you can use try_emplace:

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Pros: This method only inserts if the key does not already exist, preventing unintentional overwriting and providing better performance in scenarios where keys might already be present.

Cons: Available only from C++17 onwards.

3. Using insert (with std::pair)

Alternatively, you can also use the insert method combined with a std::pair:

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Pros: Offers a clear syntax for insertion.

Cons: Similar to emplace, it doesn't check for key existence before inserting.

4. Other Methods: insert_or_assign, and emplace_hint

Depending on your needs, you may want to explore insert_or_assign and emplace_hint as well:

insert_or_assign: This method is useful if you want to insert a new element or update an existing one.

emplace_hint: This can optimize the insertion location based on a hint.

Conclusion

In summary, when your class lacks a default constructor, you still have several effective options for inserting and filling your std::map. Depending on your requirements and the version of C++ you're using, methods like emplace, try_emplace, and others can be tailored to ensure efficient and effective handling of your data.

For your current setup, it's recommended to utilize emplace or, if available, try_emplace to maintain the integrity of your map while ensuring that you're not unnecessarily constructing objects. Choose the method that best fits your specific needs and enjoy the beauty of modern C++!

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