Скачать или смотреть Enabling C++ Constructor with std::is_floating_point T Using Template Meta-Programming

Learn how to conditionally enable C++ constructors with templates, specifically using `std::is_floating_point T ` in C++17 for efficient type handling.
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Enabling C++ Constructor using std::is_floating_point<T>

When working with template classes in C++, you might encounter a scenario where you want to enable or disable certain constructors based on the type of the template parameter. A common requirement is to enable a constructor only if the type is a floating point type (like float or double). This is exactly the kind of problem we’ll tackle today.

The Problem

Imagine you have a template class and you need a default constructor that should only work for types that are floating point. If the type is not floating point, the constructor should not be available, preventing any possible errors at compile time. This can be challenging, especially in a language like C++ where type checking and constraints are vital for code safety.

Here’s a simplified version of the template class you might have:

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Complications Encountered

The usual approach involves using SFINAE (Substitution Failure Is Not An Error), where you attempt to use std::enable_if_t to conditionally enable the constructor. However, applying this can lead to errors, especially when the constructor needs to be defined based on the type of T. In your attempts, both valid and invalid usages of your constructor might throw errors due to improper conditions being applied.

The Solution

To enable the default constructor conditionally based on whether T is a floating point type, we can leverage some features introduced in C++17. Here's how you can do it effectively:

1. Using requires Clause (C++20 and beyond)

While C++17 does not support requires, it’s worth mentioning for readers that beginning from C++20, you can use the requires clause in your constructor:

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2. Using SFINAE in C++17

Since you're working with C++17, here’s how you can implement this in a more traditional way utilizing SFINAE:

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

Explanation of the Solution

Template Parameter: Notice that we introduced another type parameter U. This is to ensure that the SFINAE condition (std::enable_if_t) can check if U is a floating point type. Here, U defaults to T.

Enable If: By using std::enable_if_t, we conditionally enable the constructor only if U is indeed a floating point type. If T is, say, int or char, the constructor effectively becomes unavailable, preventing compilation errors when you try to instantiate Thing<int>.

No Impact on Other Constructors: The SFINAE condition is evaluated only when the constructor is called. This means that if you instantiate Thing<int>, only the parameterized constructor remains available.

Conclusion

By following these patterns, you can effectively create constructor functions in your C++ templates that are conditionally available based on the type of the template parameter. This not only helps in making your code safer by preventing invalid type instantiations but also enhances code readability and maintainability.

Remember, type traits and SFINAE are powerful tools when working with templates in C++. Understanding how they operate can significantly improve how you write generic and type-safe classes in C++.

Feel free to leave feedback or share your experience with using templates in C++!