Colin.cpp
Improved template aliases with the using keyword

In C++, template aliases are a powerful feature that allows us to create alternative names for template types. They provide a way to make the code more readable and maintainable by encapsulating complex type expressions into a single, easy-to-use alias. In C++17, the using keyword was introduced to improve the syntax for defining template aliases.

What are Template Aliases?

Template aliases allow us to create a new name for an existing template type. They are similar to typedefs but are more flexible and can be used with templates.

Here’s an example of a basic template alias:

template<typename T>
using Ptr = std::shared_ptr<T>;

In the above example, we define a template alias called Ptr, which is an alias for std::shared_ptr<T>. Now, we can use Ptr<T> instead of std::shared_ptr<T> throughout our codebase, providing better readability and maintainability.

The using Keyword with Template Aliases

Prior to C++17, template aliases were typically defined using the typedef keyword. However, with the introduction of the using keyword, the syntax for defining template aliases became more consistent with other type aliases.

Here’s the same example as before, but using the using keyword:

template<typename T>
using Ptr = std::shared_ptr<T>;

Using the using keyword for template aliases has a few benefits:

  1. Consistency: The using keyword is already used for type aliases, so using it for template aliases improves consistency in the language syntax.

  2. Familiarity: Developers who are already familiar with using the using keyword for type aliases will find it easier to understand and use template aliases.

  3. Readability: The using keyword makes the code more readable and self-explanatory, especially when used with complex template types.

Limitations of Template Aliases with the using Keyword

While the using keyword provides an improved syntax for defining template aliases, it has some limitations:

  1. Template Template Parameters: The using keyword cannot be used with template template parameters. For such cases, the typedef keyword is still required.

  2. Forward Declarations: When forward declaring template aliases with the using keyword, the using directive must be repeated before the forward declaration to maintain correct scoping.

Conclusion

The using keyword in C++17 provides an improved syntax for defining template aliases. It enhances consistency, familiarity, and readability in the language. While it has some limitations, the using keyword is a valuable addition to C++ for creating more expressive and maintainable code.

References

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