Where and why do I have to put the "template" and "typename" keywords?

📝 Blog Post: Understanding the "template" and "typename" Keywords in Templates

Have you ever found yourself scratching your head when encountering the keywords "template" and "typename" in C++ templates? 🤔 Don't worry, you're not alone! In this blog post, we'll explore where and why you need to use these keywords, specifically when dealing with dependent names. We'll dive into the code snippet provided and provide easy solutions to the problem at hand. So, let's get started!

What are Dependent Names?

Before we dive into the code, let's quickly understand what dependent names are. In C++, dependent names are names that appear in templated code and depend on template arguments. These names might refer to types, functions, or variables that are not determined until a specific instantiation of the template.

Now, let's take a closer look at the code snippet you shared to identify the specific problem and work towards finding a solution.

template <typename T, typename Tail>
struct UnionNode : public Tail {
    // ...
    template<typename U> struct inUnion {
        // Q: where to add typename/template here?
        typedef Tail::inUnion<U> dummy; 
    };
    template< > struct inUnion<T> {
    };
};

template <typename T> // For the last node Tn.
struct UnionNode<T, void> {
    // ...
    template<typename U> struct inUnion {
        char fail[ -2 + (sizeof(U)%2) ]; // Cannot be instantiated for any U
    };
    template< > struct inUnion<T> {
    };
};

The Problem: "inUnion" as a Dependent Name

The problem lies in the code line: typedef Tail::inUnion<U> dummy;. The compiler is choking on the dependent name inUnion, and we need to help the compiler understand that it is a template-id.

Solution: Using "typename" and "template" Keywords

To clarify that inUnion is a template-id, we need to make use of the "typename" and "template" keywords in the appropriate places. Let's address each use case separately:

1. Inside the UnionNode struct:

Inside the UnionNode struct, you need to add the "typename" keyword before using the dependent name inUnion. Update the line as follows:

typedef typename Tail::template inUnion<U> dummy;

By using the "typename" keyword, we clarify that inUnion is a type within the dependent type Tail. The "template" keyword helps the compiler understand that inUnion is a template-id.

2. Inside the UnionNode<T, void> specialization:

For the specialization UnionNode<T, void>, you need to add the "template" keyword to indicate that inUnion is a template-id. Update the line as follows:

typedef typename template UnionNode<T, void>::template inUnion<U> dummy;

By explicitly using the "template" keyword, we inform the compiler that UnionNode<T, void>::inUnion is a template-id.

Conclusion

By strategically using the "typename" and "template" keywords, we can resolve the issue with dependent names and ensure proper compilation of our code. Remember, the "typename" keyword is used to indicate dependent types, while the "template" keyword is used to indicate dependent template-ids.

Now, armed with this knowledge, you can confidently handle similar issues that arise when working with templates in C++. Don't let template errors get the best of you! 😉

Do you have any other questions or encountered similar template problems? Share your thoughts and experiences in the comments below! Let's learn and tackle these challenges together! ✨