In modern C++ programming, it is common to use templates to write generic and reusable code. Templates allow you to write functions or classes that can work with different data types without duplicating the code. However, when using nested template declarations, you may encounter an issue with the use of right angle brackets (>). In this blog post, we will discuss this issue and provide solutions to overcome it.

The Problem

In C++, the right angle brackets (>) are used to close template declarations. However, when you have nested template declarations, the compiler may misinterpret the closing angle brackets and result in a compilation error.

template<typename T>
class Container {
    // ...
};

template<typename Key, typename Value>
class Map {
    Container<Container<Key>> data; // Compilation error!
};

In the above code snippet, we have a Map class that has a nested Container class as its member. However, when we try to declare Container<Container<Key>>, the compiler gets confused with the nested angle brackets and produces a compilation error.

Solution 1: Space between Angle Brackets

One way to resolve the issue is to add a space between the nested angle brackets. This will help the compiler differentiate between the closing brackets.

template<typename T>
class Container {
    // ...
};

template<typename Key, typename Value>
class Map {
    Container< Container<Key> > data; // Compiles successfully
};

By adding a space between Container<Key> and the closing bracket, the compiler can correctly parse the nested templates.

Solution 2: Using the typename Keyword

Another solution is to use the typename keyword before the nested type declaration. This informs the compiler that Container<Key> is a type, and not a dependent name.

template<typename T>
class Container {
    // ...
};

template<typename Key, typename Value>
class Map {
    typename Container<Container<Key>> data; // Compiles successfully
};

By using the typename keyword, we help the compiler understand that Container<Container<Key>> is a type and not a member variable.

Conclusion

When using nested template declarations in C++, it is important to be aware of the issue with the right angle brackets (>). By adding a space between the brackets or using the typename keyword, we can overcome this problem and successfully compile the code.

Remember, understanding these nuances will help you write clean and error-free code when working with templates. Happy coding!

References:

• C++ Templates - cppreference.com
• Nested templates with the template<> construct
• C++ Gotchas Part 2: Right Angle Brackets
• How stackoverflow’s Markdown is Rendered

#cpp #templates