`typename` vs. `class` in C Templates: When Do They Differ?

Template Parameters: Understanding the Distinction Between "typename" and "class"

In template programming, specifying template parameters enables the creation of generic code that can operate on different data types. Two commonly encountered parameter types are "typename" and "class." While similar in functionality, these keywords possess specific meanings and applications.

Interchangeability for Basic Template Specification

For straightforward template declarations, both "typename" and "class" are interchangeable. For instance, the following template classes are identical:

template <class T>
class Foo {};

template <typename T>
class Foo {};

Distinction for Dependent Types

"typename" is employed when referencing nested types that depend on other template parameters. This occurs in scenarios where a template class contains a type alias or typedef that depends on another parameter, as seen in this example:

template <typename param_t>
class Foo
{
    typedef typename param_t::baz sub_t;
};

"class" for Template Templates and Explicit Template Instantiation

When declaring a template template, "class" must be used. A template template allows for nesting of templates, creating a more complex parametrization structure. For instance:

template <template <typename, typename> class Container, typename Type>
class Example
{
    Container<Type, std::allocator<Type>> baz;
};

Additionally, "class" is mandatory for explicitly instantiating a template, which specifies a particular type for the template:

template class Foo<int>;

These distinctions highlight the subtle but important differences between "typename" and "class" template parameters. While generally interchangeable, the correct keyword usage is essential in specific contexts, such as dependent types, template templates, and explicit template instantiation, to ensure proper template behavior.

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