When to Use `` vs. `` in C Templates?

Understanding the Distinction between "" and "" Template Parameters

In C , templates allow programmers to create generic code that operates on various types. When defining templates, one can encounter two keywords: "" and "," both used to specify template parameters.

Interchangeability in Basic Contexts

In most cases, "" and "" can be used interchangeably. For example:

template <typename T>
class Foo {...}

is equivalent to:

template <class T>
class Foo {...}

Both declarations define a generic class "Foo" that operates on any type "T."

Specific Cases Requiring Distinction

However, there are certain instances where "" and "" do differ:

• Dependent Types: When referring to nested types that depend on other template parameters, the "typename" keyword must be used, as in:

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

• Template Templates: When defining a template that accepts other templates as parameters, the "class" keyword must be used, as in:

template <template <typename, typename> class Container, typename Type>
class Example {...}

where "Container" is a template that takes two type parameters.

• Explicit Template Instantiation: When explicitly instantiating a template, such as:

template class Foo<int>;

the "class" keyword is always used instead of "typename."

In summary, while "" and "" are often interchangeable, it is important to understand their specific usage rules when working with dependent types, template templates, and explicit template instantiation.

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