`typename` vs. `class` in C Templates: What's the Difference?

Understanding the Nuances of ""typename"" vs ""class" Template Parameters

In the realm of C templates, you may encounter two seemingly interchangeable declarations:

and

While they appear similar, there are subtle differences between these two keywords.

Basic Usage:

For specifying a simple template, both "typename" and "class" yield equivalent results:

is functionally identical to:

Dependent Types:

When referencing a nested type that relies on another template parameter, the "typename" keyword becomes crucial. Consider the following example:

Here, "typename" is used to properly resolve the nested type "baz" within the template parameter "param_t".

Template Templates:

A unique case arises when specifying a "template template." In such circumstances, the "class" keyword is mandatory:

Unlike "typename," "class" is the exclusive option for declaring template templates.

Explicit Instantiation:

The "class" keyword is essential when explicitly instantiating a template:

Conclusion:

While "typename" and "class" may seem synonymous for general template declarations, it's important to recognize their distinct applications in specific scenarios: dependent types, template templates, and explicit instantiation. By understanding these nuances, you can harness the full expressive power of C templates.

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