What are Template Template Parameters and How Do They Work in C ?

Demystifying Template Template Parameters

The enigmatic nature of template template parameters can seem daunting, but understanding their essence can greatly enhance your programming prowess. To dispel the misconceptions surrounding them, let's revisit a critical line of code:

<code class="cpp">template<template<class X> class Z = B>
class BB{};</code>

The Core Concept:
Template template parameters allow you to create templates that accept other templates as parameters. In the example above, the template class BB expects a template parameter Z that itself is a template with a single parameter X, and the default value is set to template class B.

Distinguishing Template Parameters from Templates:
To prevent ambiguity, C ensures that the template parameter Z is not mistaken for another template class Z. This distinction arises because the syntax for template template parameters closely resembles declaring a new template class.

Unveiling the Underlying Similarity:
A template template parameter, much like a regular template parameter, represents a placeholder for a specific type in the subsequent usage of the template class. The key difference is that a template template parameter represents a placeholder for a template rather than a concrete type.

Envisioning a Parallelism:
To simplify comprehension, consider the analogy of function pointers. In programming, you can define functions that accept parameters representing other functions. Similarly, template template parameters let you create templates that accept templates as parameters representing specific behaviors.

Expanding the Scope of Template Templates:
While template template templates (i.e., templates with nested template parameters) are not currently supported in C , it is not impossible to imagine their introduction in the future. Such a feature would greatly expand the expressive power of C template metaprogramming.

An Illustrative Example:
To better grasp the potential utility of template templates, consider a hypothetical graph search library. By utilizing a template template template, you could define a single search algorithm that accepts various implementations of stacks and queues, simplifying the development of complex data structures.

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