Why Can\'t C Handle Template Template Template Parameters?

Understanding Template Template Parameters

In the realm of C , template template parameters present a challenge for some programmers. Let's dispel a common misconception:

Misconception:

In the code snippet below, what prevents C from considering Z as another templated class rather than a templated parameter?

<code class="cpp">template<class T>
class B {}; // Templated class

template<template<class X> class Z = B> // Line in question
class BB{};</code>

Explanation:

Template template parameters behave similarly to normal template type parameters. However, they do not accept concrete types; instead, they accept templates themselves. Here's a simplified understanding:

• Template Type Parameters: Match concrete types (e.g., int, double, Foo, etc.)
• Template Template Parameters: Match templates (i.e., classes or structs with template parameters)

In the example, the syntax template class Z = B> defines a templated type parameter Z that expects a template class with one template parameter X.

Template Template Template Parameters (Not Possible)

It's important to note that template template template parameters are not possible in C . This is a design decision made by the standardization committee to simplify compiler implementation.

Analogy: Functions and Function Pointers

For a better understanding, draw parallels with functions and function pointers:

• Functions: Accept values as arguments
• Function Pointers: Accept functions (arguments) that accept values

Similarly, template template parameters accept templates (arguments) that accept arguments.

Example and Use Case

Although template template template parameters are not possible, let's imagine a situation where they might be useful:

Suppose we want to create a generic graph searching library with algorithms that operate on different types of stacks or queues, which in turn can be implemented with different container types. A template template template parameter could enable us to specify all these aspects in a single function call. However, since template template template parameters are not available, this use case remains a hypothetical example.

The above is the detailed content of Why Can\'t C Handle Template Template Template Parameters?. For more information, please follow other related articles on the PHP Chinese website!