What are the Different Roles of the Ellipsis (...) in C 11 Variadic Templates?

Unveiling the Ellipsis (...) Syntax in Variadic Templates

In the realm of C 11, variadic templates have emerged as a powerful tool for writing generic code. However, their syntax can present some initial quirks. One such curiosity is the role of the ellipsis (...) token in the context of variadic templates.

Syntax Rules for the Ellipsis Token

Essentially, the ellipsis (...) serves as a separator between the template parameter pack and the parameter list. When placed on the right side of an expression, it triggers the unpacking of the template parameter pack into its individual elements. Conversely, when positioned on the left side, it marks a parameter pack.

For example, in the template definition:

<code class="cpp">template< class T, class... Args >
unique_ptr<T> make_unique( Args&&... args )
{
    return unique_ptr<T>(new T(std::forward<Args>(args)...));
}</code>

the ellipsis in the template parameter list ...Args unpacks the Args parameter pack. In the function implementation, the ellipsis in std::forward(args)... plays a crucial role in forwarding the variable arguments to the constructor T.

Ambiguity of the Ellipsis Placement

Curiously, the ellipsis can appear both at the end of the template argument list (as in ...Args) and in the middle of the parameter list (as in Args&&... args).

The reason for this asymmetry lies in the varying roles of the ellipsis in these different contexts. In the template argument list, it facilitates the unpacking of the parameter pack, while in the parameter list, it indicates that the function accepts a variable number of arguments.

Additional Usage of the Ellipsis

Beyond template parameter and parameter packs, the ellipsis can also find application in other contexts:

• Array Initialization: It can be used to initialize arrays, creating a list of elements separated by commas.
• Class Inheritance: The ellipsis can be used in class definitions to derive publicly from multiple base classes.

Understanding the nuances of the ellipsis token in variadic templates is essential for harnessing their full potential. By mastering these syntax rules, developers can craft robust and extensible code that leverages the power of C 11 templates.

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