How does the ellipsis (...) token work in C 11 variadic templates?

Variadic Templates and the Ellipsis ("...") Token

In C 11, variadic templates allow for a flexible number of template parameters. When defining a variadic template, the ellipsis (...) token plays a crucial role.

Unpacking vs. Packing

The position of the ellipsis determines its function:

• Unpacking (Right Side): When used on the right-hand side of an expression, ... unpacks a template parameter pack. The unpacked patterns, separated by commas, replace the ellipsis in the actual function call.
• Packing (Left Side): When placed on the left-hand side, ... packs arguments into a template parameter pack. The packed arguments are available within the function as a single entity.

Example of Unpacking

Consider the following variadic template function:

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

When invoking make_unique with multiple arguments, the ellipsis (in std::forward(Args>(args)...) allows the function to accept a variable number of arguments. The ellipsis unpacks the Args parameter pack, resulting in the following expanded expression:

<code class="cpp">std::forward<arg0>(arg0), std::forward<arg1>(arg1), ...</arg1></arg0></code>

Ellipsis Placement in Template and Function Arguments

In the template argument list, the ellipsis is placed in the middle to indicate that the parameter pack is continued in the function parameter list. This is because there may be additional template parameters (e.g., non-variadic parameters) following the variadic parameter pack.

In the function implementation, the ellipsis is placed at the end of the expression to mark the end of the unpacked arguments. It ensures that any remaining arguments are passed without being packed into a single entity.

Additional Applications

The ellipsis can also be used in other contexts, such as:

• Creating arrays with variadic initializers
• Defining classes that inherit from multiple base classes (via a public Mixins... syntax)

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