How does the ellipsis syntax work with variadic template parameter packs in C 11?

Variadic Template Ellipsis Syntax

Variadic templates in C 11 allow for a variable number of template arguments. The syntax for the ellipsis (...) token in this context follows strict rules:

Unpacking/Packing:
The position of the ellipsis determines its purpose:

• Right side (pattern): Unpacks a template parameter pack.
• Left side (name): Creates a packed argument.

Expansion:
The pattern on the left side of ... is repeated, with comma-separated expressions replacing each unpacked pattern.

Application:
In the provided code:

<code class="cpp">return unique_ptr<T>(new T(std::forward<Args>(args)...));</code>

• std::forward: Is a non-variadic template function taking a single template parameter and argument.
• Unpacking: std::forward(args)... expands to std::forward(arg0), std::forward(arg1), ..., where arg0, arg1, ... are the individual arguments.

Reason for Ellipsis Placement:
In the template argument list and parameter list, the ellipsis is placed in the middle to achieve different expansions.

• Template argument list: The ellipsis allows for a variable number of arguments.
• Parameter list: The ellipsis ensures that the arguments are passed as a single packed argument.

Additional Notes:

• The pattern can include access specifiers (e.g., public).
• The ellipsis can be used to initialize arrays, e.g., {args, sizeof(T)}....
• The syntax is the same regardless of whether the template is a class or a function.

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