How do you use the \'...\' token to pack and unpack arguments in C variadic templates?

Syntax Rules for the "..." Token in Variadic Templates

In C , variadic templates enable the construction of templates that accept a variable number of arguments. The "..." token plays a crucial role in this context, serving as either an argument pack or a parameter unpacker.

Syntax Rules for Ellipsis Placement

The placement of the "..." token determines its function:

• Pack: When it appears on the left side of a name, "..." indicates an argument pack: ...thing // pack
• Unpack: When it appears on the right side of an expression, "..." unpacks a template parameter pack: thing... // unpack

Example: Variadic Template with "..."

Consider the following variadic template:

<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>

In this example, "..." serves as an argument pack, while the "..." in the function implementation unpacks the arguments into the args variable.

Reason for Different Ellipsis Placement

The difference in ellipsis placement between the template argument list and the parameter list is due to the distinction between argument packing and parameter unpacking. In the template argument list, "..." signifies that the parameters should be packed into a single parameter pack, while in the parameter list, "..." indicates that the arguments should be unpacked into individual parameters.

Unpacking Patterns

When "..." appears on the right side of an expression as an unpacker, it follows a specific pattern:

• The expression to the left of "..." is repeated, separated by commas
• Each repetition represents an expression that consumes a single element from the template parameter pack

Advanced Usage: Initializing Arrays

Ellipsis can also be used to initialize arrays:

<code class="cpp">struct data_info
{
     boost::any  data;
     std::size_t type_size;
};

std::vector<data_info> v{{args, sizeof(T)}...}; //pattern = {args, sizeof(T)}</code>

This initializes the vector v with values where each element is a struct containing an args and a sizeof(T) pair.

In conclusion, the "..." token in the context of variadic templates serves as both an argument pack and a parameter unpacker, following specific syntax rules for placement and unpacking patterns. Its flexible usage allows for powerful template constructs and customization.

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