Variadic Templates

Variadic Templates #

A variadic template is a template that can have an arbitrary number of parameters.

template <typename ... Args>
void variadicTemplate(Args ... args){ . . . . }

Parameter Pack #

A template parameter pack is a template parameter that accepts zero or more template arguments (non-types, types, or templates). A function parameter pack is a function parameter that accepts zero or more function arguments.

• By using the ellipsis (…), Args- or args becomes a parameter pack.
• Args is a template parameter pack; args is a function parameter pack.
• Parameter packs can only be packed and unpacked.
• If the ellipsis is at the left of Args, the parameter pack will be packed and if the ellipse is at the right of Args, the parameter pack will be unpacked.

The compiler can automatically deduce the template arguments in the case of a function template.

For example, the following classes/functions in the STL extensively use variadic templates. Variadic Templates are often used in the Standard Template Library:

• sizeof-Operator, std::tuple, std::thread, std::make_unique, std::lock

The usage of parameter packs obeys a typical pattern for class templates.

• Perform an operation on the first element of the parameter pack and recursively invoke the operation on the remaining elements.
• The recursion ends after a finite number of steps.
• The boundary condition is typically a fully specialized template.