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C++ Template Metaprogramming

Understanding Parameter Packs Expansion

Explore the different contexts in which parameter packs expand in C++ variadic templates. Understand how expansions work in template parameter and argument lists, function calls, initializers, base classes, and lambda captures to enhance template flexibility and functionality.

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Parameter packs can appear in a multitude of contexts. The form of their expansion may depend on this context. These possible contexts are listed ahead along with examples.

Lists

  • Template parameter list: The template parameter list is used when we specify parameters for a template:

C++
template <typename... T>
struct outer
{
   template <T... args>
   struct inner {};
};
outer<int, double, char[5]> a;

  • Template argument list: The template argument list is used when we specify arguments for a template:

C++
template <typename... T>
struct tag {};
template <typename T, typename U, typename ... Args>
void tagger()
{
   tag<T, U, Args...> t1;
   tag<T, Args..., U> t2;
   tag<Args..., T, U> t3;
   tag<U, T, Args...> t4;
}

  • Function parameter list: The function parameter list is ...