Variadic Template Parameter Pack

A Variadic Template Parameter Pack is a Template Metaprogramming pattern in C++ that lets a function accept any number of arguments of any types, resolved at Compile-Time.

Syntax

template <typename... Args>
void foo(Args... args) {
    bar(args...);  // pack expansion -- forwards all args to bar
}

• typename… Args declares a template parameter pack (zero or more types)
• Args… args declares a function parameter pack (one parameter per type in Args)
• args… is a pack expansion (the compiler substitutes each element)

How it works

Each call site generates a unique specialization:

foo(42, "hello", 3.14);
// Compiler generates:
// void foo(int args0, const char* args1, double args2) {
//     bar(args0, args1, args2);
// }

This is zero-cost: no runtime dispatch, no type erasure. The compiler produces exactly the code you’d write by hand for each combination of argument types.

Patterns

Fold

template <typename... Args>
auto sum(Args... args) {
    return (args + ...);  // right fold: a + (b + (c + d))
}

sizeof

template <typename... Args>
constexpr std::size_t count(Args...) {
    return sizeof...(Args);  // number of types in the pack
}