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C++ High Performance

The Variadic Template Parameter Pack

Learn how to use variadic template parameter packs in C++, including constructing them and creating functions like make_string().

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The variadic template parameter pack enables programmers to create template functions that can accept any number of arguments.

An example of a function with a variadic number of arguments

If we were to create a function that makes a string out of any number of arguments without variadic template parameter packs, we would need to use C-style variadic arguments (just like printf() does) or create a separate function for every number of arguments:

C++
auto make_string(const auto& v0) {
  auto ss = std::ostringstream{};
  ss << v0;
  return ss.str();
}
auto make_string(const auto& v0, const auto& v1) {
   return make_string(v0) + " " + make_string(v1);
}
auto make_string(const auto& v0, const auto& v1, const auto& v2) {
  return make_string(v0, v1) + " " + make_string(v2);
}
// ... and so on for as many parameters we might need

This is the intended use of our function:

C++
auto str0 = make_string(42);
auto str1 = make_string(42, "hi");
auto str2 = make_string(42, "hi", true);
std::cout<<str0<<"\n"<<str1<<"\n"<<str2<<"\n";

If we require a large number of arguments, this becomes tedious, but with a parameter pack, we can implement this as a function that accepts an ...