Parameter Packs 1

template<typename... Ts>
constexpr auto get_type_sizes(){
	return std::array<std::size_t, 	sizeof...(Ts)>{sizeof(Ts)...};
}

auto sizes = get_type_sizes<short, int, long, long long>();
  • What’s the sizeof going on here?

    • sizeof…(Ts) evaluates how many arguments in the parameter packs, 4 in this case.

    • sizeof(Ts)...expands into a comma-seperated sequence as sizeof(short), sizeof(int), sizeof(long), sizeof(long long), a number sequence.

template <typename... Ts, typename... Us>
constexpr auto multipacks(Ts... args1, Us... args2)
{
	std::cout << sizeof...(args1) << ',' << sizeof...(args2) << '\n';
}

multipacks<int>(1, 2, 3, 4, 5, 6);// 1,5
multipacks<int, int, int>(1, 2, 3, 4, 5, 6);// 3,3
multipacks<int, int, int, int>(1, 2, 3, 4, 5, 6);// 4,2
multipacks<int, int, int, int, int, int>(1, 2, 3, 4, 5, 6); // 6,0
  • A parameter pack can be followed by other parameters including more parameter packs. First group of function parameters match template parameters one by one as Ts... args1, the remainings are Us... args2.
#include<iostream>
#include<functional>
template<typename, typename>
struct func_pair;

template<typename R1, typename... A1, typename R2, typename... A2>
struct func_pair<R1(A1...), R2(A2...)>{
	std::function<R1(A1...)> f;
	std::function<R2(A2...)> g;
};

bool twice_as(int a, int b){
	return a >= b*2;
}
double sum_and_div(int a, int b, double c){
	return (a + b) / c;
}
int main(){
	func_pair<bool(int, int), double(int, int, double)> funcs{twice_as, sum_and_div };
	funcs.f(42, 12);
	funcs.g(42, 12, 10.0);
	return 0;
}
  • a class template that represents a pair of function pointers.

Reference

Template Metaprogramming with C++: Learn everything about C++ templates and unlock the power of template metaprogramming - by Marius Bancila (Author)