C++ Metaprogramming Iterating over a parameter pack
Example
Often, we need to perform an operation over every element in a variadic template parameter pack. There are many ways to do this, and the solutions get easier to read and write with C++17. Suppose we simply want to print every element in a pack. The simplest solution is to recurse:
C++11
void print_all(std::ostream& os) {
// base case
}
template <class T, class... Ts>
void print_all(std::ostream& os, T const& first, Ts const&... rest) {
os << first;
print_all(os, rest...);
}
We could instead use the expander trick, to perform all the streaming in a single function. This has the advantage of not needing a second overload, but has the disadvantage of less than stellar readability:
C++11
template <class... Ts>
void print_all(std::ostream& os, Ts const&... args) {
using expander = int[];
(void)expander{0,
(void(os << args), 0)...
};
}
For an explanation of how this works, see T.C's excellent answer.
C++17
With C++17, we get two powerful new tools in our arsenal for solving this problem. The first is a fold-expression:
template <class... Ts>
void print_all(std::ostream& os, Ts const&... args) {
((os << args), ...);
}
And the second is if constexpr, which allows us to write our original recursive solution in a single function:
template <class T, class... Ts>
void print_all(std::ostream& os, T const& first, Ts const&... rest) {
os << first;
if constexpr (sizeof...(rest) > 0) {
// this line will only be instantiated if there are further
// arguments. if rest... is empty, there will be no call to
// print_all(os).
print_all(os, rest...);
}
}
