We can validate at compile time that an input to a function is a specialization of a template. I.E the following code validates that the input for f
is some specialization of struct Holder.
template<typename T>
struct Holder<T> {...};
template<typename T>
void f(Holder<T> h) {...};
I want to validate that a set of variadic arguments are a specialization of a template. More precisely I want to differentiate between two consecutive sets of variadic arguments - a set which is a specialization of a template, and a set which isn't. Following is an example of how it might have looked like if the syntax allowed it to -
template<...Args1, ...Args2>
void f(Holder<Args1>.... args_which_are_specializations_of_Holder, Args2... args_which_are_not) {
use_holders(args_which_are_specializations_of_Holder...);
use_rest(args_which_are_not...);
return;
}
Is this possible ?
Thanks,
CodePudding user response:
It's harder to mix variadic arguments like that. You can use std::tuple
:
#include <tuple>
#include <cstdio>
template<typename T>
struct Holder {
};
template <class ...T1, class ...T2>
void f(const std::tuple<Holder<T1>...>& holders, const std::tuple<T2...>& non_holders) {
std::printf("Holder count: %zu\n"
"Non Holder count: %zu", sizeof...(T1), sizeof...(T1));
}
int main() {
Holder<int> a;
Holder<double> b;
int c;
double d;
f(std::tuple{a, b}, std::tuple{c, d});
}
CodePudding user response:
You can store the args
in a tuple
and calculate the index of the last Holder
argument, then extract the Holder
and normal arguments by index and forward them to the corresponding function.
#include <tuple>
template<class T>
constexpr bool is_holder = false;
template<class T>
constexpr bool is_holder<Holder<T>> = true;
template<class... Args>
void f(Args... args) {
constexpr auto holder_index = (is_holder<Args> ... 0);
auto args_tuple = std::tuple(args...);
[&args_tuple]<auto... Is>(std::index_sequence<Is...>) {
use_holders(std::get<Is>(args_tuple)...);
}(std::make_index_sequence<holder_index>{});
[&args_tuple]<auto... Is>(std::index_sequence<Is...>) {
use_rest(std::get<Is holder_index>(args_tuple)...);
}(std::make_index_sequence<sizeof...(Args) - holder_index>{});
}