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I am using a fold expression to print elements in a variadic pack, but how do I get a space in between each element?
Currently the output is "1 234", the desired output is "1 2 3 4"
template<typename T, typename Comp = std::less<T> >
struct Facility
{
template<T ... list>
struct List
{
static void print()
{
}
};
template<T head,T ... list>
struct List<head,list...>
{
static void print()
{
std::cout<<"\""<<head<<" ";
(std::cout<<...<<list);
}
};
};
template<int ... intlist>
using IntList = typename Facility<int>::List<intlist...>;
int main()
{
using List1 = IntList<1,2,3,4>;
List1::print();
}
771
you can that
#include <iostream>
template<typename T>
struct Facility
{
template<T head,T ... list>
struct List
{
static void print()
{
std::cout<<"\"" << head;
((std::cout << " " << list), ...);
std::cout<<"\"";
}
};
};
template<int ... intlist>
using IntList = typename Facility<int>::List<intlist...>;
int main()
{
using List1 = IntList<1,2,3,4>;
List1::print();
}
the fold expression ((std::cout << " " << list), ...) will expands to ((std::cout << " " << list1), (std::cout << " " << list2), (std::cout << " " << list3)...)
66
If you need space only between numbers (and not after the last or before the first too), you might do:
template <std::size_t... Is>
void print_seq(std::index_sequence<Is...>)
{
const char* sep = "";
(((std::cout << sep << Is), sep = " "), ...);
}
Demo
(It is similar to my "runtime version") for regular containers with for-loop.
40
In general, you use recursion for tasks like this.
You have to define what happens when there are 2 or more and 1 elements in the list and recursively fall back to those definitions:
template <int ...> struct List;
template <int First, int Second, int ... More> struct List {
static void print() {
std::cout << First << " ";
List<Second, More ...>::print();
}
};
template <int Last> struct List {
static void print() {
std::cout << Last;
}
};
You can reuse print() to achieve this behaviour. Afterall you are doing a fold operation which is by definition resursive.
Live Demo
template<T head,T ... rest_of_pack>
struct List<head , rest_of_pack...>
{
static void print_()
{
std::cout<<head<<" ";
List<rest_of_pack...>::print();
}
};
If you want to process many elements this way you might run into problems with template depth (gcc for instance has a limit of 900). Lucky for you you can use the -ftemplate-depth= option to tweak this behaviour.
You can compile with -ftemplate-depth=100000 and make it work. Note that compilation time will skyrocket (most likely) or in thhe worst case you run out of memory.
20
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