Print spaces between each element using a fold expression

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猫巷女王i

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 outpu

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4条回答

被撕碎了的回忆

2021-01-21 05:41

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;
    }
};

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伪装坚强ぢ

2021-01-21 05:44

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)...)

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醉酒成梦

2021-01-21 05:45
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
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走了就别回头了

2021-01-21 05:55
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.
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