expand and fold parameter pack in specific way

Question

#include <string>
#include <string_view>

template<typename... Args>
std::string_view concatenateBuffer(std::string &buffer, Args &&... args){
    static_assert((std::is_constructible_v<std::string_view, Args> && ...));

    buffer.clear();

    (buffer.append(std::forward<Args>(args)), ...);

    return buffer;
}


template<typename ...Ts>
std::string_view concat(std::string s, Ts ...ts){
    return concatenateBuffer(s , (("," , ts) , ...) );
}

#include <iostream>

int main(){
    std::string b;

    std::cout << concat(b, "a") << '\n';
    std::cout << concat(b, "a", "b") << '\n';
}

I have function concatenateBuffer, the version I show here is simplified, but what it do is it appends "strings" (char *, string_view, string) to buffer.

I want to do another function concat that act bit like php function implode, e.g. put separator "," between the "strings".

e.g. if you call :

    concat(b, "a")           ===> "a"
    concat(b, "a", "b")      ===> "a,b"
    concat(b, "a", "b", "c") ===> "a,b,c"

Is this possible to be done with folding parameter pack?

Answer 1

You could break the pack up to be able to insert , between the strings:

template<class T, class ...Ts>
std::string& concat(std::string& buffer, T&& t, Ts&&...ts){
     buffer.clear();

    buffer.append(std::forward<T>(t)); // first argument, no comma

    ((buffer.append(","), 
     buffer.append(std::forward<Ts>(ts))), ...);
     
    return buffer;
}