c++c++11templatesvariadic-templatesfold-expression

What is the easiest way to print a variadic parameter pack using std::ostream?


What is the easiest way to print a parameter pack, separated by commas, using std::ostream?

Example:

template<typename... Args>
void doPrint(std::ostream& out, Args... args){
   out << args...; // WRONG! What to write here?
}

// Usage:
int main(){
   doPrint(std::cout,34,"bla",15); // Should print: 34,bla,15
}

Note: It may be assumed that a corresponding overload of the << operator is available for all types of the parameter pack.


Solution

  • Without recursive calls and commas where you wanted.

    In / through parameter pack expansion:

    template <typename Arg, typename... Args>
    void doPrint(std::ostream& out, Arg&& arg, Args&&... args)
    {
        out << std::forward<Arg>(arg);
        using expander = int[];
        (void)expander{0, (void(out << ',' << std::forward<Args>(args)), 0)...};
    }
    

    DEMO


    In using fold expressions:

    template <typename Arg, typename... Args>
    void doPrint(std::ostream& out, Arg&& arg, Args&&... args)
    {
        out << std::forward<Arg>(arg);
        ((out << ',' << std::forward<Args>(args)), ...);
    }
    

    DEMO 2