C++11 Args...

This is a discussion on C++11 Args... within the C++ Programming forums, part of the General Programming Boards category; I've been reading about variadic templates, but having a hard time wrapping my mind around them. The following code is ...

  1. #1
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    C++11 Args...

    I've been reading about variadic templates, but having a hard time wrapping my mind around them.

    The following code is from:

    c++ - Call function with parameters extracted from string - Stack Overflow

    Is it possible to do something like this with a string array, string vector, stringstream, or something to that effect instead of with an istream? istreams don't fit my usage. id like to pass in a string and split argument by spaces.

    Can anyone point me in the right direction?

    Code:
    #include <map>
    #include <string>
    #include <iostream>
    #include <sstream>
    #include <functional>
    #include <stdexcept>
    #include <type_traits>
    
    
    template<class F>
    struct stream_function;
    
    
    template<class R, class... Args>
    struct stream_function<R(Args...)>{
      typedef R func_type(Args...);
      typedef std::function<func_type> function;
      static unsigned const arity = sizeof...(Args);
    
    
      template<class F>
      stream_function(F const& f)
        : _f(f) {}
    
    
      void operator()(std::istream& args, std::string* out_opt) const{
        call(args, out_opt, typename std::is_void<R>::type());
      }
    
    
    private:  
      template<class T>
      static T get(std::istream& args){
        T t; // must be default constructible
        if(!(args >> t)){
          args.clear();
          throw std::invalid_argument("invalid argument to stream_function");
        }
        return t;
      }
    
    
      void call(std::istream& args, std::string*, std::true_type) const{
        // void return
        _f(get<Args>(args)...);
      }
    
    
      void call(std::istream& args, std::string* out_opt, std::false_type) const{
        // non-void return
        if(!out_opt) // no return wanted, redirect
          return call(args, 0, std::true_type());
    
    
        R ret(_f(get<Args>(args)...));
        std::stringstream conv;
        if(!(conv << ret))
          throw std::runtime_error("bad return in stream_function");
        *out_opt = conv.str();
      }
    
    
      function _f;
    };
    
    
    typedef std::function<void(std::istream&, std::string*)> func_type;
    typedef std::map<std::string, func_type> dict_type;
    
    
    void print(){
      std::cout << "print()\n";
    }
    
    
    int add(int a, int b){
      return a + b;
    }
    
    
    int main(){
      dict_type func_dict;
      func_dict["print"] = stream_function<void()>(print);
      func_dict["add"] = stream_function<int(int,int)>(add);
    
    
      for(;;){
        std::cout << "Which function should be called?\n";
        std::string tmp;
        std::cin >> tmp;
        auto it = func_dict.find(tmp);
        if(it == func_dict.end()){
          std::cout << "Invalid function '" << tmp << "'\n";
          continue;
        }
        tmp.clear();
        try{
          it->second(std::cin, &tmp);
        }catch(std::exception const& e){
          std::cout << "Error: '" << e.what() << "'\n";
          std::cin.ignore();
          continue;
        }
        std::cout << "Result: " << (tmp.empty()? "none" : tmp) << '\n';
      }
    }
    Last edited by tmes; 11-13-2012 at 09:19 PM. Reason: code formatting fix.

  2. #2
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    Oct 2008
    Posts
    1,262
    The class is written to take "std::istream" to process the arguments. std::istream is not specific to std::cin, std::istringstream is derived from std::istream as well. So you should just be able to replace:
    it->second(std::cin, &tmp);
    With:
    std::istringstream args("1 2");
    it->second(args, &tmp);
    (At least for the "add" function)

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