Question

This the below program i have written for some test.

class tgsetmap
{
public:
std::map<std::string,std::string> tgsetlist;
void operator<<(const char *str1,const char *str2)
{
  tgsetlist.insert( std::map<std::string,std::string>::value_type(str1,str2));
}

};


int main()
{

tgsetmap obj;

obj<<("tgset10","mystring");

obj.tgsetlist.size();
}

This throws a compilation error:

“test.cc”, line 10: Error: Illegal number of arguments for tgsetmap::operator<<(const char, const char*).
“test.cc”, line 22: Error: The operation “tgsetmap << const char*” is illegal.
2 Error(s) detected.*

Am i wrong some where?

Answer 1

You can’t force operator<< to take two arguments on right-hand side. The following code:

obj<<("tgset10","mystring");

does not work as a function call with two arguments but instead just uses the , operator. But it’s probably not what you are interested in.

If you need to pass two arguments to the << operator, you need to wrap them in some other (single) type. For example, you could use the standard std::pair, i.e. std::pair<const char*, const char*>.

But note that the operator<< should also return some reasonable type suitable for << chaining. That would probably be a tgsetmap& in your case. The following version should work fine:

#include <map>
#include <string>
#include <iostream>

class tgsetmap
{
public:
    typedef std::map<std::string, std::string> list_type;
    typedef list_type::value_type item_type;

    list_type tgsetlist;

    tgsetmap& operator<<(item_type item)
    {
        tgsetlist.insert(item);
        return *this;
    }
};

int main()
{
    tgsetmap obj;

    obj << tgsetmap::item_type("tgset10","mystring")
        << tgsetmap::item_type("tgset20","anotherstring");

    std::cout << obj.tgsetlist.size() << std::endl;
}

Note that I’ve added typedefs to not have to repeat the type names over and over again. I’ve also made operator<< return a tgsetmap& so that << could be chained (used like in the modified main() above). And finally, I’ve reused the std::map<...>::value_type to make it simpler but you could also use any other type of your own.

---

But I believe that you may prefer using a regular method instead. Something like:

void add(const char *str1, const char *str2)
{
    tgsetlist.insert( std::map<std::string, std::string>::value_type(str1, str2));
}

(inside the class declaration), and then:

obj.add("tgset10", "mystring");