string concatenate operator with stringstream

inc.hpp

#include <iostream> 
#include <sstream>
#include <string>

class SSWrapper
{
public:
    static SSWrapper& instance() 
    {
        if(!inst)
			inst = new SSWrapper;
        return *inst;
    }

	template<typename T>
	SSWrapper& add(T& val)
	{
		ss << val;
		return *this;
	}

	operator std::string()
	{
		auto ret = ss.str();
		ss.str("");
		return ret;
	}

private:
	static SSWrapper *inst;
	SSWrapper() {}
    SSWrapper( const SSWrapper& );  
    SSWrapper& operator=( SSWrapper& );

	std::stringstream ss;
};

template<typename T1, typename T2>
SSWrapper& operator%(T1 lft, T2 rgt)
{
	return SSWrapper::instance().add(lft).add(rgt);
}

template<typename T2>
SSWrapper& operator%(SSWrapper& lft, T2 rgt)
{
	return lft.add(rgt);
}

std::ostream& operator<<(std::ostream& os, SSWrapper& wrp)
{
	return os << std::string(wrp);
}

main.cpp

#include <iostream> 
#include <string>
#include <sstream>

#include "inc.h"

SSWrapper* SSWrapper::inst=nullptr;

using namespace std;

int main() 
{
    string b{"b"};

    std::string c = 'a' % b;
    cout << c << endl;

    cout << 'a' % b % 1 << endl;

    system("pause");
}