operator overloading multiple operands C++

Question

I have coded NumberS class, it is just for demonstration. I overloaded the operator+. It is OK when applying the operator with 2 operands, but applying with more operands producted undefined result. The code is below

NumberS.h

#include <string>
#include <iostream>
using namespace std;

class NumberS
{
private:
    long long number;
    void assign(long long);
public:
    NumberS();
    NumberS(long long);
    void operator=(string);
    NumberS& operator+(string);
    friend ostream& operator<<(ostream &os,NumberS numb)
    {
        os<<to_string(numb.number);
        return os;
    }
};

int main()
{
    NumberS number(5);
    number="10";
    cout<<number<<"\n";//print out 10
    number=number+"12";
    cout<<number<<"\n";//print out 22
    number=number+"5" +"6";//<-----   wrong result occured here, I want it to be 33
    cout<<number<<"\n";
    getchar();
}

NumberS.cpp

#include "NumberS.h"

NumberS::NumberS()
{
    number=0;
}
void NumberS::assign(long long numb)
{
    number=numb;
}
NumberS::NumberS(long long numb)
{
    assign(numb);
}
void NumberS::operator=(string str)
{
    assign(stoi(str));
}
NumberS& NumberS::operator+(string str)
{
    NumberS s;
    s.operator=(str);
    s.assign(number+s.number);
    return s;
}

the undefined result occured at the line number=number+"5" +"6";, I expect the result to be 33 please help me to get it to the right way

Answer 1

NumberS& operator+=(string s)&{
  NumberS num=std::move(s);
  this->assign(number+s.number);
  return *this;
}
friend NumberS operator(NumberS lhs, string s) {
  lhs += std::move(s);
  return lhs;
}

Answer 2

NumberS::operator+ returns by reference, but you're returning s, it's a local variable and it'll be destroyed when get out of operator+, the returned reference is always dangled. After that any dereference on it leads to UB, anything is possible.

For your case return-by-value would be fine.

NumberS NumberS::operator+(string str)
{
    NumberS s;
    s.operator=(str);
    s.assign(number+s.number);
    return s;
}

BTW: Most compilers give warning for this case, such as clang:

prog.cc:54:12: warning: reference to stack memory associated with local variable 's' returned [-Wreturn-stack-address]
    return s;
           ^
1 warning generated.