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

class A
{
private:
   static std::string s;
public:
   void modify_string();
};

 std::string A::s = "default";

void A::modify_string()
{
s = "something";  // Error here. 
}

int main() {
	A a;
a.modify_string();
	// your code goes here
	return 0;
}