#include <iostream>

int main() {

    std::string gs;

    class Base {
    public:
        void add_string(const std::string &s)
        {
            gs += s + "\n";
            //std::cout << s << "\n";
        }
        virtual void report();
    };

    class : public Base {
    public:
        void report() override
        {
            add_string("report one");
        }
    } one;

    class: public Base {
    public:
        void report() override
        {
            add_string("report two");
        }
    } two;

    one.report();
    two.report();
    std::cout << gs;
    return 0;
}

-------------------------------------

test.cpp: In member function ‘void main()::Base::add_string(const string&)’:
test.cpp:11:4: error: use of local variable with automatic storage from containing function
    gs += s + "\n";
    ^~
test.cpp:5:14: note: ‘std::__cxx11::string gs’ declared here
  std::string gs;
              ^~