Your commands don't link your main with my_operator.so
. You need something like the following:
g++ -Wall -Wextra -c -fPIC -o my_operator.o my_operator.cpp
g++ -shared -o libmy_operator.so my_operator.o
g++ -Wall -Wextra -c -o main.o main.cpp
g++ -o main main.o -lmy_operator
./main