foo.cc: In function ‘int main()’:
foo.cc:8:15: warning: ‘distance’ is used uninitialized [-Wuninitialized]
    8 |         printf("%.3lf\n", distance);
      |         ~~~~~~^~~~~~~~~~~~~~~~~~~~~
foo.cc:5:28: note: ‘distance’ was declared here
    5 |         double x1,y1,x2,y2,distance;
      |                            ^~~~~~~~