🚀 go-pugleaf

i just finished installing the libg++ library for c++ and i want to test the
compiler to see if it is working. when i try to compile the following little
test file using gcc,
i get these errors:
	collect2: ld returned 2 exit status
	ld: Undefined symbol
	  String::length(void)
	  String::set_to(char *)
	  String::read(void)

what do these errors mean i did or didn't do? 
(i'm new at c++)
below are the files...

/* this is the beginning of file String.h */
const int max_string_length = 128;
class String{
public:
  void set_to(char*);
  int length();
  int read();
  void print();

private:
  char text[max_string_length+1];
};
/* this is the end of file String.h */

/* and the beginning of file test.cc */
#include "String.h"
#include<stdio.h>

main(int, char *[])
{
  String s, t;
  s.set_to("hello world");
  printf("Length of s is %d\n", s.length());
  printf("Enter string t:	");
  t.read();
  printf("Length of t is %d\n",t.length());
  return 0;
}
/* this is the end of file test.cc */