[user02@FC4 Lesson10]$ cat -n proto.cpp

   1  #include <iostream>
   2  using namespace std;
   3
   4  float average_value(int, int);  // Function prototype
   5
   6  int main()
   7  {
   8     cout << "The average of 2000 and 2 is " << average_value(2000, 2) << endl;
   9
  10     return 0;
  11  }
  12
  13  float average_value(int a, int b)
  14  {
  15     return((a + b) / 2.0);
  16  }

[user02@FC4 Lesson10]$ g++ -o proto proto.cpp
[user02@FC4 Lesson10]$ ./proto
The average of 2000 and 2 is 1001
[user02@FC4 Lesson10]$

---

[user02@FC4 Lesson10]$ cat -n proto2.cpp

   1  #include <iostream>
   2  using namespace std;
   3
   4  //float average_value(int, int);  // Function prototype
   5
   6  int main()
   7  {
   8     cout << "The average of 2000 and 2 is " << average_value(2000, 2) << endl;
   9
  10     return 0;
  11  }
  12
  13  float average_value(int a, int b)
  14  {
  15     return((a + b) / 2.0);
  16  }

[user02@FC4 Lesson10]$ g++ -o proto2 proto2.cpp
proto2.cpp: In function ?eint main()?f:
proto2.cpp:8: error: ?eaverage_value?f was not declared in this scope
[user02@FC4 Lesson10]$ ./proto2
-bash: ./proto2: No such file or directory
[user02@FC4 Lesson10]$