Q:

Structure prototype? 

How do I put a struct in a separate file?  I can do it with functions by putting the function prototype in a header file e.g. file.h and the function body in a file like file.cpp and then using the include directive #include "file.h" in the source file with main.  Can anybody give a simple example of doing the same thing with a structure like the one below?  I'm using dev-c++. 
struct person{
  string name;
  double age;
  bool sex;
};

A:

Just declare 
struct person;

It is called class forward declaration.  In C++, structs are classes with all members public by default. 

A:

If you're talking about a struct declaration:
person.h :
#ifndef PERSON_H_
#define PERSON_H_
struct person{ 
  string name; 
  double age; 
  bool sex; 
};
#endif

Then you just include person.h in the .cpp files where you need that struct. 
If you're talking about a (global) variable of the struct:
person.h :
#ifndef PERSON_H_
#define PERSON_H_
struct person{ 
  string name; 
  double age; 
  bool sex; 
};
extern struct Person some_person;
#endif

And in one of your .cpp files you need this line, at global scope,that holds the definition for 'some_person'
struct Person some_person;

Now every .cpp file that needs to access the global 'some_person' variable can include the person.h file. 