Stroustrup 5.5 References

this i the example programme. it has many errors, i am not able to
make anything out it:

---------- PROGRAMME -----------
/* Stroustrup, 5.5 references

STATEMENT:
the basic idea is that a "string" has an associated floating-point
value
associated with it, wrapped in a "Struct" named "Pair"."pairs_list"
is
a "Vector" of different "Pair(s)"

then we define a function named "get_value()" which keeps a "Pair"
in "pair_list" for each different string that has been given to it
as input. If the "input" matches with some "string" already present,
then the value corresponding to that "string" is returned, otherwise
that string is added with corresponding value 0 (zero) and 0 is
returned.

in "main()", we aks of ro input and just print the whole structure
out

*/

#include<iostream>
#include<string>
#include<vector>

struct Pair;
double& get_value(const std::string&);
std::vector<Pair> pairs_list;

int main()
{
  std::string get_name;

  while(std::cin >> get_name)
    get_value(get_name)++;

  for(std::vector<Pair>::const_iterator p=pairs_list.begin(); p !=
pairs_list.end(); ++p)
    {
      std::cout << p->name
        << " : "
        << p->value
        << std::endl;
    }

  return 0;
}

struct Pair
{
  std::string name;
  double val;
}

double& get_value(const std::string& s)
{
  /* maintains aset of Pairs, if "s" is found
     returns its corresponding value */

  const int my_val = 0;

  for(int i=0; i < pair_list.size(); i++)
    if(s == pairs_list[i].name)
      return pairs_list[i].val;

  Pair p = { s, my_val };
  pairs_list.push_back(p); // adds pair at the end

  return pairs_list[pairs_list.size() - 1].val; // actually return 0
(zero)
}

----------- OUTPUT -------------------
[arch@voodo tc++pl]$ g++ -ansi -pedantic -Wall -Wextra
5.5_references.cpp
5.5_references.cpp: In function 'int main()':
5.5_references.cpp:39: error: invalid use of undefined type 'const
struct Pair'
5.5_references.cpp:24: error: forward declaration of 'const struct
Pair'
5.5_references.cpp:41: error: invalid use of undefined type 'const
struct Pair'
5.5_references.cpp:24: error: forward declaration of 'const struct
Pair'
5.5_references.cpp: At global scope:
5.5_references.cpp:54: error: new types may not be defined in a return
type
5.5_references.cpp:54: note: (perhaps a semicolon is missing after the
definition of 'Pair')
5.5_references.cpp:54: error: two or more data types in declaration of
'get_value'
5.5_references.cpp: In function 'Pair& get_value(const std::string&)':
5.5_references.cpp:54: error: new declaration 'Pair& get_value(const
std::string&)'
5.5_references.cpp:25: error: ambiguates old declaration 'double&
get_value(const std::string&)'
5.5_references.cpp: In function 'Pair& get_value(const std::string&)':
5.5_references.cpp:61: error: 'pair_list' was not declared in this
scope
5.5_references.cpp:63: error: invalid initialization of reference of
type 'Pair&' from expression of type 'double'
5.5_references.cpp:68: error: invalid initialization of reference of
type 'Pair&' from expression of type 'double'
[arch@voodo tc++pl]$