Re: Stroustrup 5.5 References

From:

Markus Moll <markus.moll@esat.kuleuven.be>

Newsgroups:

comp.lang.c++

Date:

Wed, 28 Mar 2007 11:28:22 +0200

Message-ID:

<460a3537$0$23133$9b4e6d93@newsspool1.arcor-online.net>

Hi

arnuld wrote:

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

Well, it's all in the error messages...

#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

Here, you try to access a Pair, but the definition of Pair is still
unknown, thus:

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'

<< " : "
<< p->value

The same again:
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'

         << std::endl;
    }

  return 0;
}

5.5_references.cpp: At global scope:

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

double& get_value(const std::string& s)

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')

Indeed, your definition of Pair is missing a semicolon. (Notice that you
could e.g. define a Pair-object p by "struct Pair { [...] } p;". The
same is _not_ allowed for functions, but to the compiler, it looks like
that is what you want to do: struct Pair { [...] } [...] get_value(const
std::string& s) { [...] })

And furthermore, it looks like you give two return types, Pair and
double&, so...

5.5_references.cpp:54: error: two or more data types in declaration of
'get_value'

Of course, this differs from the previous declaration only in the return
type and you cannot overload functions on return types:

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&)'

{
  /* 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++)

^^^^^^^^^
Wait, it was pair!s!_list before! What is pair_list?

5.5_references.cpp:61: error: 'pair_list' was not declared in this scope

    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)
}

Finally, there are some errors due to previous errors:
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'

Remember: the compiler is confused about the return type of get_value!

hth
Markus