problem with libpqxx's connection class

Hello, I am having trouble using libpqxx's connection. It
may be a 
misunderstanding on my part.

I want a persistent connection, a connection to postgresql
database 
server I can use throughout the lifetime of my program. As
we know, 
connecting and disconnecting from a server can be expensive.
It seems I 
simply cann't do this with libpqxx. For example,

#include <pqxx/pqxx>
class mydbh{
  private:
    pqxx::connection c;
  public:
    mydbh(){
         c = pqxx::connection("");
    }
};

I simply can't compile this code due to connection class
design 
decision. Is there a workaround of this problem? Or am I
missing 
something completely?

Thanks,
Fei
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On Thu, June 28, 2007 03:57, Fei Liu wrote:

> I want a persistent connection, a connection to
postgresql database
> server I can use throughout the lifetime of my program.
As we know,
> connecting and disconnecting from a server can be
expensive. It seems I
> simply cann't do this with libpqxx. For example,

Yes you can!  The problem is just a simple misunderstanding
about C++
syntax.  It looks like you're probably coming from a Java
background, so
I'll explain on that assumption.


> #include <pqxx/pqxx>
> class mydbh{
>   private:
>     pqxx::connection c;
>   public:
>     mydbh(){
>          c = pqxx::connection("");
>     }
> };

In C++, unlike Java, when you create a variable of object
type (such as
the member variable "pqxx::connection c;") then
you get an instance of the
object, not a reference.  So you don't need to assign c in
your
constructor.  The object is already there.

Also, when you write a constructor in C++, you initialize
base-class
objects and members with a special syntax.  So unlike Java,
those
constructor calls come *before* the constructor body.

So in this case, you'd do:

  mydbh() :
    c("")
  {
  }

Of course, if you're using the empty connection string, you
can just use
the default constructor for the connection:

  mydb() : c() {}

Which is also what the compiler assumes as the definition
for your
constructor.  So in your example, you can even make things
work by leaving
out your constructor entirely!  This class definition does
exactly the
same things as your example above (although of course you'll
want to add
your own member functions):

#include <pqxx/pqxx>
class mydbh{
    pqxx::connection c;
};

Done.  The connection connects when you create this object. 
If you want
to have a constructor that lets you specify your own
connection string,
you add:

  explicit mydbh(const std::string &connstring) :
c(connstring) {}

The "explicit" keyword says that this is not
providing an implicit
conversion from string to mydbh.  That will help protect you
from subtle
mistakes in code using your mydbh class.


> I simply can't compile this code due to connection
class design
> decision. Is there a workaround of this problem? Or am
I missing
> something completely?

It has nothing to do with the connection class design.  This
is a common
problem for C++ beginners to run into.


Jeroen


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