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Version 133 (modified by jtv, 8 years ago) (diff)

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C++ connector for PostgreSQL

libpqxx is the official C++ client API for PostgreSQL, the enterprise-strength open-source relational database. (If "PostgreSQL" is too verbose, call it by its shorter name, postgres).

If you are writing software in C++ that needs to access databases managed by postgres—on just about any platform—then libpqxx is the library you use. It is the standard C++ language binding for the postgres RDBMS.

The source code for libpqxx is available under the BSD license, so you're free to download it, pass it on to others, change it, sell it, include it in your own code, and share your changes with anyone you choose. No charge, no catch. In most cases you'll want a pre-built package provided by a package maintainer for your platform, and distributed through your normal package management infrastructure.

News

2011-04-14: Declaring victory over site outages

Last month we identified and made a configuration change that our hosting provider required for the server's last operating system upgrade. We haven't had any outages since. Phew!

2011-03-28: We're retooling

After some intense back-and-forth with our hosting provider over the weekend, another potential source of our recurring site outages has been identified and fixed. Here's hoping!

Meanwhile, the script that implements the daily test runs is being rewritten. It's going to be a lot faster, but there will be some teething problems as we work on it. This may affect daily snapshot builds.

2011-03-28: gcc 4.6.0

A new gcc series was released on Friday. A few minor warnings in the libpqxx code have been fixed.

Finding Everything

Where What
Sales Pitch Why this library should interest you
Using This Site The various services offered by this development site
Download Page Source archives (no binaries; those depend on your individual platform)
FAQ Frequently Asked Questions, and their answers
Online Documentation Wiki and copies of packaged documentation
Packagers Page Information for maintainers of libpqxx packages
Consulting? Where can I get professional development help?
Bug Tracker Known bugs and requests (as in View Tickets option in top button bar)
Reporting Bugs How to report a problem or request a new feature
Mailing Lists Hosted on pgFoundry site
Database Notes Notes and tips about postgres
Performance Tips Figure out and solve performance problems
Other Projects Other open-source development projects hosted here
libpqxx Elsewhere Sites where libpqxx is registered as a project
Author and Contributors Who made all this?

For issues not suitable for the mailing list or bug tickets, contact the author as jtv at xs4all.nl.

Also, you may want to have a look at the other open source projects hosted on this site.

Technical Overview

This library works on top of the C-level API library, libpq. It comes with postgres. You will link your program with both libpqxx and libpq, in that order.

Coding with libpqxx revolves around transactions. Transactions are a central concept in database management systems, but they are widely under-appreciated among application developers. In libpqxx, they're fundamental.

With conventional database APIs, you issue commands and queries to a database session or connection, and optionally create the occasional transaction. In libpqxx you start with a connection, but you do all your SQL work in transactions that you open in your connection. You commit each transaction when it's complete; if you don't, all changes made inside the transaction get rolled back.

There are several types of transactions with various "quality of service" properties; if you don't really want to use transactions at all, one of the available transaction types is called nontransaction. This transaction type provides basic non-transactional behaviour. (This is sometimes called "autocommit": it commits every command right away).

Every command or query returns a result. Your query fetches its result data immediately when you execute it, and stores it in the result. Internally the result object is really just a smart pointer to the data, so you can copy it around very cheaply. Don't check your result for errors; failures show up as regular C++ exceptions.

Result objects can be kept around for as long as they are needed, completely separate from the connections and transactions that originated them. You can access the rows in a result using standard iterators, or more like an array using numerical indexes. Inside each row you can access the fields by standard iterators, numerical indexes, or using column names.

Here's a simple example:

#include <pqxx/pqxx>

pqxx::result get_employees(pqxx::transaction_base &txn)
{
  return txn.exec("SELECT id, name, department FROM Employee");
}

void add_employee(pqxx::transaction_base &txn, std::string name, int department)
{
  txn.exec(
    "INSERT INTO Employee(name, department) "
    "VALUES (" + txn.quote(name) + ", " + txn.quote(department) + ")");
}

int find_employee(pqxx::transaction_base &txn, std::string name)
{
  pqxx::result match = txn.exec("SELECT id FROM Employee WHERE name = " + txn.quote(name));

  if (match.empty())
    throw EmployeeNotFound("We have nobody by the name of " + name);
  if (match.size() > 1)
    throw AmbiguousName("We have more than one employee named " + name);

  return match[0][0].as<int>();
}

With these functions somewhere in your code, a simple C++ program to add an employee could look like this:

#include <iostream>
#include <pqxx/pqxx>
#include "my_functions"

int main(int argc, char *argv[])
{
  if (argc != 3)
  {
    std::cerr
      << "Give me an employee name and a department name.  No more, no less."
      << endl;
    return 2;
  }

  try
  {
    int department;

    // Use a libpqxx helper to interpret our department id string as an int.
    pqxx::from_string(argv[2], department);

    // Connect to the database.  Use default host, database, password etc.
    pqxx::connection conn;

    // Start a transaction.  With libpqxx we always work in a transaction.
    pqxx:: work txn(conn);
    add_employee(txn, argv[1], department);

    // Commit.  If we neglect to do this (e.g. because the program crashed)
    // then our changes will be rolled back.
    txn.commit();
  }
  catch (const std::exception &e)
  {
    // Our transaction's destructor has already been called, so our change
    // is implicitly rolled back before we get here.

    // Report error & exit with error code.
    std::cerr << e.what() << std::endl;
    return 1;
  }
}

How do you build your program? That depends on your system and compiler. On a typical Unix-like system, you might do:

c++ add_employee.cxx -lpqxx -lpq

This should work on most GNU/Linux systems (Debian, Fedora, Gentoo, Red Hat, Slax, Ubuntu, etc.), BSD systems (FreeBSD, NetBSD, OpenBSD), and so on, as long as you have libpqxx, libpq, and a C++ compiler installed. (If your C++ compiler has a different name on the command line, use that instead of "c++"). It may also work on Apple OS X or Microsoft Windows.


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