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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

2013-01-20: New point releases

Three new minor updates are available for download: 3.0.3 for the 3.0 series, 3.1.1 for the 3.1 series, and 4.0.1 for the 4.0 series. Get them at the download page. Most of the changes are for compatibility with newer compilers, C++11, and different operating systems.

It's hard to make guarantees with C++, but you may find the binaries compatible with their respective previous versions. It is recommended that you use the 4.0.1 version where possible; the older versions will have problems with changes in more recent PostgreSQL versions.

The updates also include support for the REPEATABLE READ isolation level. This is the new name for the old SERIALIZABLE level. Newer PostgreSQL backends have a new, fully standards-compliant implementation for SERIALIZABLE. If your binary uses the old SERIALIZABLE isolation level and you re-link it to a newer libpqxx, without recompiling, it will get REPEATABLE READ on newer backends; behaviour will be identical to what the application was originally written for. Or if you recompile the program against a newer libpqxx, it will get the new fully-compliant SERIALIZABLE isolation.

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. 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.

Example

Can't have a database example without an Employee table. Here's a simple program: find an employee by name, and raise their salary by 1 whatever-it-is-they-get-paid-in.

This example is so simple that anything that goes wrong crashes the program. You won't need to do that much more to fix that, but we'll get to it later.

#include <iostream>
#include <pqxx/pqxx>

int main(int, char *argv[])
{
  pqxx::connection c("dbname=company user=accounting");
  pqxx::work txn(c);

  pqxx::result r = txn.exec(
    "SELECT id "
    "FROM Employee "
    "WHERE name =" + txn.quote(argv[1]));

  if (r.size() != 1)
  {
    std::cerr
      << "Expected 1 employee with name " << argv[1] << ", "
      << "but found " << r.size() << std::endl;
    return 1;
  }

  int employee_id = r[0][0].as<int>();
  std::cout << "Updating employee #" << employee_id << std::endl;

  txn.exec(
    "UPDATE EMPLOYEE "
    "SET salary = salary + 1 "
    "WHERE id = " + txn.quote(employee_id));

  txn.commit();
}

For more realistic examples with detailed explanations, see CodeExamples. Also, see the  API reference for details about what everything does.

Building your libpqxx program

The details depend on your system and compiler. On a typical Unix-like system, you might do:

c++ add_employee.cxx -I/usr/local/include/ -lpqxx -lpq

Remember to keep the -lpqxx and -lpq in that order! Otherwise the linker will complain bitterly about missing functions with names starting with "PQ."

The -I/usr/local/include/ tells the compiler where the directory with header files for libpqxx is. So in this example, the header files would be in /usr/local/include/pqxx/.

This should work on most GNU/Linux systems (Debian, Fedora, Gentoo, Red Hat, Slax, Ubuntu, etc.), BSD systems (FreeBSD, NetBSD, OpenBSD), vaguely Unix-like systems such as Apple OS X, 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 works differently on Microsoft Windows, though there are development environments out there that behave more like a Unix system.


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