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The 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. Also, no guarantees.

News

2017-07-23: Releasing 5.1.0, and moving to C++11

This new release will be the last one to support pre-C++11 compilers. I repeat: C++11 will be required from now on.

2016-09-22: Moving to Github

I haven't been keeping up with the email, issue trackers, and so on. The project is moving to github to reduce my workload:

https://github.com/jtv/libpqxx

Finding Everything

Where What
Download Page Source archives (no binaries; those depend on your individual platform)
Online Documentation Wiki and copies of packaged documentation
Github page Get source, report bugs, submit patches, request changes
Author and Contributors Who made all this?

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 really don't want a transaction, 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.

Brief 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 -lpqxx -lpq

Remember to keep the -lpqxx and -lpq in that order! Otherwise the linker will complain bitterly about missing functions like PQconnectdb and PQexec.

If libpqxx is installed in a nonstandard location, such as /usr/local, you may need to add options like -I/usr/local/include (to make the compiler find headers pqxx/* in /usr/local/include/pqxx), and/or -L/usr/local/lib (to make the linker find the library in /usr/local/lib).

This should work on most GNU/Linux systems (Mint, 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.

Handling errors

Errors are exceptions, derived from std::exception, just like you'd expect. So you can handle database errors like all others:

#include <iostream>
#include "my-db-code.hxx"

int main(int argc, char *argv[])
{
  try
  {
    do_db_work(trans);
  }
  catch (const std::exception &e)
  {
    std::cerr << e.what() << std::endl;
    return 1;
  }
}

Of course libpqxx also defines its own exception hierarchy for errors it throws, so you can handle those specially if you like:

#include <iostream>
#include <pqxx/except>
#include "my-db-code.hxx"

int main(int argc, char *argv[])
{
  try
  {
    do_db_work(trans);
  }
  catch (const std::exception &e)
  {
    std::cerr << e.what() << std::endl;
    return 1;
  }
  catch (const pqxx::sql_error &e)
  {
    std::cerr
        << "Database error: " << e.what() << std::endl
        << "Query was: " << e.query() << std::endl;
    return 2;
  }
}

Just one caveat: not all platforms support throwing an exception in a shared library and catching it outside that shared library. It's probably a good habit to use static libraries instead.

Complete example

Here's a more complete example, in C++11, showing iteration and direct field access. Download it, or view it in colour, here.

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

/// Query employees from database.  Return result.
pqxx::result query()
{
  pqxx::connection c{"dbname=company user=accounting"};
  pqxx::work txn{c};

  pqxx::result r = txn.exec("SELECT name, salary FROM Employee");
  for (auto row: r)
    std::cout
      // Address column by name.  Use c_str() to get C-style string.
      << row["name"].c_str()
      << " makes "
      // Address column by zero-based index.  Use as<int>() to parse as int.
      << row[1].as<int>()
      << "."
      << std::endl;

  // Not really needed, since we made no changes, but good habit to be
  // explicit about when the transaction is done.
  txn.commit();

  // Connection object goes out of scope here.  It closes automatically.
  return r;
}


/// Query employees from database, print results.
int main(int, char *argv[])
{
  try
  {
    pqxx::result r = query();

    // Results can be accessed and iterated again.  Even after the connection
    // has been closed.
    for (auto row: r)
    {
      std::cout << "Row: ";
      // Iterate over fields in a row.
      for (auto field: row) std::cout << field.c_str() << " ";
      std::cout << std::endl;
  }
  catch (const std::exception &e)
  {
    std::cerr << "Error: " << e.what() << std::endl;
    return 1;
  }
  catch (const pqxx::sql_error &e)
  {
    std::cerr << "SQL error: " << e.what() << std::endl;
    std::cerr << "Query was: " << e.query() << std::endl;
    return 2;
  }
}

Results and result rows have all the member functions you expect to find in a container: front()/back(), size(), index operator, and so on. You can't modify the result's contents, however.

If you're on an older version of C++, the example gets a little harder to write:

  1. There's no auto, so specify types explicitly.
  2. You won't be able to iterate using the simple for syntax. You'll need iterators and r.begin()/r.end().
  3. No move semantics, so you can't just return a pqxx::result on the stack.

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Last modified 6 days ago Last modified on Jul 23, 2017, 1:05:26 PM

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