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.

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. :-)

Finding Everything

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News

2020-05-16: libpqxx 7.1.0 released

Easier, faster, neater: the transaction classes now have a stream method. You specify a query and the C++ types to which you want the fields converted, and you get to iterate over the rows. And thanks to C++14 structured bindings, you can read the fields straight into separate local variables:

for (auto const [id, name]:
    tx.stream<int, std::string_view>("SELECT id, name FROM thing"))
{
    process(id, name);
}

For super-fast access to a field's text-format contents, convert it to std::string_view. It'll give you an immediate reference to the buffer containing the field. Beware though: the buffer only holds that text for the one iteration. The next iteration will overwrite it.

There's a new field in nullness, and if you define your own conversions for additional C++ types, you'll need to define it. It's called always_null, and it should be false — except for types which always represent a null, in which case it should be true.

I've also made string_traits<TYPE>::size_buffer() a noexcept function. If you define any custom conversions, you'll need to do the same.

2020-05-11: Site rewrite

Oops.

Something went wrong in updating the pqxx.org server. That's what you get for running the same web server for a decade and a half (give or take) without a sysop.

But every cloud has a silver lining. I'm re-creating the development wiki as a static 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 really don't want a transaction, one of the available transaction types is called nontransaction. It provides basic non-transactional behaviour. (This is sometimes called "autocommit": it commits every successful command right away).

You can execute queries in different ways: the transaction's exec method receives all result data from the server and then returns a result object. It acts like a standard C++ container of rows, and the rows in turn are standard C++ containers of fields. You can keep a result object around for as long as you like, even after you close the transaction and even after you close the connection.

Alternatively, you can stream your query, iterating over its result rows. This is generally faster, but of course your processing can be interrupted if you lose your network connection while reading data.

Either way, don't do any if statements to check for errors. If something goes wrong, you will get an exception.

Quick example

Can't have a database example without an Employee table. Here's a simple application: 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 a lot more to fix that, but we'll get to it later.

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

int main(int, char *argv[])
{
  pqxx::connection c{"postgresql://accounting@localhost/company"};
  pqxx::work txn{c};

  // Normally we'd query the DB using txn.exec().  But for querying just one
  // single value, we can use txn.query_value() as a shorthand.
  //
  // Use txn.quote() to escape and quote a C++ string for use as an SQL string
  // in a query's text.
  int employee_id = txn.query_value<int>(
    "SELECT id "
    "FROM Employee "
    "WHERE name =" + txn.quote(argv[1]));

  std::cout << "Updating employee #" << employee_id << '\n';

  // Update the employee's salary.  Use exec0() to perform a query and check
  // that it produces an empty result.  If the result does contain data, it
  // will throw an exception.
  //
  // The ID is an integer, so we don't need to escape and quote it when using
  // it in our query text.  Just convert it to its PostgreSQL string
  // representation using to_string().
  txn.exec0(
    "UPDATE EMPLOYEE "
    "SET salary = salary + 1 "
    "WHERE id = " + pqxx::to_string(employee_id));

  // Make our change definite.
  txn.commit();
}

You'll find more detailed explanations and reference-style docs on the ReadTheDocs site.

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's macOS, 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, and 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 (std::exception const &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 (pqxx::sql_error const &e)
  {
    std::cerr
        << "Database error: " << e.what() << std::endl
        << "Query was: " << e.query() << std::endl;
    return 2;
  }
  catch (std::exception const &e)
  {
    std::cerr << e.what() << std::endl;
    return 1;
  }
}

Just one caveat: not all platforms support throwing an exception in a shared library and catching it outside that shared library. Unless you're building for Windows, it's probably a good habit to use static libraries instead.

Complete example

Here's a more complete example, showing iteration and direct field access.

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

/// Query employees from database.  Return result.
pqxx::result query()
{
  pqxx::connection c{"postgresql://accounting@localhost/company"};
  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 (pqxx::sql_error const &e)
  {
    std::cerr << "SQL error: " << e.what() << std::endl;
    std::cerr << "Query was: " << e.query() << std::endl;
    return 2;
  }
  catch (std::exception const &e)
  {
    std::cerr << "Error: " << e.what() << std::endl;
    return 1;
  }
}

Results and result rows have all the member functions you expect to find in a container: begin()/end(), front()/back(), size(), index operator, and so on. The contents are immutable.


GTF Contributor