README.md

libpqxx

Welcome to libpqxx, the C++ API to the PostgreSQL database management system.

Home page: http://pqxx.org/development/libpqxx

Find libpqxx on Github: https://github.com/jtv/libpqxx

Documentation on Read The Docs: https://libpqxx.readthedocs.io

Compiling this package requires PostgreSQL to be installed – or at least the C headers and library for client development. The library builds on top of PostgreSQL’s standard C API, libpq, though your code won’t notice.

If you’re getting the code straight from the Git repo, the head of the master branch represents the current development version. Releases are tags on commits in master. For example, to get version 7.1.1:

    git checkout 7.1.1

Upgrade notes

The 7.x versions require at least C++17. Make sure your compiler is up to date. For libpqxx 8.x you will need at least C++20.

Also, 7.0 makes some breaking changes in rarely used APIs:

  • There is just a single connection class. It connects immediately.
  • Custom connection classes are no longer supported.
  • It’s no longer possible to reactivate a connection once it’s been closed.
  • The API for defining string conversions has changed.

If you’re defining your own type conversions, 7.1 requires one additional field in your nullness traits.

Building libpqxx

There are two different ways of building libpqxx from the command line:

  1. Using CMake, on any system which supports it.
  2. On Unix-like systems, using a configure script.

“Unix-like” systems include GNU/Linux, Apple macOS and the BSD family, AIX, HP-UX, Irix, Solaris, etc. Even on Microsoft Windows, a Unix-like environment such as WSL, Cygwin, or MinGW should work.

You’ll find detailed build and install instructions in BUILDING-configure.md and BUILDING-cmake.md, respectively.

And if you’re working with Microsoft Visual Studio, have a look at Gordon Elliott’s Easy-PQXX Build for Windows Visual Studio project.

Documentation

Building the library, if you have the right tools installed, generates HTML documentation in the doc/ directory. It is based on the headers in include/pqxx/ and text in include/pqxx/doc/. This documentation is also available online at readthedocs.

Programming with libpqxx

Your first program will involve the libpqxx classes connection (see the pqxx/connection.hxx header), and work (a convenience alias for transaction<> which conforms to the interface defined in pqxx/transaction_base.hxx).

These *.hxx headers are not the ones you include in your program. Instead, include the versions without filename suffix (e.g. pqxx/connection). Those will include the actual .hxx files for you. This was done so that includes are in standard C++ style (as in <iostream> etc.), but an editor will still recognize them as files containing C++ code.

Continuing the list of classes, you may also need the result class (pqxx/result.hxx). In a nutshell, you create a pqxx::connection based on a Postgres connection string (see below), create a pqxx::work (a transaction object) in the context of that connection, and run one or more queries and/or SQL commands on that.

Depending on how you execute a query, it can return a stream of std::tuple (each representing one row); or a pqxx::result object which holds both the result data and additional metadata: how many rows your query returned and/or modified, what the column names are, and so on. A pqxx::result is a container of pqxx::row, and a pqxx::row is a container of pqxx::field.

Here’s an example with all the basics to get you going:

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

    int main()
    {
        try
        {
            // Connect to the database.  You can have multiple connections open
            // at the same time, even to the same database.
            pqxx::connection c;
            std::cout << "Connected to " << c.dbname() << '\n';

            // Start a transaction.  A connection can only have one transaction
            // open at the same time, but after you finish a transaction, you
            // can start a new one on the same connection.
            pqxx::work tx{c};

            // Query data of two columns, converting them to std::string and
            // int respectively.  Iterate the rows.
            for (auto [name, salary] : tx.query<std::string, int>(
                "SELECT name, salary FROM employee ORDER BY name"))
            {
                std::cout << name << " earns " << salary << ".\n";
            }

            // For large amounts of data, "streaming" the results is more
            // efficient.  It does not work for all types of queries though.
            //
            // You can read fields as std::string_view here, which is not
            // something you can do in most places.  A string_view becomes
            // meaningless when the underlying string ceases to exist.  In this
            // one situation, you can convert a field to string_view and it
            // will be valid for just that one iteration of the loop.  The next
            // iteration may overwrite or deallocate its buffer space.
            for (auto [name, salary] : tx.stream<std::string_view, int>(
                "SELECT name, salary FROM employee"))
            {
                std::cout << name << " earns " << salary << ".\n";
            }

            // Execute a statement, and check that it returns 0 rows of data.
            // This will throw pqxx::unexpected_rows if the query returns rows.
            std::cout << "Doubling all employees' salaries...\n";
            tx.exec0("UPDATE employee SET salary = salary*2");

            // Shorthand: conveniently query a single value from the database.
            int my_salary = tx.query_value<int>(
                "SELECT salary FROM employee WHERE name = 'Me'");
            std::cout << "I now earn " << my_salary << ".\n";

            // Or, query one whole row.  This function will throw an exception
            // unless the result contains exactly 1 row.
            auto [top_name, top_salary] = tx.query1<std::string, int>(
                R"(
                    SELECT name, salary
                    FROM employee
                    WHERE salary = max(salary)
                    LIMIT 1
                )");
            std::cout << "Top earner is " << top_name << " with a salary of "
                      << top_salary << ".\n";

            // If you need to access the result metadata, not just the actual
            // field values, use the "exec" functions.  Most of them return
            // pqxx::result objects.
            pqxx::result res = tx.exec("SELECT * FROM employee");
            std::cout << "Columns:\n";
            for (pqxx::row_size_type col = 0; col < res.columns(); ++col)
                std::cout << res.column_name(col) << '\n';

            // Commit the transaction.  If you don't do this, the database will
            // undo any changes you made in the transaction.
            std::cout << "Making changes definite: ";
            tx.commit();
            std::cout << "OK.\n";
        }
        catch (std::exception const &e)
        {
            std::cerr << "ERROR: " << e.what() << '\n';
            return 1;
        }
        return 0;
    }

Connection strings

Postgres connection strings state which database server you wish to connect to, under which username, using which password, and so on. Their format is defined in the documentation for libpq, the C client interface for PostgreSQL. Alternatively, these values may be defined by setting certain environment variables as documented in e.g. the manual for psql, the command line interface to PostgreSQL. Again the definitions are the same for libpqxx-based programs.

The connection strings and variables are not fully and definitively documented here; this document will tell you just enough to get going. Check the PostgreSQL documentation for authoritative information.

The connection string consists of attribute=value pairs separated by spaces, e.g. “user=john password=1x2y3z4”. The valid attributes include:

  • host — Name of server to connect to, or the full file path (beginning with a slash) to a Unix-domain socket on the local machine. Defaults to “/tmp”. Equivalent to (but overrides) environment variable PGHOST.
  • hostaddr — IP address of a server to connect to; mutually exclusive with “host”.
  • port — Port number at the server host to connect to, or socket file name extension for Unix-domain connections. Equivalent to (but overrides) environment variable PGPORT.
  • dbname — Name of the database to connect to. A single server may host multiple databases. Defaults to the same name as the current user’s name. Equivalent to (but overrides) environment variable PGDATABASE.
  • user — User name to connect under. This defaults to the name of the current user, although PostgreSQL users are not necessarily the same thing as system users.
  • requiressl — If set to 1, demands an encrypted SSL connection (and fails if no SSL connection can be created).

Settings in the connection strings override the environment variables, which in turn override the default, on a variable-by-variable basis. You only need to define those variables that require non-default values.

Linking with libpqxx

To link your final program, make sure you link to both the C-level libpq library and the actual C++ library, libpqxx. With most Unix-style compilers, you’d do this using these options: -lpqxx -lpq

while linking. Both libraries must be in your link path, so the linker knows where to find them. Any dynamic libraries you use must also be in a place where the loader can find them when loading your program at runtime.

Some users have reported problems using the above syntax, however, particularly when multiple versions of libpqxx are partially or incorrectly installed on the system. If you get massive link errors, try removing the “-lpqxx” argument from the command line and replacing it with the name of the libpqxx library binary instead. That’s typically libpqxx.a, but you’ll have to add the path to its location as well, e.g. /usr/local/pqxx/lib/libpqxx.a. This will ensure that the linker will use that exact version of the library rather than one found elsewhere on the system, and eliminate worries about the exact right version of the library being installed with your program..

Описание

The official C++ client API for PostgreSQL.