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https://github.com/jtv/libpqxx

The official C++ client API for PostgreSQL.
https://github.com/jtv/libpqxx

cplusplus cplusplus-17 databases postgres postgresql

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The official C++ client API for PostgreSQL.

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README 

        
libpqxx

=======



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



Home page: [

    http://pqxx.org/development/libpqxx

](http://pqxx.org/development/libpqxx/)



Find libpqxx on Github: [

    https://github.com/jtv/libpqxx

](https://github.com/jtv/libpqxx)



Documentation on Read The Docs: [

    https://libpqxx.readthedocs.io

](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:



```sh

    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`](./BUILDING-configure.md)

and [`BUILDING-cmake.md`](./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

](https://github.com/GordonLElliott/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](https://libpqxx.readthedocs.io).



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 `` 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:



```c++

    #include 

    #include 



    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(

                "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(

                "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(

                "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(

                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..