Working with Databases in C++: PostgreSQL and MongoDB in Practice

Introduction: Why does a C++ developer need databases?

C++ is one of the most powerful system programming languages. However, even in high-load projects, it is rarely possible to do without data storage. Whether it's a game server, a financial application, or an IoT system — you will need reliable storage. In this article, we will cover two popular approaches: the relational DBMS PostgreSQL and the document-oriented MongoDB. You will learn how to connect them to a C++ project, perform basic operations, and avoid common mistakes.

1. PostgreSQL in C++: The Classic Relational Database

PostgreSQL is a mature, reliable DBMS with excellent support for transactions and complex queries. To work with it from C++, the libpqxx library is most commonly used — the official C++ wrapper over libpq.

Installation and Setup

Install libpqxx via a package manager or build from source:

# Ubuntu/Debiansudo apt-get install libpqxx-dev

# macOS (Homebrew)brew install libpqxx

In CMakeLists.txt add:

find_package(PQXX REQUIRED)target_link_libraries(your_project pqxx::pqxx)

Connecting to the Database and a Simple SELECT

Let's look at a minimal example — retrieving a list of users:

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

int main() {    try {        // Connection string: "host=... dbname=... user=... password=..."        pqxx::connection conn("host=localhost dbname=testdb user=admin password=secret");        pqxx::work txn(conn);

        // Execute the query        pqxx::result result = txn.exec("SELECT id, name, email FROM users LIMIT 10");

        for (auto row : result) {            std::cout << "ID: " << row[0].as<int>()                      << ", Name: " << row[1].as<std::string>()                      << ", Email: " << row[2].as<std::string>() << std::endl;        }

        txn.commit(); // Commit the transaction    } catch (const std::exception &e) {        std::cerr << "Error: " << e.what() << std::endl;        return 1;    }    return 0;}

Note: all operations are wrapped in a try-catch block — libpqxx throws exceptions on any failure.

Parameterized Queries (Security)

Never concatenate strings for value substitution — this opens the door to SQL injection. Use prepared statements:

pqxx::work txn(conn);// Preparationconn.prepare("insert_user", "INSERT INTO users (name, email) VALUES ($1, $2)");

// Execution with parameterstxn.exec_prepared("insert_user", "Ivan Petrov", "ivan@example.com");txn.commit();

2. MongoDB in C++: NoSQL Flexibility

MongoDB stores data in BSON format (binary JSON) and is well-suited for tasks where the data schema may change. The official driver for C++ is mongocxx (built on top of mongoc).

Installing the mongocxx Driver

Building from source is the most reliable method. Quick instructions:

# Install dependencies: libmongoc, libbson, cmakegit clone https://github.com/mongodb/mongo-cxx-driver.gitcd mongo-cxx-driver/buildcmake .. -DCMAKE_BUILD_TYPE=Release -DBSONCXX_POLY_USE_MNMLSTC=ONmake -j4sudo make install

In CMakeLists.txt:

find_package(libmongocxx REQUIRED)target_link_libraries(your_project mongocxx::mongocxx)

Connecting and Inserting a Document

#include <iostream>#include <bsoncxx/json.hpp>#include <mongocxx/client.hpp>#include <mongocxx/instance.hpp>#include <mongocxx/uri.hpp>

int main() {    // Initialize the driver (once per application)    mongocxx::instance inst{};

    // Con