Working with Databases in C++: PostgreSQL and MongoDB

Introduction: Why connect databases to C++

Modern C++ applications rarely function without persistent data storage. Whether it's a game server, telemetry system, or high-load backend — you will need interaction with a DBMS. Two fundamentally different approaches dominate the market: relational (SQL) and document-oriented (NoSQL) databases.

In this article, we will cover how to work with PostgreSQL (via the libpqxx library) and MongoDB (via the official mongo-cxx-driver). You will learn how to set up a connection, perform CRUD operations, and handle errors.

1. PostgreSQL and C++: the relational approach

PostgreSQL is a powerful object-relational DBMS with excellent support for transactions and complex queries. For C++, the recommended library is libpqxx — a modern, type-safe wrapper over libpq.

1.1 Installing libpqxx

Installation depends on your OS:

• Ubuntu/Debian: sudo apt install libpqxx-dev
• macOS (Homebrew): brew install libpqxx
• Windows (vcpkg): vcpkg install libpqxx

1.2 Connecting and executing queries

Consider a minimal program for connecting to a database, creating a table, and inserting data:

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

int main() {    try {        // Connecting to the database        pqxx::connection conn("dbname=testdb user=postgres password=secret hostaddr=127.0.0.1 port=5432");        if (conn.is_open()) {            std::cout << "Connected to PostgreSQL: " << conn.dbname() << std::endl;        }

        // Creating a transaction        pqxx::work txn(conn);

        // Creating a table        txn.exec("CREATE TABLE IF NOT EXISTS users ("                 "id SERIAL PRIMARY KEY,"                 "name VARCHAR(100) NOT NULL,"                 "email VARCHAR(255) UNIQUE NOT NULL"                 ");");

        // Inserting data (with parameterization for safety)        txn.exec_params("INSERT INTO users (name, email) VALUES ($1, $2);",                        "Alice", "alice@example.com");

        // Committing the transaction        txn.commit();

        std::cout << "Data inserted successfully." << std::endl;

    } catch (const std::exception &e) {        std::cerr << "Error: " << e.what() << std::endl;        return 1;    }    return 0;}

1.3 Reading data

To retrieve data, use iterators over the query result:

pqxx::work txn(conn);pqxx::result result = txn.exec("SELECT id, name, email FROM users;");

for (auto row : result) {    int id = row["id"].as<int>();    std::string name = row["name"].as<std::string>();    std::string email = row["email"].as<std::string>();    std::cout << id << ": " << name << " <" << email << ">" << std::endl;}

2. MongoDB and C++: the document-oriented approach

MongoDB stores data in BSON format (binary JSON). For C++, the official driver mongo-cxx-driver is used, built on top of the libmongoc library.

2.1 Installing mongo-cxx-driver

• Ubuntu: sudo apt install libmongoc-1.0-0 libbson-1.0-0 libmongocxx-dev
• macOS: brew install mongo-cxx-driver
• Building from source: Requires cmake, libmongoc, and libbson.

2.2 Connecting and CRUD operations

Example of connecting to a local MongoDB instance and inserting a document:

#include <iostream>#include <bsoncxx/b