...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
This example assumes you have gone through the setup.
/** * This example demonstrates how to use callbacks when using async functions. * This can be a good choice when targeting a standard lower than C++20. * This example uses the 'boost_mysql_examples' database, which you * can get by running db_setup.sql. */ #include <boost/mysql/any_connection.hpp> #include <boost/mysql/connect_params.hpp> #include <boost/mysql/diagnostics.hpp> #include <boost/mysql/results.hpp> #include <boost/mysql/row_view.hpp> #include <boost/mysql/with_params.hpp> #include <boost/asio/io_context.hpp> #include <boost/system/error_code.hpp> #include <functional> #include <iostream> #include <memory> // When using callbacks, we usually employ error codes instead of exceptions. using boost::system::error_code; namespace mysql = boost::mysql; namespace asio = boost::asio; // Prints a database employee to stdout void print_employee(mysql::row_view employee) { std::cout << "Employee '" << employee.at(0) << " " // first_name (string) << employee.at(1) << "' earns " // last_name (string) << employee.at(2) << " dollars yearly\n"; // salary (double) } // A session object, containing all variables that need to be kept alive for our session. // We will use a shared_ptr to ensure that all these variables are kept alive // until the last callback is executed class session : public std::enable_shared_from_this<session> { mysql::connect_params conn_params; // MySQL credentials and other connection config mysql::any_connection conn; // Represents the connection to the MySQL server mysql::results result; // A result from a query mysql::error_code final_error; // Will be set in case of error mysql::diagnostics diag; // Will be populated with info about server errors const char* company_id; // The ID of the company whose employees we want to list. Untrusted. public: session( asio::io_context& ctx, const char* server_hostname, const char* username, const char* password, const char* company_id ) : conn(ctx), company_id(company_id) { conn_params.server_address.emplace_host_and_port(server_hostname); conn_params.username = username; conn_params.password = password; conn_params.database = "boost_mysql_examples"; } // Accessor for error information, so main can access it error_code get_error() const { return final_error; } const boost::mysql::diagnostics& get_diagnostics() const { return diag; } // Initiates the callback chain void start() { // Will call on_connect when the connect operation completes. // The session object is kept alive with the shared_ptr that shared_from_this produces conn.async_connect( conn_params, diag, std::bind(&session::on_connect, shared_from_this(), std::placeholders::_1) ); } void on_connect(error_code ec) { // If there was an error, stop the callback chain if (ec) { final_error = ec; return; } // Initiate the query execution. company_id is an untrusted value. // with_params will securely compose a SQL query and send it to the server for execution. // Returned rows will be read into result. // We use the callback chain + shared_ptr technique again conn.async_execute( mysql::with_params( "SELECT first_name, last_name, salary FROM employee WHERE company_id = {}", company_id ), result, diag, std::bind(&session::on_execute, shared_from_this(), std::placeholders::_1) ); } void on_execute(error_code ec) { // If there was an error, stop the callback chain if (ec) { final_error = ec; return; } // Print the rows returned by the query for (boost::mysql::row_view employee : result.rows()) { print_employee(employee); } // Notify the MySQL server we want to quit and then close the socket conn.async_close(diag, std::bind(&session::finish, shared_from_this(), std::placeholders::_1)); } void finish(error_code err) { final_error = err; } }; void main_impl(int argc, char** argv) { if (argc != 4 && argc != 5) { std::cerr << "Usage: " << argv[0] << " <username> <password> <server-hostname> [company-id]\n"; exit(1); } // The execution context, required to run I/O operations. boost::asio::io_context ctx; // The company_id whose employees we will be listing. This // is user-supplied input, and should be treated as untrusted. const char* company_id = argc == 5 ? argv[4] : "HGS"; // Create the session object and launch it auto sess = std::make_shared<session>(ctx, argv[3], argv[1], argv[2], company_id); sess->start(); // Run the callback chain until it completes ctx.run(); // Check for errors if (error_code ec = sess->get_error()) { std::cerr << "Error: " << ec << ": " << ec.message() << '\n' << "Server diagnostics: " << sess->get_diagnostics().server_message() << std::endl; exit(1); } } int main(int argc, char** argv) { try { main_impl(argc, argv); } catch (const std::exception& err) { std::cerr << "Error: " << err.what() << std::endl; return 1; } }