...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 stackful coroutines 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. * You need to link your program to Boost.Context to use asio::spawn. * * 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/error_with_diagnostics.hpp> #include <boost/mysql/results.hpp> #include <boost/mysql/row_view.hpp> #include <boost/mysql/with_diagnostics.hpp> #include <boost/mysql/with_params.hpp> #include <boost/asio/io_context.hpp> #include <boost/asio/spawn.hpp> #include <iostream> namespace mysql = boost::mysql; namespace asio = boost::asio; 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) } /** * The main coroutine. It will suspend every time we call one of the asynchronous functions, saving * all information it needs for resuming. When the asynchronous operation completes, * the coroutine will resume in the point it was left. * We need to pass the yield object to async functions for this to work. */ void coro_main( const char* server_hostname, const char* username, const char* password, const char* company_id, asio::yield_context yield ) { // Represents a connection to the MySQL server. // The connection will use the same executor as the coroutine mysql::any_connection conn(yield.get_executor()); // The hostname, username, password and database to use mysql::connect_params conn_params; conn_params.server_address.emplace_host_and_port(server_hostname); conn_params.username = username; conn_params.password = password; conn_params.database = "boost_mysql_examples"; // Connect to server. with_diagnostics turns thrown exceptions // into error_with_diagnostics, which contain more info than regular exceptions conn.async_connect(conn_params, mysql::with_diagnostics(yield)); // 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. mysql::results result; conn.async_execute( mysql::with_params( "SELECT first_name, last_name, salary FROM employee WHERE company_id = {}", company_id ), result, mysql::with_diagnostics(yield) ); // Print the employees for (boost::mysql::row_view employee : result.rows()) { print_employee(employee); } // Notify the MySQL server we want to quit, then close the underlying connection. conn.async_close(mysql::with_diagnostics(yield)); } 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 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"; // The execution context, required to run I/O operations. asio::io_context ctx; // Launch the coroutine asio::spawn( ctx, [argv, company_id](asio::yield_context yield) { coro_main(argv[3], argv[1], argv[2], company_id, yield); }, // If any exception is thrown in the coroutine body, rethrow it. [](std::exception_ptr ptr) { if (ptr) { std::rethrow_exception(ptr); } } ); // Calling run will actually execute the coroutine until completion ctx.run(); } int main(int argc, char** argv) { try { main_impl(argc, argv); } catch (const boost::mysql::error_with_diagnostics& err) { // You will only get this type of exceptions if you use with_diagnostics. // Some errors include additional diagnostics, like server-provided error messages. // Security note: diagnostics::server_message may contain user-supplied values (e.g. the // field value that caused the error) and is encoded using to the connection's character set // (UTF-8 by default). Treat is as untrusted input. std::cerr << "Error: " << err.what() << '\n' << "Server diagnostics: " << err.get_diagnostics().server_message() << std::endl; return 1; } catch (const std::exception& err) { std::cerr << "Error: " << err.what() << std::endl; return 1; } }