...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 UPDATE statements, * transactions and semicolon-separated queries. * * The program updates the first name of an employee given their ID * and prints their full details. * * It uses Boost.Pfr for reflection, which requires C++20. * You can backport it to C++14 if you need by using Boost.Describe. * * 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/error_with_diagnostics.hpp> #include <boost/mysql/pfr.hpp> #include <boost/mysql/results.hpp> #include <boost/mysql/resultset_view.hpp> #include <boost/mysql/row_view.hpp> #include <boost/mysql/static_results.hpp> #include <boost/mysql/with_params.hpp> #include <boost/asio/awaitable.hpp> #include <boost/asio/co_spawn.hpp> #include <boost/asio/io_context.hpp> #include <cstdint> #include <iostream> #include <string> #include <string_view> #include <tuple> namespace mysql = boost::mysql; namespace asio = boost::asio; // As in the previous tutorial, this struct models // the data returned by our SELECT query. It should contain a member // for each field of interest, with a matching name. struct employee { std::string first_name; std::string last_name; }; // The main coroutine asio::awaitable<void> coro_main( std::string_view server_hostname, std::string_view username, std::string_view password, std::int64_t employee_id, std::string_view new_first_name ) { // Create a connection. // Will use the same executor as the coroutine. mysql::any_connection conn(co_await asio::this_coro::executor); // The server host, username, password and database to use. // Setting multi_queries to true makes it possible to run several // semicolon-separated queries with async_execute. mysql::connect_params params; params.server_address.emplace_host_and_port(std::string(server_hostname)); params.username = std::move(username); params.password = std::move(password); params.database = "boost_mysql_examples"; params.multi_queries = true; // Connect to the server co_await conn.async_connect(params); // Perform the update and retrieve the results: // 1. Begin a transaction block. Further updates won't be visible to // other transactions until this one commits. // 2. Perform the update. // 3. Retrieve the employee we just updated. Since we're in a transaction, // this will be the employee we just updated (if any), // without the possibility of other transactions interfering. // 4. Commit the transaction and make everything visible to other transactions. // If any of the previous steps fail, the commit won't be run, and the // transaction will be rolled back when the connection is closed. // MySQL returns one resultset for each query, so we pass 4 params to static_results mysql::static_results< std::tuple<>, // START TRANSACTION doesn't generate rows std::tuple<>, // The UPDATE doesn't generate rows mysql::pfr_by_name<employee>, // The SELECT generates employees std::tuple<> // The COMMIT doesn't generate rows > result; co_await conn.async_execute( mysql::with_params( "START TRANSACTION;" "UPDATE employee SET first_name = {0} WHERE id = {1};" "SELECT first_name, last_name FROM employee WHERE id = {1};" "COMMIT", new_first_name, employee_id ), result ); // We've run 4 SQL queries, so MySQL has returned us 4 resultsets. // The SELECT is the 3rd resultset. Retrieve the generated rows. // employees is a span<const employee> auto employees = result.rows<2>(); if (employees.empty()) { std::cout << "No employee with ID = " << employee_id << std::endl; } else { const employee& emp = employees[0]; std::cout << "Updated: employee is now " << emp.first_name << " " << emp.last_name << std::endl; } // Notify the MySQL server we want to quit, then close the underlying connection. co_await conn.async_close(); } void main_impl(int argc, char** argv) { if (argc != 6) { std::cerr << "Usage: " << argv[0] << " <username> <password> <server-hostname> <employee-id> <new-first-name>\n"; exit(1); } // Create an I/O context, required by all I/O objects asio::io_context ctx; // Launch our coroutine asio::co_spawn( ctx, [=] { return coro_main(argv[3], argv[1], argv[2], std::stoi(argv[4]), argv[5]); }, // 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(); std::cout << "Done\n"; } int main(int argc, char** argv) { try { main_impl(argc, argv); } catch (const boost::mysql::error_with_diagnostics& err) { // 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() << ", error code: " << err.code() << '\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; } }