...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 is analogous to the synchronous tutorial, but uses async functions * with C++20 coroutines, instead. */ #include <boost/mysql/any_address.hpp> #include <boost/mysql/any_connection.hpp> #include <boost/mysql/error_with_diagnostics.hpp> #include <boost/mysql/results.hpp> #include <boost/asio/awaitable.hpp> #include <boost/asio/co_spawn.hpp> #include <boost/asio/io_context.hpp> #include <boost/asio/this_coro.hpp> #include <exception> #include <iostream> namespace mysql = boost::mysql; namespace asio = boost::asio; /** * The main coroutine. * It must have a return type of asio::awaitable<T>. * Our coroutine does not communicate any result back, so T=void. * * The coroutine 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 use the same program structure as in the sync world, replacing * sync functions by their async equivalents and adding co_await in front of them. */ asio::awaitable<void> coro_main( mysql::any_connection& conn, std::string_view server_hostname, std::string_view username, std::string_view password ) { // The hostname, username, password and database to use. // TLS is used by default. mysql::connect_params params; params.server_address.emplace_host_and_port(std::string(server_hostname)); params.username = username; params.password = password; // Connect to the server co_await conn.async_connect(params); // Issue the SQL query to the server const char* sql = "SELECT 'Hello world!'"; mysql::results result; co_await conn.async_execute(sql, result); // Print the first field in the first row std::cout << result.rows().at(0).at(0) << std::endl; // Close the connection co_await conn.async_close(); } void main_impl(int argc, char** argv) { if (argc != 4) { std::cerr << "Usage: " << argv[0] << " <username> <password> <server-hostname>\n"; exit(1); } // The execution context, required to run I/O operations. asio::io_context ctx; // Represents a connection to the MySQL server. mysql::any_connection conn(ctx); // Enqueue the coroutine for execution. // This does not wait for the coroutine to finish. asio::co_spawn( // The execution context where the coroutine will run ctx, // The coroutine to run. This must be a function taking no arguments // and returning an asio::awaitable<T> [&conn, argv] { return coro_main(conn, argv[3], argv[1], argv[2]); }, // Callback to run when the coroutine completes. // If any exception is thrown in the coroutine body, propagate it to terminate the program. [](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 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() << '\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; } }