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//
// Copyright (c) 2019-2025 Ruben Perez Hidalgo (rubenperez038 at gmail dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//
//[example_callbacks
/**
* 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;
}
}
//]
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