Boost C++ Libraries

This example assumes you have gone through the setup.

/**
 * This example demonstrates how to insert several records in a single
 * SQL statement using format_sql. The implementation is generic,
 * and can be reused to batch-insert any type T with Boost.Describe metadata.
 *
 * The program reads a JSON file containing a list of employees
 * and inserts it into the employee table. It uses Boost.JSON and
 * Boost.Describe to parse the file.
 *
 * This example uses C++20 coroutines. If you need, you can backport
 * it to C++14 (required by Boost.Describe) by using callbacks, asio::yield_context
 * or sync functions instead of coroutines.
 *
 * 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/format_sql.hpp>
#include <boost/mysql/results.hpp>
#include <boost/mysql/sequence.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 <boost/describe/class.hpp>
#include <boost/describe/members.hpp>
#include <boost/describe/modifiers.hpp>
#include <boost/json/parse.hpp>
#include <boost/json/value_to.hpp>
#include <boost/mp11/list.hpp>
#include <boost/mp11/tuple.hpp>

#include <array>
#include <cstddef>
#include <fstream>
#include <iostream>
#include <string>
#include <string_view>

namespace describe = boost::describe;
namespace mp11 = boost::mp11;
namespace mysql = boost::mysql;
namespace asio = boost::asio;
namespace json = boost::json;

/**
 * An example Boost.Describe struct. Our code will work with any struct like this,
 * as long as it has metadata as provided by BOOST_DESCRIBE_STRUCT.
 * We will use this type as an example.
 */
struct employee
{
    std::string first_name;
    std::string last_name;
    std::string company_id;
    std::int64_t salary;  // in dollars per year
};
BOOST_DESCRIBE_STRUCT(employee, (), (first_name, last_name, company_id, salary))

// Retrieves all public data members from a Boost.Describe struct, including inherited ones.
// This is a Boost.Mp11 compatible type list.
template <class T>
using public_members = describe::describe_members<T, describe::mod_public | describe::mod_inherited>;

// The number of public members of a Boost.Describe struct
template <class T>
constexpr std::size_t num_public_members = mp11::mp_size<public_members<T>>::value;

// Gets the member names of a struct, as an array of strings.
// For employee, generates
// {"first_name", "last_name", "company_id", "salary"}
template <class T>
constexpr std::array<std::string_view, num_public_members<T>> get_field_names()
{
    return mp11::tuple_apply(
        [](auto... descriptors) {
            return std::array<std::string_view, num_public_members<T>>{{descriptors.name...}};
        },
        mp11::mp_rename<public_members<T>, std::tuple>()
    );
}

// A formatting function that generates an insert field list for any struct T with
// Boost.Describe metadata.
// For example, employee{"John", "Doe", "HGS", 20000} generates the string
// "('John', 'Doe', 'HGS', 20000)"
struct insert_struct_format_fn
{
    template <class T>
    void operator()(const T& value, mysql::format_context_base& ctx) const
    {
        // Convert the struct into a std::array of formattable_ref
        // formattable_ref is a view type that can hold any type that can be formatted
        auto args = mp11::tuple_apply(
            [&value](auto... descriptors) {
                return std::array<mysql::formattable_ref, num_public_members<T>>{
                    {value.*descriptors.pointer...}
                };
            },
            mp11::mp_rename<public_members<T>, std::tuple>()
        );

        // Format them as a comma-separated sequence
        mysql::format_sql_to(ctx, "({})", args);
    }
};

// Reads a file into memory
std::string read_file(const char* file_name)
{
    std::ifstream ifs(file_name);
    if (!ifs)
        throw std::runtime_error("Cannot open file: " + std::string(file_name));
    return std::string(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>());
}

// The main coroutine
asio::awaitable<void> coro_main(
    std::string_view server_hostname,
    std::string_view username,
    std::string_view password,
    const std::vector<employee>& employees
)
{
    // Create a connection.
    // Will use the same executor as the coroutine.
    mysql::any_connection conn(co_await asio::this_coro::executor);

    // The hostname, username, password and database to use
    mysql::connect_params params;
    params.server_address.emplace_host_and_port(std::string(server_hostname));
    params.username = username;
    params.password = password;
    params.database = "boost_mysql_examples";

    // Connect to the server
    co_await conn.async_connect(params);

    // Run the query. Placeholders ({}) will be expanded before the query is sent to the server.
    // We use sequence() to format C++ ranges as comma-separated sequences.
    mysql::results result;
    co_await conn.async_execute(
        mysql::with_params(
            "INSERT INTO employee ({::i}) VALUES {}",
            get_field_names<employee>(),
            mysql::sequence(std::ref(employees), insert_struct_format_fn())
        ),
        result
    );

    // 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 != 5)
    {
        std::cerr << "Usage: " << argv[0] << " <username> <password> <server-hostname> <input-file>\n";
        exit(1);
    }

    // Read our JSON file into memory
    auto contents = read_file(argv[4]);

    // Parse the JSON. json::parse parses the string into a DOM,
    // and json::value_to validates the JSON schema, parsing values into employee structures
    auto values = json::value_to<std::vector<employee>>(json::parse(contents));

    // We need one value to insert, at least
    if (values.empty())
    {
        std::cerr << argv[0] << ": the JSON file should contain at least one employee" << std::endl;
        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], values); },
        // 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 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;
    }
}