Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for an old version of Boost. Click here to view this page for the latest version.
PrevUpHomeNext

Metadata

This example demonstrates how to use the available metadata in a results object. It employs synchronous functions with exceptions as error handling. See this section for more info on error handling.

This example assumes you have gone through the setup.

#include <boost/mysql/error_with_diagnostics.hpp>
#include <boost/mysql/handshake_params.hpp>
#include <boost/mysql/metadata_mode.hpp>
#include <boost/mysql/results.hpp>
#include <boost/mysql/tcp_ssl.hpp>

#include <boost/asio/io_context.hpp>
#include <boost/asio/ip/tcp.hpp>
#include <boost/asio/ssl/context.hpp>

#include <iostream>

#define ASSERT(expr)                                          \
    if (!(expr))                                              \
    {                                                         \
        std::cerr << "Assertion failed: " #expr << std::endl; \
        exit(1);                                              \
    }

void main_impl(int argc, char** argv)
{
    if (argc != 4)
    {
        std::cerr << "Usage: " << argv[0] << " <username> <password> <server-hostname>\n";
        exit(1);
    }

    // I/O context and connection. We use SSL because MySQL 8+ default settings require it.
    boost::asio::io_context ctx;
    boost::asio::ssl::context ssl_ctx(boost::asio::ssl::context::tls_client);
    boost::mysql::tcp_ssl_connection conn(ctx, ssl_ctx);

    // By default, string metadata (like column names) won't be retained.
    // This is for efficiency reasons. You can change this setting by calling
    // connection::set_meta_mode. It will affect any subsequent queries and statement executions.
    conn.set_meta_mode(boost::mysql::metadata_mode::full);

    // Hostname resolution
    boost::asio::ip::tcp::resolver resolver(ctx.get_executor());
    auto endpoints = resolver.resolve(argv[3], boost::mysql::default_port_string);

    // TCP and MySQL level connect
    boost::mysql::handshake_params params(
        argv[1],                // username
        argv[2],                // password
        "boost_mysql_examples"  // database to use; leave empty or omit the parameter for no
                                // database
    );
    conn.connect(*endpoints.begin(), params);

    // Issue the query
    const char* sql = R"(
        SELECT comp.name AS company_name, emp.id AS employee_id
        FROM employee emp
        JOIN company comp ON (comp.id = emp.company_id)
    )";
    boost::mysql::results result;
    conn.execute(sql, result);

    /**
     * results objects allow you to access metadata about the columns in the query
     * using the meta() function, which returns span-like object containing metadata objects
     * (one per column in the query, and in the same order as in the query).
     * You can retrieve the column name, type, number of decimals,
     * suggested display width, whether the column is part of a key...
     * These metadata objects are owned by the results object.
     */
    ASSERT(result.meta().size() == 2);

    // clang-format off
    const boost::mysql::metadata& company_name = result.meta()[0];
    ASSERT(company_name.database() == "boost_mysql_examples");  // database name
    ASSERT(company_name.table() == "comp");  // the alias we assigned to the table in the query
    ASSERT(company_name.original_table() == "company");   // the original table name
    ASSERT(company_name.column_name() == "company_name");  // the name of the column in the query
    ASSERT(company_name.original_column_name() == "name");  // the name of the physical column in the table
    ASSERT(company_name.type() == boost::mysql::column_type::varchar);  // we created the column as a VARCHAR
    ASSERT(!company_name.is_primary_key());     // column is not a primary key
    ASSERT(!company_name.is_auto_increment());  // column is not AUTO_INCREMENT
    ASSERT(company_name.is_not_null());         // column may not be NULL

    const boost::mysql::metadata& employee_id = result.meta()[1];
    ASSERT(employee_id.database() == "boost_mysql_examples");  // database name
    ASSERT(employee_id.table() == "emp");  // the alias we assigned to the table in the query
    ASSERT(employee_id.original_table() == "employee");  // the original table name
    ASSERT(employee_id.column_name() == "employee_id");   // the name of the column in the query
    ASSERT(employee_id.original_column_name() == "id");  // the name of the physical column in the table
    ASSERT(employee_id.type() == boost::mysql::column_type::int_);  // we created the column as INT
    ASSERT(employee_id.is_primary_key()); // column is a primary key
    ASSERT(employee_id.is_auto_increment()); // we declared the column as AUTO_INCREMENT
    ASSERT(employee_id.is_not_null()); // column cannot be NULL
    // clang-format on

    // Close the connection
    conn.close();
}

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 encoding
        // (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;
    }
}

PrevUpHomeNext