![]() |
Home | Libraries | People | FAQ | More |
We have already covered the static interface basics in the tutorials. This section expands on this topic.
To use the static interface, we must first define a data structure that describes the shape of our rows. We have several options:
struct
with BOOST_DESCRIBE_STRUCT
to enable reflection on it.
pfr_by_name
or pfr_by_position
to use PFR automatic reflection capabilities.
std::tuple
.
We will work with the following table:
CREATE TABLE employee( id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, first_name VARCHAR(100) NOT NULL, last_name VARCHAR(100) NOT NULL, salary INT UNSIGNED, company_id CHAR(10) NOT NULL, FOREIGN KEY (company_id) REFERENCES company(id) );
![]() |
Note |
---|---|
For simplicity, code snippets use C++20 coroutines. You can use the techniques
described here with lower C++ standards by using sync functions, callbacks
or |
Let's start with Boost.Describe. If we want to query the first three fields, we can define our row type like this:
// We can use a plain struct with ints and strings to describe our rows. struct employee_v1 { int id; std::string first_name; std::string last_name; }; // This must be placed at the namespace level. It adds reflection capabilities to our struct BOOST_DESCRIBE_STRUCT(employee_v1, (), (id, first_name, last_name))
And write the following to query our table:
mysql::static_results<employee_v1> result; co_await conn.async_execute("SELECT id, first_name, last_name FROM employee LIMIT 10", result); for (const employee_v1& emp : result.rows()) { // Process the employee as required std::cout << "ID: " << emp.id << ": " << emp.first_name << ' ' << emp.last_name << "\n"; }
Note that static_results::rows
returns a boost::span
object, which is a C++11 backport of
std::span
. The span points into memory owned by
the static_results
object.
Care must be taken not to use this view object after the static_results
goes out of scope.
Columns in the query are matched to fields in the struct by name. If a struct field cannot be matched to any query column, an error is issued. Extra columns in the query are ignored.
If your query contains columns with names that don't qualify as C++ identifiers, you can use SQL aliases. For example, given this struct:
struct statistics { std::string company; double average; double max_value; }; BOOST_DESCRIBE_STRUCT(statistics, (), (company, average, max_value))
You can write your query as:
// Summing 0e0 is MySQL way to cast a DECIMAL field to DOUBLE constexpr const char* sql = R"%( SELECT IFNULL(AVG(salary), 0.0) + 0e0 AS average, IFNULL(MAX(salary), 0.0) + 0e0 AS max_value, company_id AS company FROM employee GROUP BY company_id )%"; mysql::static_results<statistics> result; co_await conn.async_execute(sql, result);
The static interface will try to validate as soon as possible that the provided row type is compatible with the schema returned by the server. This process is known as metadata checking, and is performed before reading any data. The following checks are performed:
std::int32_t
is compatible with TINYINT
(1 byte integer), but not with
BIGINT
(8 byte integer).
For a full list of allowable field types, refer
to this table.
NULL
, your type
must account for it. You can use std::optional<T>
or boost::optional<T>
for these columns.
Let's add the salary
field
to our employee query. We might try the following:
// If we try to query the employee table with this struct definition, // an error will be issued because salary can be NULL in the database, // but not in the C++ type struct employee_v2 { int id; std::string first_name; std::string last_name; unsigned salary; }; BOOST_DESCRIBE_STRUCT(employee_v2, (), (id, first_name, last_name, salary))
However, this won't work because of the nullability check. In this case, the correct definition would be:
struct employee_v3 { int id; std::string first_name; std::string last_name; std::optional<unsigned> salary; // salary might be NULL in the database }; BOOST_DESCRIBE_STRUCT(employee_v3, (), (id, first_name, last_name, salary))
If you're using C++20 or above, you can use Boost.PFR to reflect types without
the BOOST_DESCRIBE_STRUCT
macro:
// employee_v4 doesn't contain any metadata - we're not using BOOST_DESCRIBE_STRUCT here struct employee_v4 { int id; std::string first_name; std::string last_name; std::optional<unsigned> salary; };
PFR reflection can be enabled in Boost.MySQL by using pfr_by_name
:
// pfr_by_name is a marker type. It tells static_results to use // Boost.PFR for reflection, instead of Boost.Describe. mysql::static_results<mysql::pfr_by_name<employee_v4>> result; // As with Boost.Describe, query fields are matched to struct // members by name. This means that the fields in the query // may appear in any order. co_await conn.async_execute("SELECT * FROM employee LIMIT 10", result); // Note that result.rows() is a span of employee_v4 objects, // rather than pfr_by_name<employee_v4> objects. employee_v4 // is the underlying row type for pfr_by_name<employee_v4> for (const employee_v4& emp : result.rows()) { // Process the employee as required std::cout << "ID: " << emp.id << ": " << emp.first_name << ' ' << emp.last_name << "\n"; }
Note that pfr_by_name
is what we call a marker type - an empty type that tells
classes like static_results
how to reflect a type. If no marker type is used, Boost.Describe is used to
retrieve reflection data for struct types.
pfr_by_position
is similar to pfr_by_name
,
but will match columns in the query to struct fields by position, rather than
name. It only requires C++17 to work. For instance:
// pfr_by_position is another marker type. // Fields in employee_v4 must appear in the same order as in the query, // as matching will be done by position. mysql::static_results<mysql::pfr_by_position<employee_v4>> result; co_await conn.async_execute("SELECT id, first_name, last_name, salary FROM employee", result); // The underlying row type is employee_v4 for (const employee_v4& emp : result.rows()) { // Process the employee as required std::cout << "ID: " << emp.id << ": " << emp.first_name << ' ' << emp.last_name << "\n"; }
Please refer to this table for a comparison with Boost.Describe.
You can also use std::tuple
s as row types. This can be handy for
simple queries:
mysql::static_results<std::tuple<std::int64_t>> result; co_await conn.async_execute("SELECT COUNT(*) FROM employee", result); std::cout << "Number of employees: " << std::get<0>(result.rows()[0]) << "\n";
Fields in tuples are matched to query columns by order. The query must return as many columns as fields the tuple has, at least. Any extra trailing columns in the query are ignored.
You can use both with the dynamic interface. Please refer to the sections on multi-resultset operations and multi-function operations for more information.
Should I use Boost.Describe, Boost.PFR or tuples? Each one has its advantages and drawbacks. This table may help you decide:
Technique |
Sample code |
Minimum C++ standard |
Comments |
Feature test macro |
---|---|---|---|---|
Boost.Describe |
// Definition should be at namespace scope struct employee { int id; std::string first_name; std::string last_name; }; BOOST_DESCRIBE_STRUCT(employee, (), (id, first_name, last_name))
// Usage mysql::static_results<employee> result; conn.execute("SELECT first_name, last_name, id FROM employee", result); |
C++14 |
|
|
Boost.PFR using names |
// Definition should be at namespace scope struct employee { int id; std::string first_name; std::string last_name; };
// Usage mysql::static_results<mysql::pfr_by_name<employee>> result; conn.execute("SELECT first_name, last_name, id FROM employee", result); |
C++20 |
|
|
Boost.PFR using field position |
// Definition should be at namespace scope struct employee { int id; std::string first_name; std::string last_name; };
// Usage mysql::static_results<mysql::pfr_by_position<employee>> result; conn.execute("SELECT id, first_name, last_name FROM employee", result); |
C++17 |
|
|
Standard tuples |
using tuple_t = std::tuple<int, std::string, std::string>; mysql::static_results<tuple_t> result; conn.execute("SELECT id, first_name, last_name FROM employee", result); |
C++14 |
|
|
Note that using the static interface always requires C++14, at least. The
BOOST_MYSQL_CXX14
test macro
is defined only if the static interface is supported. Including the static
interface headers on an unsupported compiler doesn't cause any error, but classes
like static_results
and static_execution_state
are not defined. The test macro is brought on scope by any of the static interface
headers.
All the types used within your Describe structs or tuples must be within the
following table. A Describe struct or tuple composed of valid field types models
the StaticRow
concept.
The following table is a reference of the C++ types that can be used in a
StaticRow
and their compatibility
with MySQL database types:
C++ type |
Compatible with... |
---|---|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Any type compatible with |
|
Any type compatible with |