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
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
In the context of this library, metadata refers to information describing a
column retrieved by the execution of a SQL query. The metadata
class contains information about an individual column.
You may access metadata using results::meta
or execution_state::meta.
There is a metadata object
per column retrieved by the query. The metadata objects are present even if
no row was returned by the query (e.g. a SELECT
on an empty table).
connection
objects have an associated metadata_mode
that describes how to handle metadata when running a query or a statement:
connection::meta_mode
is metadata_mode::minimal (the default), the library will
retain the minimal amout of data required to run the operation. Additional
information, like column names, won't be retained. Unless you are using
metadata explicitly, you should keep this default, as it consumes slightly
less memory.
connection::meta_mode
is metadata_mode::full, the library will retain all the
information provided by the server, including column names.
Only the metadata
members that are strings (database, table and field names) are affected by
this setting. You may change this setting using connection::set_meta_mode.
For example:
// By default, a connection has metadata_mode::minimal results result; conn.execute("SELECT 1 AS my_field", result); string_view colname = result.meta()[0].column_name(); // colname will be empty because conn.meta_mode() == metadata_mode::minimal BOOST_TEST(colname == ""); // If you are using metadata names, set the connection's metadata_mode conn.set_meta_mode(metadata_mode::full); conn.execute("SELECT 1 AS my_field", result); colname = result.meta()[0].column_name(); BOOST_TEST(colname == "my_field");