SOCI provides mechanisms to bind local buffers for input and output data.
Note: The Oracle documentation uses two terms: defining (for instructing the library where the output data should go) and binding (for the input data and input/output PL/SQL parameters). For the sake of simplicity, SOCI uses the term binding for both of these.
Binding output data (into)
into expression is used to add binding information to
int count; sql << "select count(*) from person", into(count); string name; sql << "select name from person where id = 7", into(name);
In the above examples, some data is retrieved from the database and transmitted into the given local variable.
There should be as many
into elements as there are expected columns in the result (see dynamic resultset binding for the exception to this rule).
Binding input data (use)
use expression associates the SQL placeholder (written with colon) with the local data:
int val = 7; sql << "insert into numbers(val) values(:val)", use(val);
In the above statement, the first "val" is a column name (assuming that there is appropriate table
numbers with this column), the second "val" (with colon) is a placeholder and its name is ignored here, and the third "val" is a name of local variable.
To better understand the meaning of each "val" above, consider also:
int number = 7; sql << "insert into numbers(val) values(:blabla)", use(number);
Both examples above will insert the value of some local variable into the table
numbers - we say that the local variable is used in the SQL statement.
There should be as many
use elements as there are parameters used in the SQL query.
Object lifetime and immutability
SOCI assumes that local variables provided as
use elements live at least as long at it takes to execute the whole statement.
In short statement forms like above, the statement is executed sometime at the end of the full expression and the whole process is driven by the invisible temporary object handled by the library.
If the data provided by user comes from another temporary variable, it might be possible for the compiler to arrange them in a way that the user data will be destroyed before the statement will have its chance to execute, referencing objects that no longer exist:
// Dangerous code! string getNameFromSomewhere(); sql << "insert into person(name) values(:n)", use(getNameFromSomewhere());
In the above example, the data passed to the database comes from the temporary variable that is a result of call to
getNameFromSomewhere - this should be avoided and named variables should be used to ensure safe lifetime relations:
// Safe code string getNameFromSomewhere(); string name = getNameFromSomewhere(); sql << "insert into person(name) values(:n)", use(name);
It is still possible to provide
const data for use elements.
Note that some database servers, like Oracle, allow PL/SQL procedures to modify their in/out parameters - this is detected by the SOCI library and an error is reported if the database attempts to modify the
use element that holds
The above example can be ultimately written in the following way:
// Safe and efficient code string getNameFromSomewhere(); string const& name = getNameFromSomewhere(); sql << "insert into person(name) values(:n)", use(name);
Binding by position
If there is more output or input "holes" in the single statement, it is possible to use many
use expressions, separated by commas, where each expression will be responsible for the consecutive "hole" in the statement:
string firstName = "John", lastName = "Smith"; int personId = 7; sql << "insert into person(id, firstname, lastname) values(:id, :fn, :ln)", use(personId), use(firstName), use(lastName); sql << "select firstname, lastname from person where id = :id", into(firstName), into(lastName), use(personId);
In the code above, the order of "holes" in the SQL statement and the order of
use expression should match.
Binding by name
The SQL placeholders that have their names (with colon) can be bound by name to clearly associate the local variable with the given placeholder.
This explicit naming allows to use different order of elements:
string firstName = "John", lastName = "Smith"; int personId = 7; sql << "insert into person(id, firstname, lastname) values(:id, :fn, :ln)", use(firstName, "fn"), use(lastName, "ln"), use(personId, "id");
or bind the same local data to many "holes" at the same time:
string addr = "..."; sql << "update person" " set mainaddress = :addr, contactaddress = :addr" " where id = 7", use(addr, "addr");
Bulk operations allow the user to bind, as into or use element, whole vectors of objects.
This allows the database backend to optimize access and data transfer and benefit from the fact that
std::vector stores data in contiguous memory blocks (the actual optimization depends on the backend and the capability of the underlying data base server).
It is possible to
use the vector as a data source:
std::vector<int> v; // ... sql << "insert into t ...", use(v);
as well as a destination:
std::vector<int> v; v.resize(100); sql << "select ...", into(v);
In the latter case the initial size of the vector defines the maximum number of data elements that the user is willing to accept and after executing the query the vector will be automatically resized to reflect that actual number of rows that were read and transmitted. That is, the vector will be automatically shrunk if the amount of data that was available was smaller than requested.
It is also possible to operate on the chosen sub-range of the vector:
std::vector<int> v; // ... std::size_t begin = ...; std::size_t end = ...; sql << "insert into t ...", use(v, begin, end); // or: sql << "select ...", into(v, begin, end);
Above, only the sub-range of the vector is used for data transfer and in the case of
into operation, the
end variable will be automatically adjusted to reflect the amount of data that was actually transmitted, but the vector object as a whole will retain its initial size.
Bulk operations can also involve indicators, see below.
Bulk operations support user-defined data types, if they have appropriate conversion routines defined.