# Contracts

A common problem that arises when wrapping C libraries is that of maintaining reliability and checking for errors. The fact of the matter is that many C programs are notorious for not providing error checks. Not only that, when you expose the internals of an application as a library, it often becomes possible to crash it simply by providing bad inputs or using it in a way that wasn't intended.

This chapter describes SWIG's support for software contracts. In the context of SWIG, a contract can be viewed as a constraint that is attached to a declaration. For example, you can easily attach argument checking rules, check the output values of a function and more. When one of the rules is violated by a script, a runtime exception is generated rather than having the program continue to execute.

## The %contract directive

Contracts are added to a declaration using the %contract directive. Here is a simple example:

>     %contract sqrt(double x) {
>     require:
>         x >= 0;
>     ensure:
>         sqrt >= 0;
>     }

>     ...
>     double sqrt(double);

In this case, a contract is being added to the `sqrt()` function. The `%contract` directive must always appear before the declaration in question. Within the contract there are two sections, both of which are optional. The `require:` section specifies conditions that must hold before the function is called. Typically, this is used to check argument values. The `ensure:` section specifies conditions that must hold after the function is called. This is often used to check return values or the state of the program. In both cases, the conditions that must hold must be specified as boolean expressions.

In the above example, we're simply making sure that sqrt() returns a non-negative number (if it didn't, then it would be broken in some way).

Once a contract has been specified, it modifies the behavior of the resulting module. For example:

>     >>> example.sqrt(2)
>     1.4142135623730951
>     >>> example.sqrt(-2)
>     Traceback (most recent call last):
>       File "", line 1, in ?
>     RuntimeError: Require assertion violation, in function sqrt( arg1>=0)
>     >>>

## %contract and classes

## Constant aggregation and %aggregate_check

## Notes

Contract support was implemented by Songyan (Tiger) Feng and first appeared in SWIG-1.3.20.