+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. + +
+
+%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) +>>> +