Safe C++

Chapter 14. General Testing Principles

Although it is impossible to test code without concrete knowledge of what a particular program does, and how, there are nevertheless some general principles of testing that are useful to follow. Correctly designed and implemented code must produce the right answer when given correct inputs. Furthermore, when given incorrect ones, the program should not silently die, crash, or get stuck, but should diagnose the problem—where, why, and if necessary, when the error happened—and then either gracefully terminate or return to the initial state from which it can process the next input. Testing must include everything from unit tests of each single class, to unit tests of groups of classes working together, to a test of the whole application.

To the extent possible, you should try to create a reproducible test that leads to the same results when repeated. This can be a challenge when dealing with multi-threaded applications, when the timing of events between different threads is an issue, but even in cases like that it is usually possible to convert tests of some parts of the code to a single-threaded mode where the results should be totally deterministic.

In order to test multiple classes, organize them in a hierarchy such that some classes are considered more “basic” than others. In other words, the classes on one level of the hierarchy can make calls only to the classes on the same level or below, not above. Then the sequence of testing ...