The Friend-the-tests Pattern

The Friend-the-tests pattern involves friending test fixture classes from the class under test. Chromium has a macro named FRIEND_TEST_ALL_PREFIXES that makes this convenient to do.

Note: Friending test classes is usually not the best way to provide testing access to a class. It makes it far too easy to unintentionally depend on the class's implementation rather than its interface. It is better to use a proper testing interface than to friend test classes. That testing interface might be the TestApi pattern, or might be a simple subclass of the class under test which exposes normally-protected functionality publicly.

Use this pattern when:

A test needs simple access to the internals of a class, such as access to individual fields or methods.

Don't use this pattern when:

• Many different test classes need access to a class's internals: you will have to friend too many things and the class under test will end up coupled with too many unit tests.
• You're doing it to test private methods: in general, you should not separately test private methods; the correctness of the private methods should be observable from the correctness of the public methods, and if a piece of private behavior has no effect on the public behavior of a class, it might actually be unnecessary.

Alternatives / See also:

• TestApi
• ForTesting methods
• "Test the contract, not the implementation"
• Discussion thread on cxx@

How to use this pattern:

class Foo {
 public:
  // ... public API ...

 private:
  FRIEND_TEST_ALL_PREFIXES(FooTest, TestSomeProperty);
  FRIEND_TEST_ALL_PREFIXES(FooTest, TestABehavior);

  // or if every test in the suite needs private access:
  friend class FooTest;
};