C++ – Interfaces vs Templates for dependency injection in C++


To be able to unit test my C++ code I usually pass the constructor of the class under test one or several objects that can be either "production code" or fake/mock objects (let's call these injection objects). I have done this either by

  1. Creating an interface that both the "production code" class and the fake/mock class inherits.
  2. Making the class under test a template class that takes the types of the injection objects as template parameters, and instances of the injection objects as parameters to the constructor.

Some random thoughts:

  • Until we have concepts (C++0x), only documentation and parameter naming will hint what to provide the class under test (when using templates).
  • It is not always possible to create interfaces for legacy code
  • The interface is basically only created to be able to do dependency injection
  • In the same way: templating the class under test is done only to enable dependency injection

What are your thoughts? Are there other solutions to this problem?

Best Solution

With C++, there's another option - you give your mock classes exact same names as the real classes, and when linking your unit tests, just link them with mock object/library files instead of real ones.