A common, but ugly, pattern in Gecko's C++ code is:

#ifdef DEBUG
  bool ok =

Why do this? Assertions are (usually) only checked in DEBUG builds. In release builds, there's no point in declaring the ok variable, because it's not going to be used. Worse, declaring a variable but not using it generates a compiler warning, and unused variables can additionally incur a small cost from being allocated a stack slot and/or register (depending on how the compiler optimizes the code). So, we write these ugly little warts to avoid compiler warnings and small runtime overhead from unused variables.

Using DebugOnly<T>

The DebugOnly<T> helper provides a cleaner way to avoid these compiler warnings and the runtime overhead mentioned above. With DebugOnly, the example above could be rewritten as:

    DebugOnly<bool> ok = FunctionWithReturnValueThatShouldAlwaysBeTrue();

In DEBUG builds, the ok variable behaves as if it had been declared bool ok, so it can be used in assertions etc. as in the example above. And in release builds, the ok variable doesn't occupy storage space or incur a runtime overhead. In fact, any attempt to use the value of ok in a release build generates a compiler error.

Warning: DebugOnly<T> is a "stack class", variables of which are only intended to be declared on the stack.  If you declare one as a member variable of a class, for example, it will behave differently than described above.  Don't do that!

See also