Effective C++ “35. Minimize compilation dependenci

问题:

In this chapter Scott Meyer mentioned a few technique to avoid header files dependency. The main goal is to avoid recompiling a cpp file if changes are limited to other included header files.

My questions are:

• In my past projects I never paid attention to this rule. The compilation time is not short but it is not intolerable. It could have more to do with the scale (or the lack of) of my projects. How practical is this tip today given the advance in the compiler technology (e.g. clang)?

• Where can I find more examples of the use of this techniques? (e.g. Gnome or other OSS projects)

P.S. I am using the 2nd edition.

回答1:

I don't think compiler technology has advanced particularly. clang is not some piece of magic - if you have dependencies then and you make changes, then dependent code will have to be recompiled. This can take a very, very long time - read hours, or even days for a big project, so people try to minimise such dependencies where possible.

Having said that, it is possible to overdo things - making all classes into PIMPLs, forward declaring everything, etc. Doing this just leads to obfuscated code, and should be avoided whenever possible.

回答2:

Reducing compilation times is a red herring, and a form of premature optimization. Reorganizing your code to reduce compilation times (when this matters) can be done, but at a somehow great cost.

As for Gnome, Gnome has a "private pointer" in every GObject. This implements the pimpl idiom. This reduces dependencies between source files, and allow for some form of encapsulation. There are fewer compile time problems for C projects.

Modern C++ designs make heavy use of templates, which inevitably make your compilation times skyrocket. Using the pimpl idiom and forward declaring classes (instead of including a header, where possible) reduces the logical dependencies between translation units (this is a good thing), but in many situations do not really help with compilation times.

Using boost greatly increase compilation times (beware if you indirectly include boost headers in many source files), and many C++ projects use it.

I should mention also the thin template idiom is often used to reduce code bloat with templates.