What I Have Against Precompiled Headers

Precompiled headers is a technique to reduce compilation time when your #includes take a long time. Basically, you stuff all your #includes that rarely change into one single file, which you then include in all your other files. You then tell the compiler to precompile this header, so it takes a negligible amount of time to include later on.

Instead of doing:

//my_file.cpp
#include "my_file.h"
#include "3rdparty/fancy_stuff.h"
#include "boost/expensive_lib.h"
#include <string>

you do

//my_file.cpp
#include "stdafx.h" //Common name for precompiled headers in Visual Studio
#include "my_file.h"

//stdafx.h
#include "boost/expensive_lib.h"
#include "3rdparty/fancy_stuff.h"
#include <string>

Say compiling "boost/expensive_lib.h" and "3rdparty/fancy_stuff.h" takes three seconds, and you have twenty files including these, the files themselves beeing trivial to compile, taking half a second each. That means compilation will take 20*(3s + 0.5s) = 70 seconds. With precompiled headers, this takes you 20*0.5s + 3s = 13 seconds. And the next time you compile, stdafx.h is already compiled, so it only takes you 10. That’s roughly an order of magnitude quicker.

A great improvement, no?

While it improves compilation time, it certainly does not improve readability and maintainability. Personally, I think of it as a bit of an ugly hack.

There are at least three problems with this:

1: As your project evolves, stdafx.h can get quite crowded. Come refactoring-time, you want to remove #includes which are no longer in use. While this can be challenging enough in a large cpp-file, imagine having one file holding includes from tens of others. Now you don’t only have to look through the current file to see if a certain #include can be removed, you have to look through you entire project.

2: stdafx.h now has to be the first header you include, which violates best pratice for include order.

3: Precompiled headers make .cpp-files quicker to compile. But what about when a header itself depends on other expensive headers? Normally, these need to be included in the header itself, leading to long compilation time for everyone using this header. A popular technique with precompiled headers is to not include these in the header. Since all .cpp-files will have included stdafx.h before including this header, everything will be ok. In addition to the problems laid out in 2, this poses a bigger problem when this header-file is included in .cpp-files in other projects, or even other solutions. If someone now wants to include my my_header.h, it is suddenly their responsibility to include headers that my_header.h uses internally.

That about sums it up. Consider precompiled headers added to my list of “necessary evils in C++”.

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