Precompiled Headers In C++

Usually when you include a header file in any C++ file, all of the contents of the header file is copied and compiled into the C++ file. This isn't so bad for a single header file, but if the header file also included several other header files, they also have to be compiled. So if you have a large project, you can expect the compile times to be very long without using precompiled headers. By precompiling headers, it converts a set of headers into a single file that uses the binary format in which the compiler can read and this is only done once. This however doesn't work if you add header files that you will be constantly or even occasionally changing as it means that the precompiled header now has to be recompiled every time a change is made.

Precompiled headers are usually used for headers that are going to be used often throughout the project such as the standard libary or any external libraries that you might be using. This is because they aren't going to change (unless there is a major issue with a version of an external library, which causes you to update that library). You can also use it for your own libraries but as mentioned above it is only worth doing if that library you have created is complete and isn't going to change or isn't likely to change.

There is a disadvantage to using precompiled headers and that is that it can hide the dependancies of a C++ file. Normally by just including whatever header files in a C++ file, you can easily tell what it requires. This means that if you were to reuse that C++ file within another project, you already know what else you will need if you don't already have it within that project. However, this is not the case with precompiled headers as it most likely has a ton of header files that are being used throughout the project but not specifically for the C++ file that you want to reuse. Therefore, if you did reuse that file you would have to figure out the header files that the file depends on.

Also if you are only going to use a header file in a single C++ file then it is not worth adding to the precompiled headers file. This is because that header file is more than likely going to be abstracted into your own api for you to use. An example of this would be the GLFW header file.

Below is a useful video on the subject, in which these notes are based on, but also the set up needed to use precompiled headers within a C++ project.