Visual studio forces to include precompiled header file in all compilation units of the project?

Question

When compiler compiles source (e.g. *.cpp) file, it creates object file (e.g. *.o), so that later it will be linked to other .o and .so (.lib files for Windows) files and will constitute the executable file.

Now for analogical situation for not compiling header files each time it creates some .pch files so that it will be linked it by the linker then.

Now if in the scope of a Visula Studio project it is defined a precompiled header, then why Visual Studio complains with an error (e.g. **fatal error C1010: unexpected end of file while looking for precompiled header. Did you forget to add '#include "stdafx.h"' to your source?**) that the header file is not included into the .cpp file.

By summarizing, here are my questions:

1. Why in each .cpp file the precompiled header of the project is necessary?
2. How does the requirement of presence of precompiled header in each compilation unit optimizes the compilation process? In other words, what is the benefit of this requirement? (It could be up to the user to decide where to include, where not!)
3. If precompiled header is included into a .cpp file, which uses only 2% of what is in .pch file, then the remaining 98% will be added to corresponding .o file to?

Answer 1

Why in each .cpp file the precompiled header of the project is necessary?

Because you asked for it. If you don't want to use it then you need to change the option for the .cpp file. Right-click it, Properties, C/C++, Precompiled Headers, "Create/Use" = "Not using precompiled headers". The default setting is "Use". There's little point in doing this.

How does the precompiled header in each compilation unit optimizes the compilation process?

By not having to parse the #includes. Particularly useful when you #include <windows.h>. Time saved is in the order of seconds, on large projects with hundreds of .cpp files that adds up to many minutes. It is by far the cheapest way to speed up the compiler with no loss of quality on the generated code.

then the remaining 98% will be added to corresponding .o file to?

Of course not.

Answer 2

1. It's not actually necessary to include the precompiled header in every compilation unit. You can set the "not using precompiled header" setting for a single file in the C/C++->Precompiled Headers properties section for that file. This is a lot of work though, and I've never known anyone to do it in production code.
2. The optimization is that the precompiled header is built just once and the whole shebang is reused for all compilation units without recompiling / re-including (sort of). If you have a set of files that are included a lot of times this can save much compilation time. See The Care And Feeding of Precompiled Headers too.
3. No, you don't get superfluous file contents, just like you don't when static-linking.

Answer 3

1. I guess your question means "why can't the compiler assume that this header is always there if it is mandatory". The reason is that VS does not want to deviate from the standard so much. If your .cpp file is using something from a header file, it must include it. This way your file will compile exactly the same even if you turn off precompiled headers (only it will take longer).
2. As others have said, you can explicitly disable precompiled headers for individual files if you want. The idea is that you should only include in the precompiled header those elements that you are using in most, if not all of your files.
3. No, the resulting object code should be the same regardless of the header being precompiled or not.