PRECOMPILED_CXX - why do we use this option?

Hi,

At my work, I saw bunch of other projects doing PRECOMPILED_CXX=1 in their corresponding source configuration files.

I searched on MSDN, and it gave me the following information.

Quote:

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This macro definition indicates whether the associated precompiled header will be used with files written in C or C++.

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I have a program written in C++, but I forgot to put the PRECOMPILED_CXX=1 in the build settings, but it compiled fine, and then worked fine. So what exactly is the reason why we should set this macro when building C++ project?

Code:

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# My source configuration file
...
PRECOMPILED_INCLUDE=pch.h
...

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Another interesting thing I noticed. When I don't set the macro, the compiler won't complain when I only include pch.h in .h files and not in their corresponding .cpp files. But it will complain as expected when I set the macro. Does it have to do with C vs C++ pre-compiled header usage? I am just beginning to learn C/C++ build settings, so any help would be appreicated!

Quote:

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Originally Posted by donglee

I have a program written in C++, but I forgot to put the PRECOMPILED_CXX=1 in the build settings, but it compiled fine, and then worked fine. So what exactly is the reason why we should set this macro when building C++ project?

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Because the documentation says so. It might work now, but could break in the future.

Do not taunt the MS build! ;)