在什么情况下必须编译cpp代码并将其链接到exe中

Question

I am using MSVC++ with visual studio 2012, to build basic unmanaged c++ x86/x64 Windows and Console subsystem programs. (exception handling turned off)

I have a rather large project and when curious about the final exe size I decided to see how big it would be if I changed my main() function to just do nothing (so not calling or referencing any of the other code. I noticed that the exe size was still a lot bigger than when I went and removed all the cpp files from the project.

So I was wondering what types of C++ code would cause code to be linked in even if the main function does nothing. One example would be having a global variable in one of your cpp files of type A, where A was a class/struct. In this case I would assume that A's constructor and destructor would have to be included, as well as the constructors and destructor's of the types of all of A's members.

In visual studio I have all optimizations turned on including the linker ones to eliminate functions/data that is never referenced, and comdat folding. and the cpp code generation to enable comdats/function level linking.

Update: I am not using any C++11 or above functionality. Though I'm not sure if my build environment is set up to build for c++11 or above, or if this would make any difference.

Answer 1

Technically, a global doesn't need to be included just because it has a constructor. C++ follows a graph model, where main acts as the root Translation Unit. Function calls from one TU to another form the edges of the graph. A standalone TU might be skipped.

MSVC++ has another option, to use not the TU but individual functions as the nodes in the graph (/COMDAT). In combination with /OPT:ICF this reduces the executable size even further. Leaving out functions is a win by itself, but also can remove additional dependencies.

The downside of this model is that certain self-registration schemes are unportable, as the class doing the self-registration is not a dependency of anything and thus subject to removal.