Alright, so this is a topic where I have very little exposure. So I feel like I need some sort of guidance to understand it all.
This has happened to me more than once. I will sometimes follow the breakpoints in visual studio which eventually lead me to STL code. For example, this is code from Math.h
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_Check_return_ _ACRTIMP short __cdecl _dclass(_In_ double _X);
_Check_return_ _ACRTIMP short __cdecl _ldclass(_In_ longdouble _X);
_Check_return_ _ACRTIMP short __cdecl _fdclass(_In_ float _X);
I don't know what it does, but the part that bothers me most is how unusual the syntax is.
For example, what does "_In_" do and why do they care to surround every input parameter with it? I never do that with my code.
Moreover, what are these functions even returning? I see like 3 or 4 variables or keywords that I've never seen before: "_Check_return_, _ACRTIMP, __cdecl"? Plus, why is the short there?
And you shouldn't. In C++, anything that has two underscores in a row (__cdecl) and anything tha tbegins with an underscore followed by a capital (_Check_return_, _ACRTIMP, _In_) are symbols reserved for use by the internals of the standard library, for their internal purposes. (see http://en.cppreference.com/w/cpp/language/identifiers#In_declarations )
Here you see a few different internal Microsoft-only things
_Check_return_ and _In_ are part of The Microsoft source-code annotation language.