When does the compiler do inline a function? C++

The compiler chooses. See, for instance inline specifier - cppreference.com[^].

A) Inlining is code optimization, and modern compilers are already way better at code optimization than programmers*. Because of that, leaving the decision to the compiler is almost always the best option. This wasn't always the case, so, in the past, it made sense for compiler builders to ask programmers for a hint, in the form of the inline keyword. C++11 however recognizes that this is no longer sensible.

B) virtual functions can not be inlined at compile-time, because which function has to be called has to be decided at run-time.**

P.S.:
*: unless you are the programmer who implemented the compiler optimization algorithms
**: many compilers will not inline even non-virtual functions if the function implementation is placed in a separate file. The reason is that the implementation file is typically not visible to any other compilation unit, and it would take a lot of extra effort to find the implementation code every time the compiler finds a call to that function. Modern compilers still may do that, but there's no guarantee.

OK -
A) is answered - ist Compiler Task like I guessed
B) was wether it makes sense to put short functions into declarations or not
in some forums I read that the Compiler cannot optimize when funcion is not in same file like the class declaration:

@Stefan_Lang: good Input, never read about that point of view.

Now I wrote a short test: seperated files or function Body in class declaration etc... using the Switches @Jochen Arndt told us.

At all: the Compiler does what it thinks is the best, also funbodies outside class declaration were optimized and inlined