inline in C++ and compiler

!! Specific on frequently used methods like getter & setter. !!

I have no idea when the keyword inline should be used. Ofc I know what it does, but I still have no idea.

According to an interview with Bjarne Stroustrup he said:

My own rule of thumb is to use inlining (explicitly or implicitly) only for simple one- or two-line functions that I know to be frequently used and unlikely to change much over the years. Things like the size() function for a vector. The best uses of inlining is for function where the body is less code than the function call and return mechanism, so that the inlined function is not only faster than a non-inlined version, but also more compact in the object core: smaller and faster.

But I often read that the compiler automatically inline short functions like getter, setter methods (in this case getting the size() of a vector).

Can anyone help?

Edit:

Coming back to this after years and more experience the high performance C+++ programming, inline can indeed help. Working in the games industry even forceinline sometimes makes a difference, since not all compilers work the same. Some might inline automatically some don't. My advice is if you work on frameworks, libraries or any heavily used code consider the use of inline, but this is just general advice anyway since you want such code to be fully optimized for any compiler. Always using inline might not be the best, because you'll also need the class definition for this part of the code. Sometimes this can increase compilation times if you can't use forward declarations anymore.

another hint: you can use C++14 auto return type deduction even with seperating the function definition:

MyClass.h

class MyClass
{
    int myint;
public:
    auto GetInt() const;
}  

inline auto MyClass::GetInt() const { return myint; }

all in one .h file.

I'm answering the question my self!: Solution: After a few performance tests, the rule of thumb from Stroustrup is right! inlining Short functions like the .size() from vector can improve the performance (.size() calls are used frequently). But the impact is only noticeable for FREQUENTLY used functions. If a getter/setter method is used a lot, inlining it might increase the performance.

Stroustrup:

Don’t make statements about “efficiency” of code without first doing time measurements. Guesses about performance are most unreliable.