What does 'inline' mean?

[Steven Schveighoffer]

On 6/8/20 4:42 AM, evilrat wrote:
> On Monday, 8 June 2020 at 06:14:44 UTC, Manu wrote:
>>
>> I think a first part of the conversation to understand, is that since 
>> D doesn't really have first-class `inline` (just a pragma, assumed to 
>> be low-level compiler control), I think most people bring their 
>> conceptual definition over from C/C++, and that definition is a little 
>> odd (although it is immensely useful), but it's not like what D does.
>>
>> In C/C++, inline says that a function will be emit to the binary only 
>> when
>> it is called, and the function is marked with internal linkage (it is not
>> visible to the linker from the symbol table)
>> By this definition; what inline REALLY means is that the function is not
>> placed in the binary where it is defined, it is placed in the binary 
>> where
>> it is CALLED, and each CU that calls an inline function receives their 
>> own
>> copy of the function. From here; optimisers will typically inline the 
>> call
>> if they determine it's an advantage to do so.
>>
> 
> Sorry for sticking in my nose, but in result inline in C++ simply tells 
> the linker "this symbol could be present multiple times, just pick 
> whichever one of them and done with it" so it won't complain about 
> multiple symbols anymore.
> And that's why you have to slap inline to functions that have body in 
> header in order to build.
> 
> But I'm not very certain if that's all about it, not using C++ that much 
> since then, and never to that truly deep extent where any loose bit can 
> destroy performance, not beyond fixing the client's code and some crappy 
> pet projects.

This was not true the last time I found "inline abuse".

At a previous company, there was a type where a function was inline or 
not depending on whether you defined a macro. And when you defined that 
macro, the implementation was DIFFERENT (and different in a way that 
calling the non-inline version would be a bug).

Which meant that if the linker was free to choose whichever 
implementation it wanted, it would defeat the purpose of the inline 
mechanism (BTW, this was the first time I ever experienced an inline 
function, and had to research what it meant).

I think Manu's description is accurate, but I also thought it actually 
inlined the function call as well, even without an optimizer. Hard to 
tell when the end result isn't obvious.

-Steve