It is also an optimization hint, but AFAIK, modern compiler ignore it.
Need a way to make inlining heuristics ignore whether a function is inline https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93008
(Bug saw a few updates recently, that's how I remembered.)
As a workaround, if you need the linkage aspect of the inline keyword, you currently have to write fake templates instead. Not great.
It's not just "you can have multiple definitions of the same function" but rather a promise that the function doesn't need to be address/pointer equivalent between translation units. This is arguably more important than inlining directly because it means the compiler can fully deduce how the function may be used without any LTO or other cross translation unit optimisation techniques.
Of course you could still technically expose a pointer to the function outside a TU but doing so would be obvious to the compiler and it can fall back to generating a strictly conformant version of the function. Otherwise however it can potentially deduce that some branches in said function are unreachable and eliminate them or otherwise specialise the code for the specific use cases in that TU. So it potentially opens up alternative optimisations even if there's still a function call and it's not inlined directly.
Traditionally you'd use `static` for that use case, wouldn't you?
After all, `inline` can be ignored, `static` can't.
I can see exactly one use for an effect like that: static variables within the function.
Are there any other uses?
No, its purpose was and is still to specify a preference for inlining. The C++ standard itself says this:
> The inline specifier indicates to the implementation that inline substitution of the function body at the point of call is to be preferred to the usual function call mechanism.