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"inline" is confusing in C++, as it is not really about inlining. Its purpose is to allow multiple definitions of the same function. It is useful when you have

• wredue 3y ago
  I believe the wording I’ve seen is that compilers may not respect the inline keyword, not that it is ignored.
• fweimer 3y ago
  GCC does not ignore inline for inlining purposes: Need a way to make inlining heuristics ignore whether a function is inline https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93008 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.
• lqr 3y ago
  10 years ago it was already folklore that compilers ignore the "inline" keyword when optimizing, but that was false for clang/llvm: https://stackoverflow.com/questions/27042935/are-the-inline-keyword-and-inlining-optimization-separate-concepts/27044943#27044943 https://stackoverflow.com/questions/27042935/are-the-inline-...
• CuriousCosmic 3y ago
  The thing with `inline` as an optimisation is that it's not about optimising by inlining directly. It's a promise about how you intend to use the function. 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.
• ack_complete 3y ago
  > "inline" is confusing in C++, as it is not really about inlining. Its purpose is to allow multiple definitions of the same function. 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. https://eel.is/c++draft/dcl.inline https://eel.is/c++draft/dcl.inline
• lelanthran 3y ago
  > It is useful when you have a function defined in a header file, because if included in several source files, it will be present in multiple object files, and without "inline" the linker will complain of multiple definitions. Traditionally you'd use `static` for that use case, wouldn't you? After all, `inline` can be ignored, `static` can't.
  • pjmlp 3y ago
    No, because that would make it internal to each object file, while what you want is for all object files to see the same memory location.
    • lelanthran 3y ago
      > No, because that would make it internal to each object file, while what you want is for all object files to see the same memory location. I can see exactly one use for an effect like that: static variables within the function. Are there any other uses?
      • pjmlp 3y ago
        Global variables and the magic of a build system based on C semantics.