C++: Associative map containers with compile-time lookup

Post by hogliux » Tue, Oct 23, 2018, 10:41 AM UTC

quotemstr wrote: post #9

hogliux wrote: post #7

Earlier quoted context omitted.

Yes with the optional run-time lookup there is an extra two machine instructions (a cmp and jne) needed to check if this is the first time accessing the value. It is exactly as fast as a global variable if you don't need the optional run-time lookup. As stated in the talk, the main use case for a map like this is to cache objects which are expensive to load/compute (again something like `getFont` comes to mind). In t…

Not exactly as fast. Without -Bsymbolic, a global variabile with global symbol visibility gets its own GOT entry, meaning access to your global goes through an indirection table that supports ELF symbol interposition. (Which, IMHO, was a bad idea, but you can't unbreak an egg.) With a data structure like this (or with a conventional "struct globals"), there's one symbol at the ELF level to look up instead of one per…

Post by nemoniac » Tue, Oct 23, 2018, 10:45 AM UTC

Post by de_watcher » Tue, Oct 23, 2018, 11:09 AM UTC

nemoniac wrote: post #13

C++ is getting more and more like Lisp.

Post by w8rbt » Tue, Oct 23, 2018, 11:14 AM UTC

carlmr wrote: post #14

nemoniac wrote: post #13

C++ is getting more and more like Lisp.

A secret cabal of Lisp programmers took over the C++ steering committee.

Post by jstimpfle » Tue, Oct 23, 2018, 11:45 AM UTC

hogliux wrote: post #5

petters wrote: post #2

I can see how `semi::map` could be useful, but `semi::static_map` just seems to be an alternate syntax for global variables.

Yes, in a way you are right: the compiler really does boil it down to global variables. However, `semi::static_map` gives you some additional features, which may be useful for a lot of situations 1) `semi::static_map` gives you the optional run-time look-up with the same API (`semi::static_map::get`). Looking up a global variable with a runtime key isn't straight-forward. This makes it particularly nice to use an API…

    enum {
        KEY_FOO,
        KEY_BAR,
        KEY_BLAH,
        NUM_KEYS,
    };

    struct KeyInfo {
        const char *name;
        int info1;
        float info2;
    };

    const static struct KeyInfo keyInfo[NUM_KEYS] = {
    #define MAKE(x, y, z) [x] = { #x, y, z }
        MAKE( KEY_FOO,   1,   1.0 ),
        MAKE( KEY_BAR,   7,   3.5 ),
        MAKE( KEY_BLAH, 42, 127.2 ),
    #undef MAKE
    };

    void print_key(int key)
    {
        printf("%d's name is %s\n", key, keyInfo[key].name);
    }