Initialize static const std::map during compile time?

Question

I have a look up table in my C++ program and for now I have to initialize it at the beginning of the program using something like this:

static const map<string, int> m;
m["a"] = 1;
m["b"] = 2;
...

I am just wondering if there is anyway I can make this initialization process happen at compile time rather than run time? I understand this has very small impact of performance to my program. I am just curious that with in the scope of current C++11/14/17 semantic it is possible or not.

Initialization will happen at runtime regardless of what you do but you can use an [list initialization](http://en.cppreference.com/w/cpp/language/list_initialization) to make it easier and more idiomatic. — Captain Obvlious, Jul 25 '17 at 13:58
Allocation of dynamic memory is one of the things not allowed in `constexpr` routines, and `std::map` can't avoid it. — Ben Voigt, Jul 25 '17 at 13:58
`static const map m{{"a", 1}, {"b", 2}};` but it is still at runtime. — Jarod42, Jul 25 '17 at 13:59
You could use `std::array, xx>`, you can't use a map for `constexpr`. — Hatted Rooster, Jul 25 '17 at 13:59
@bopersson While closing as duplicate is great, closing as duplicate of a wrong answer seems unwise. — Yakk - Adam Nevraumont, Jul 25 '17 at 14:20
@Yakk - There are several answers, some of which are correct (and more upvoted than the selected answer). — Bo Persson, Jul 25 '17 at 14:23
@BoPersson Sure, but cannot we find one where the checkmark isn't misplaced? Failing that, we should make one. A poor cannonical dupe target is poor. — Yakk - Adam Nevraumont, Jul 25 '17 at 14:24

score 6 · Accepted Answer · answered Aug 17 '17 at 15:50

No, you can't initialize the std::map with data in compile time!

However, you can use this "fancier" initializer if you prefer, then you can have your data in a const std::map, in case this is what you are trying to do.

static const map<string, int> m = {
    { "a", 1 },
    { "b", 2 }
};

But AGAIN, this WILL NOT initialize the std::map itself in compile time. Behind the scenes, std::map will do the job in runtime.

Initialize static const std::map during compile time?

1 Answers1