Initialize const static std::map with unique_ptr as value

Question

The question is similar with Initialize static std::map with unique_ptr as value . But in my question map should be readonly. so, how can i initilize static const map, where value is std::unique_ptr:

static const std::map<int, std::unique_ptr<MyClass>>

In this case initialization inside static function doesn't work,

std::map<int, std::unique_ptr<MyClass>> init()
{
    std::map<int, std::unique_ptr<MyClass>> mp;
    mp[0] = std::make_unique<MyClass>();
    mp[1] = std::make_unique<MyClass>();
    //...etc
    return mp;
}

Initilization via initilization list doesn't work as well

    const std::map<int, std::unique_ptr<MyClass>> = {
        { 0, std::make_unique<MyClass>() }

First, what is the specific compilation error you are getting? Second, please provide motivation for your static const map. It seems like there would be a much easier way to accomplish your requirements this using other design patterns. Also, given @felix 's comment, I think you have a typo with your description of "Initialization via initialization list...". — jhill515, Mar 29 '19 at 14:36
The initialization list requires copy construction, so doesn't work with unique pointers. Known issue on SO. — Matthieu Brucher, Mar 29 '19 at 14:44
Possible duplicate of [How to uniform initialize map of unique\_ptr?](https://stackoverflow.com/questions/17180673/how-to-uniform-initialize-map-of-unique-ptr) — Matthieu Brucher, Mar 29 '19 at 14:45

BiagioF · Accepted Answer · 2019-03-29T15:43:38.310

You cannot directly instantiate a const map with initializer list of std::unique_ptr because:

constructing from std::initializer-list copies its contents.

and you cannot copy std::unique_ptr.

Therefore, you can "dynamically" create the map (inside a function) and move the content into a const map.

That is:

using Map = std::map<int, std::unique_ptr<int>>;

static Map GenMap() {
  Map m;
  m[0] = std::make_unique<int>(0);
  // ...
  return m;
}

static const Map globalStaticMap = GenMap();

Lambda can avoid defining the static function making your code more compact.

static const Map globalStaticMap = []() {
  Map m;
  m[0] = std::make_unique<int>(0);
  // ...
  return m;
}();

An immediately-invoked lambda can be great here too. – Quentin Mar 29 '19 at 15:23 — Quentin, Mar 29 '19 at 15:23

Initialize const static std::map with unique_ptr as value

1 Answers1