Initialize static std::map with unique_ptr as valu

问题:

How can one initialize static map, where value is std::unique_ptr?

static void f()
{
    static std::map<int, std::unique_ptr<MyClass>> = {
        { 0, std::make_unique<MyClass>() }
    };
}

Of course this does not work (copy-ctor of std::unique_ptr is deleted).

Is it possible?

回答1:

The Problem is that constructing from std::initializer-list copies its contents. (objects in std::initializer_list are inherently const). To solve your problem: You can initialize the map from a separate function...

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;
}

And then call it

static void f()
{
    static std::map<int, std::unique_ptr<MyClass>> mp = init();
}

See it Live On Coliru

回答2:

Writing bespoke crestion code seems boring and gets in the way of clarity.

Here is reasonably efficient generic container initialization code. It stores your data in a temporary std::array like an initializer list does, but it moves out instead of making it const.

The make_map takes an even number of elements, the first being key the second value.

template<class E, std::size_t N>
struct make_container_t{
  std::array<E,N> elements;
  template<class Container>
  operator Container()&&{
    return {
      std::make_move_iterator(begin(elements)),
      std::make_move_iterator(end(elements))
    };
  }
};
template<class E0, class...Es>
make_container_t<E0, 1+sizeof...(Es)>
make_container( E0 e0, Es... es ){
  return {{{std::move(e0), std::move(es)...}}};
}

namespace details{
  template<std::size_t...Is, class K0, class V0, class...Ts>
  make_container_t<std::pair<K0,V0>,sizeof...(Is)>
  make_map( std::index_sequence<Is...>, std::tuple<K0&,V0&,Ts&...> ts ){
    return {{{
      std::make_pair(
        std::move(std::get<Is*2>(ts)),
        std::move(std::get<Is*2+1>(ts))
      )...
    }}};
  }
}
template<class...Es>
auto make_map( Es... es ){
  static_assert( !(sizeof...(es)&1), "key missing a value?  Try even arguments.");
  return details::make_map(
    std::make_index_sequence<sizeof...(Es)/2>{},
    std::tie( es... )
  );
}

This should reduce it to:

static std::map<int, std::unique_ptr<MyClass>> bob = 
  make_map(0, std::make_unique<MyClass>());

... barring typos.

Live example.

回答3:

another way to do this is to use a lambda. it's the same as using a separate function but puts the map's initialisation closer to the action. In this case I've used a combination of an auto& and decltype to avoid having to name the type of the map, but that's just for fun.

Note that the argument passed into the lambda is a reference to an object that has not yet been constructed at the point of the call, so we must not reference it in any way. It's only used for type deduction.

#include <memory>
#include <map>
#include <utility>

struct MyClass {};


static auto& f()
{
  static std::map<int, std::unique_ptr<MyClass>> mp = [](auto& model)
  {
    auto mp = std::decay_t<decltype(model)> {};
    mp.emplace(0, std::make_unique<MyClass>());
    mp.emplace(1, std::make_unique<MyClass>());
    return mp;
  }(mp);
  return mp;
}

int main()
{
  auto& m = f();
}

Here's another way. In this case we've passed a temporary into the lambda and relied on copy elision/RVO.

#include <memory>
#include <map>
#include <utility>

struct MyClass {};

static auto& f()
{
  static auto mp = [](auto mp)
  {
    mp.emplace(0, std::make_unique<MyClass>());
    mp.emplace(1, std::make_unique<MyClass>());
    return mp;
  }(std::map<int, std::unique_ptr<MyClass>>{});
  return mp;
}

int main()
{
  auto& m = f();
}

And yet another way, using a lambda capture in a mutable lambda.

#include <memory>
#include <map>
#include <utility>

struct MyClass {};

static auto& f()
{
  static auto mp = [mp = std::map<int, std::unique_ptr<MyClass>>{}]() mutable
  {
    mp.emplace(0, std::make_unique<MyClass>());
    mp.emplace(1, std::make_unique<MyClass>());
    return std::move(mp);
  }();
  return mp;
}

int main()
{
  auto& m = f();
}