I am playing around with c++ and faced this question

if I have a variant that looks like this:
using Fruit = variant<Apple, Tomato>

and a vector of unique_ptr of variant:
vector<unique_ptr<Fruit>> fruits

how can I move a unique_ptr<Apple> into the fruits vector?

here is an example piece of code:

#include <variant>
#include <memory>

using namespace std;

struct Apple {};
struct Tomato {};

using Fruit = variant<Apple, Tomato>;

int main() {
    vector<unique_ptr<Fruit>> fruits;
    unique_ptr<Apple> apple = make_unique<Apple>();
    fruits.push_back(unique_ptr<Fruit>(move(apple))); // error here
}

on the call to push_back I get this error:

No matching conversion for functional-style cast from 'remove_reference_t<unique_ptr<Apple, default_delete<Apple>> &>' (aka 'std::unique_ptr<Apple>') to 'unique_ptr<Fruit>' (aka 'unique_ptr<variant<Apple, Tomato>>')

if I change the line to fruits.push_back(unique_ptr<Apple>(move(apple))); I get this error:

No matching member function for call to 'push_back'

and if I change it to fruits.emplace_back(unique_ptr<Apple>(move(apple))); no error occurs.

so is using emplace_back the right choice here? why does this error occur? I am assuming it is because I can’t cast a unique_ptr<VariantMemberType> to unique_ptr<VariantType>?