无法复制到nullptr的unique_ptr

Question

It looks like creating nested vectors of unique_ptr to null throws attempting to reference a deleted function . I believe this is because it's trying to copy the vector unique_ptr(nullptr)'s and unique_ptr(nullptr) is uncopyable.

#include <memory>
#include <vector>
struct Foo {
};
int main() {
    std::vector<std::vector<std::unique_ptr<Foo>>> foo(5, std::vector<std::unique_ptr<Foo>>(5));
}

https://onlinegdb.com/SkvGkVYoQ

I'm not sure how to proceed. I just need a multi-dimensional array of nullptr's, and it'd be swell if they'd be unique - shared_ptr isn't needed other than fixing this issue.

Answer 1

std::unique_ptr is simply not copyable. That's unrelated to the nullptr . Simplest workaround is to just use a single dimensional array and map the dimensions.

Then you can do something like:

for (std::size_t j = 0; i < 5; ++j) {
    for (std::size_t i = 0; i < 5; ++i) {
        std::size_t index = j*5+i;
        foo.emplace_back(std::make_unique<Foo>());
    }
}

(You could apply a similar pattern with nested std::vector s but this way it's probably better anyway in regards to cache locality etc.)

Answer 2

if you ever need to nest multiple vectors due to each nested vector element within the vector having a different size, then you can use the std::move to move unique ptrs from one vector to another.

    std::vector<std::unique_ptr<Foo>> container1;
    std::vector<std::unique_ptr<Foo>> container2;
    for (int i = 0; i < 10; ++i)
    {
        container1.emplace_back(std::make_unique<Foo>());
    }

    for (int i = 0; i < 2; ++i)
    {
        container2.emplace_back(std::make_unique<Foo>());
    }

    std::vector<std::vector<std::unique_ptr<Foo>>> containers;
    containers.emplace_back(std::move(container1));
    containers.emplace_back(std::move(container2));

    return 0;

Mapping is the fastest solution though.