How to initialize a vector of a structure with unique_ptr?

Question

How to initialize a vector of a structure with unique_ptr? For example:

#include <memory>
#include <vector>

using namespace std;

struct A
{
    int i;
    unique_ptr<int> p;
};

int main()
{
    vector<A> v{ { 10, make_unique<int>(10) } };
    // error above: cannot convert from initializer-list to vector<A>

    return 0;
}

score 2 · Accepted Answer · answered Apr 03 '14 at 00:26

2

vector<A> v{ { 10, make_unique<int>(10) } };

Within the inner braces of the statement above you're aggregate initializing an instance of A, so far so good.

But now there's no way to move this instance out of the initializer list, the only this you can do is copy the object. However, copying will fail because the compiler generated copy constructor for A is implicitly deleted because of the unique_ptr data member. Hence the compilation error.

The only way around this is to not use a braced-init-list. Instead construct an instance of A and then use

v.push_back(std::move(a));

answered Apr 03 '14 at 00:26

Praetorian

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Or you could create a copy constructor for A. – AndyG Apr 03 '14 at 00:35
@AndyG Sure, but how do you handle a `unique_ptr` within a copy-constructor? Making a copy of the object being pointed to seems kinda odd, but you could do that. – Praetorian Apr 03 '14 at 00:37
Yes, via copying. In OPs case, we're only copying an int. – AndyG Apr 03 '14 at 00:40

How to initialize a vector of a structure with unique_ptr?

1 Answers1