Question

VS2010

I have a structure that has a unique_ptr inside. I then have a vector of this structure. If I try to resize (or use reserve on) the vector, I get compilation errors. Below is a stripped-down example that still exhibits the problem.

struct Test
{
    unique_ptr<int> pValue;
};

void test()
{
    // this doesn't compile
    vector<Test> testVector;
    testVector.resize(5);

    // this also doesn't compile
    testVector.reserve(5);

    // this, of course, compiles
    vector<unique_ptr<int>> testVector2;
    testVector2.resize(5);
    testVector2.reserve(5);
}

The errors I get are complaints about accessing private members of unique_ptr (the assignment operators). The compiler is trying to dynamically construct Test &Test::operator =(const Test &) and Test::Test(const Test &). I don’t see why a resize operation would need to call either of these functions (why doesn’t it just call the default constructor if it needs to?). Both of them present problems because it isn’t possible to make either with unique_ptr due to the const.

Answer 1

I hate to interrupt this conversation you’re having with yourself. 🙂

But the answer is that VS2010 does not yet fully implement the C++11 spec (which would require a bit of time travel). Test should have a defaulted noexcept move constructor that calls the unique_ptr move constructor. VS2010 does not implement this implicit Test move constructor. If it did, your complete example would compile and run as expected. vector will use noexcept move constructors to move things around when it needs to.