How to check if a std::unique_ptr is null, if it is in a std::vector?

Question

I have a vector<unique_ptr<BaseClass>>, and I am adding new items to it by calling vec.push_back(std::make_unique<DerivedClass>()).

How can I check for nullptr using operator bool()?

I tried to use vec.back() directly, like so:

if((!vec.empty() && vec.back())
{
  // yay!
}
else
{
  //nay!
}

but it always returns with false, regardless of the contents of the pointer.

Answer 1

As you can read from here, if the vector is empty, it's UB. If it's not your case, as you can read from here instead, unique_ptr has a operator bool() which checks whether an object is currently managed by the unique_ptr

So, with:

vector.empty();

You can check if the vector has elements, and with:

vector<unique_ptr<something>> vec;
vec.push_back(make_unique<something>());
if(vec.front()){ // example
    // do something
}

you check whether the first unique_ptr is pointing to an object or not.

PS: if you always use vec.push_back(std::make_unique<DerivedClass>()), you will never have a unique_ptr that holds a nullptr