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

user_1467600 0
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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.

c++14 stdvector pointers smart-pointers c++
2 Answers

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

user_123617000

The @Berto99's answer has mentioned the problem(i.e. UB) of calling the std::vector::back for a empty std::vector.

In addition, like @RemyLebeau mentioned, if you use the std::make_unique, it will always return the std::unique_ptr of an instance of type T(i.e. BaseClass).

I would like to add something to your actual question. If you want to check anything regarding the last insertion, you could use std::vector::emplace_back, which returns (since C++17) the references to the inserted element.

std::vector<std::unique_ptr<BaseClass>> vec;
auto& lastEntry = vec.emplace_back(std::make_unique<BaseClass>());

if (lastEntry) // pointer check
{
    // do something with the last entry!
}

As a plus over std::vector::push_back, your std::unique_ptr<BaseClass> will be constructed in-place.

user_96098400
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