CODEWITHSUNDEEP
c++unique-ptr
Hemant Bhargava
Hemant Bhargava

Reputation: 3585

Constructor deleted in unique_ptr

Was reading more about smart pointers and came across the concept of constructor getting deleted when you copy one unique_ptr to another. What exactly is that concept?

#include<iostream>
#include<memory>

class Person {
  public:
  int e;
  Person(int e) : e(e) { }
};

int main() {
  std::unique_ptr<Person> p (new Person(5));
  // Below line seems to be deleting constructor and thus error in compiling.
  std::unique_ptr<Person> q  = p;
}

std::move semantics are working fine though.

Upvotes: 3

Views: 11947

Answers (4)

Ravinder Rawat
Ravinder Rawat

Reputation: 1

copy construtor and copy assignment operator are deleted. So, unique pointer can't copy and assign to other object.

Upvotes: 0

Build Succeeded
Build Succeeded

Reputation: 1150

The unique_ptr class:

The class satisfies the requirements of MoveConstructible and MoveAssignable, but of neither CopyConstructible nor CopyAssignable.

The copy constructor and copy assignment operator do not work in the case of unique_ptr. The unique_ptr holds the ownership exclusively. It cannot be shared.

https://en.cppreference.com/w/cpp/memory/unique_ptr

Upvotes: 0

Persixty
Persixty

Reputation: 8589

Normal copy semantics will result in both q and p holding a pointer to the same Person object and both their destructors will then delete the same object.

That's invalid. You should delete the same allocation exactly once.

Move semantics however are permitted to modify the object being copied. In the case of std::unique_ptr<> the object is set to "empty" (i.e. assigned nullptr).

Upvotes: 1

drRobertz
drRobertz

Reputation: 3638

Since a unique pointer should be unique, it cannot be copied. It can only be moved.

Hence, the copy constructor is deleted.

Upvotes: 10

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