Can a unique_ptr take a nullptr value?
Solution 1
It will work.
From Paragraphs 20.7.1.2.3/8-9 of the C++11 Standard about the unique_ptr<>
class template:
unique_ptr& operator=(nullptr_t) noexcept
;Effects:
reset()
.Postcondition:
get() == nullptr
This means that the definition of class template unique_ptr<>
includes an overload of operator =
that accepts a value of type nullptr_t
(such as nullptr
) as its right hand side; the paragraph also specifies that assigning nullptr
to a unique_ptr
is equivalent to resetting the unique_ptr
.
Thus, after this assignment, your A
object will be destroyed.
Solution 2
More common case:
#include <iostream>
#include <string>
#include <memory>
class A {
public:
A() {std::cout << "A::A()" << std::endl;}
~A() {std::cout << "A::~A()" << std::endl;}
};
class B {
public:
std::unique_ptr<A> pA;
B() {std::cout << "B::B()" << std::endl;}
~B() { std::cout << "B::~B()" << std::endl;}
};
int main()
{
std::unique_ptr<A> p1(new A());
B b;
b.pA = std::move(p1);
}
Output:
A::A()
B::B()
B::~B()
A::~A()
This code example can be non-intuitive:
#include <iostream>
#include <string>
#include <memory>
class A {
public:
A() {std::cout << "A::A()" << std::endl;}
~A() {std::cout << "A::~A()" << std::endl;}
};
class B {
public:
std::unique_ptr<A> pA;
B() {std::cout << "B::B()" << std::endl;}
~B()
{
if (pA)
{
std::cout << "pA not nullptr!" << std::endl;
pA = nullptr; // Will call A::~A()
}
std::cout << "B::~B()" << std::endl;
}
};
int main()
{
std::unique_ptr<A> p1(new A());
B b;
b.pA = std::move(p1);
}
Output:
A::A()
B::B()
pA not nullptr!
A::~A()
B::~B()
Zhen
I'm a Game Programmer at soul but currently I work as System Administrator. To see me rant: http://malcodigo.blogspot.com To see me walk: http://zhenpaseando.blospot.com To just see me: Napoli, Italia. My best known work: Doodle Hex for Nintendo DS, Cell & Love for Android and Altamira II Supercomputer for the RES.
Updated on July 16, 2020Comments
-
Zhen almost 4 years
Is this code fragment valid? :
unique_ptr<A> p(new A()); p = nullptr;
That is, can I assign
nullptr
to aunique_ptr
? or it will fail?I tried this with the g++ compiler and it worked, but what about other compilers?