C++ unique_ptr 可以采用 nullptr 值吗？
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Can a unique_ptr take a nullptr value?
提问by Zhen
Is this code fragment valid? :
这个代码片段有效吗？：
unique_ptr<A> p(new A());
p = nullptr;
That is, can I assign nullptrto a unique_ptr? or it will fail?
也就是说，我可以分配nullptr给 aunique_ptr吗？还是会失败？
I tried this with the g++compiler and it worked, but what about other compilers?
我用g++编译器尝试了这个并且它工作了，但是其他编译器呢？
回答by Andy Prowl
It will work.
它会起作用。
From Paragraphs 20.7.1.2.3/8-9 of the C++11 Standard about the unique_ptr<>class template:
来自 C++11 标准关于unique_ptr<>类模板的第 20.7.1.2.3/8-9 段：
unique_ptr& operator=(nullptr_t) noexcept;Effects:
reset().Postcondition:
get() == nullptr
unique_ptr& operator=(nullptr_t) noexcept;效果：
reset()。后置条件：
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 nullptrto a unique_ptris equivalent to resetting the unique_ptr.
这意味着类模板的定义unique_ptr<>包括一个重载，operator =它接受一个类型nullptr_t（例如nullptr）的值作为它的右手边；该段落还指定分配nullptr给 aunique_ptr等效于重置unique_ptr。
Thus, after this assignment, your Aobject will be destroyed.
因此，在此分配之后，您的A对象将被销毁。
回答by mrgloom
More common case:
更常见的情况：
#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:
输出：
A::A()
B::B()
B::~B()
A::~A()
This code example can be non-intuitive:
此代码示例可能不直观：
#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:
输出：
A::A()
B::B()
pA not nullptr!
A::~A()
B::~B()
