调用`shared_ptr.get（）`vs复制初始化，有什么区别？

Question

first version print use_count = 2;

shared_ptr<int> s{make_shared<int>(15)};
  auto b = s;
  cout<<s.use_count()<<endl;
  auto c = s.get();
  cout<<s.use_count()<<endl;
  cout<<*c<<endl;  

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second version use_count = 3;

shared_ptr<int> s{make_shared<int>(15)};
  auto b = s;
  cout<<s.use_count()<<endl;
  auto c = s;
  cout<<s.use_count()<<endl;
  cout<<*c<<endl;  

---

Question:

• Why is the behavior different between the two versions?

Answer 1

Introduction

Every time you make a copy of a shared_ptr the use-count is increased since you then have an additional handle to the underlying resource being tracked.

In order to obtain the value of the underlying pointer, shared_ptr has a member-function named get . This function will return the address of the tracked resource, it will not create an additional shared_ptr tracking the same pointer.

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Potential dangers

In your first snippet you effectively have 2 instances of shared_ptr that refers to the same resource, but in the latter you have three.

The difference being;

• auto a = s will create a copy of s , and the type of a is that of s (a shared_ptr )
• auto b = s.get () will initialize b with the address of the tracked resource managed by s , hence it will be a raw-pointer .

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If you don't know exactly what you are doing, calling .get () might be dangerous, as can be seen in the following snippet.

int * ptr = nullptr;

{
  std::shared_ptr<int> sp { make_shared<int> (15) };
  ptr = sp.get ();
}

std::cout << *ptr << std::endl; // (A), dangling pointer

Inside our nested scope we create a shared_ptr named sp , and we ask it to keep track of an int with dynamic storage duration created from our call to make_shared .

We obtain the address of this int by calling sp.get () , and assign it to ptr .

When the scope of sp ends, sp will release the resource managed since there's no more *shared_ptr*s that refers to it.

By trying to print the value of *ptr we are invoking undefined behavior in ( A ) since the value isn't available, it was destroyed with the destruction of sp .