C ++如何正确复制指针的容器（向量）？

Question

A few times I've stumbled across the scenario where I have a container of pointers that needs to be copied.

Let's say we have the following class hierarchy:

• Student (base class)

  • Freshman (subclass)
  • Sophmore (subclass)
  • Junior (subclass)
  • Senior (subclass)

• StudentService

The StudentService class has a std::vector<Student*> students field and the following constructor:

StudentService::StudentService(std::vector<Student*> students) {
   // code
}

It won't be correct to just use the std::vector::operator= operator and write this->students = students , because that will only copy the pointer addresses and so if someone from the outside deletes the objects pointed to by those pointers, then the StudentService class would suffer as a result.

The solution is to loop through each pointer in the students parameter and create a new dynamic object, something like this:

for(int i = 0; i < students.size(); i++) {
   this->students.at(i) = new Student(*students.at(i));
}

But even that is not proper due to the fact that it will create ONLY Student objects. And we know that a Student can be a Freshman, Sophmore, Junior or Senior. So here is my question: what's the best solution to this problem?

I guess one way would be to place a private enum field inside each Student class and have 4 if-else statements checking what type of Student it is and then creating a new dynamic object based on that like so:

 for(int i = 0; i < students.size(); i++) {
   if(students.at(i).getType() == FRESHMAN) {
      this->students.at(i) = new Freshman(*students.at(i));
   } else if(students.at(i).getType() == SOPHMORE) {
      this->students.at(i) = new Sophmore(*students.at(i));
   } else if {
   // and so on...
   }
}

But this still seems quite cumbersome, so what would you suggest?

Answer 1

You're looking for the Clone pattern. Add a clone() virtual function to Student, which is overridden in each descendant and creates the appropriate copy. Then write deep copy of containers as you've correctly specified.

Edit: my work assumption is that your Freshman, etc. classes are descending from Student. If not, use a variant<> and apply a copy visitor.

Answer 2

Resolving ownership issues

If you deem to shared the Student -s between your modules, then you are facing an ownership issue, and I would recommend using vector of std::shared_ptr<Student> -s to solve it.

If you have a std::vector<std::shared_ptr<Student>> , you can pass it to anyone you want. The receiver can copy the vector using an assignment operator and later on any objects he adds/removes won't affect the original container, just like you seem to desire.

Resolving cloning issues

If you are interested in each module having its own copy of a Student -s vector, you are facing a cloning issue.

You could resolve it by adding the following method to your classes:

class Student {
    [..]
    virtual Student * clone() const = 0; // Assuming Student is abstract, otherwise implement as well
};

class Freshman : public Student {
    [..]
    virtual Freshman * clone() const { return new Freshman(*this); }
};

// Same for other derived classes...

And then using std::transform to copy the vector:

// students is the original std::vector<Student *>
std::vector<Student *> copy(students.size());
std::transform(students.begin(), students.end(), copy.begin(), [](Student * s) -> Student * { return s->clone(); });

BTW, it's Sophomore, not Sophmore...