CODEWITHSUNDEEP
c++constructorcopy
jeromintus
jeromintus

Reputation: 481

working with parameter from the constructor

I have following problem: whats the best way to work on with a object which i got as parameter from my constructor? I don't want to work with the copy because, the initial object won't change.

Obj obj;

myclass::myclass(Obj& obj)
{
  this->obj = obj; //copy
}

void myclass::doSometh()
{
 obj.add(....); //working with the copy
}

My solution would be to use a pointer:

Obj* obj;

void myclass::myclass(Obj& obj)
{
  this->obj = &obj; 
}

void myclass::doSometh()
{
 obj->add(....); 
}

or 

    Obj* obj;

   myclass::myclass(Obj* obj)
    {
      this->obj = obj; 
    }

    void myclass::doSometh()
    {
     obj->add(....); 
    }

what do you thing about my solution? or are there better solutions?

Upvotes: 1

Views: 90

Answers (2)

Chris Drew
Chris Drew

Reputation: 15334

There is nothing wrong with using a pointer member-variable to avoid making a copy. But you do need to be confident that the object being pointed to, Obj, will outlive the MyClass which contains the pointer.

Also, I recommend using the constructor initializer list to initialize the pointer directly:

class MyClass {
 private:
  Obj* obj_;
 public:
  MyClass(Obj* obj) : obj_(obj) { }
};

In a similar way you could also have a reference member-variable but it restricts what you can do with the class. e.g it will not be assignable:

class MyClass {
 private:
  Obj& obj_;
 public:
  MyClass(Obj& obj) : obj_(obj) { }
};

If you are not confident of the lifetime of the object being pointed to consider std::weak_ptr or std::shared_ptr:

#include <memory>

struct Obj { };

class MyClass {
 private:
  std::weak_ptr<Obj> obj_;
 public:
  MyClass(std::weak_ptr<Obj> obj) : obj_(std::move(obj)) { }
  void doSomething() {
    std::shared_ptr<Obj> obj = obj_.lock();
    if (obj) {
      // do something with obj...
    }   
  }
};

int main() {
  std::shared_ptr<Obj> obj = std::make_shared<Obj>();
  MyClass mc(obj);
}

Upvotes: 3

clcto
clcto

Reputation: 9648

Just use a reference:

 Obj& obj;

 myclass::myclass( Obj& obj ) : obj(obj) {}

Upvotes: 1

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