Member Avatar for ihatestarch

Studying for the final and can't remember the details behind the copy constructor. I know what it does, just don't know the "why" behind some stuff.

Here is an example of a copy constructor I have.

Person::Person(const Person& aPerson) //m_variable=private variables
{
m_name = aPerson.get_name(); 

m_pBirthDate = new Date; 
m_pBirthDate->set_year(aPerson.get_birthYear());
m_pBirthDate->set_month(aPerson.get_birthMonth());
m_pBirthDate->set_day(aPerson.get_birthDay());
}

Why is there an "m_pBirthDate =new Date"? I get everything else is copying over the info to the new variable... is it just pointing to a new date block to create the copy?

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Dani AI

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A short, practical addendum to the helpful replies from , and .

If a class "owns" a heap object via a raw pointer, the copy constructor must create a separate heap allocation so the two objects do not share the same memory. In modern C++ there are three common, safer approaches to handle this:

  • Prefer value semantics: make Date a plain member (for example Date m_birthDate;). If Date is small and copyable, the compiler-generated copy will do the right thing and you avoid manual memory management.
  • Use smart pointers: std::unique_ptr<Date> expresses exclusive ownership; std::shared_ptr<Date> allows shared ownership. Smart pointers remove most manual memory pitfalls, though they change copying semantics (unique_ptr is not copyable).
  • If you must use raw owning pointers, follow the Rule of Three (or Rule of Five in C++11+): implement a destructor, copy constructor, and copy-assignment operator (and move operations where appropriate). For assignment, consider the copy-and-swap idiom to get strong exception safety.

Practical tips: always free owned memory in the destructor, allocate before you change the current object when implementing assignment, and prefer RAII (resources tied to object lifetime). For formal details see the C++ reference on copy constructors and the Rule of Three: C++ copy constructor and Rule of Three.

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Member Avatar for Frederick2

A Copy Constructor has to completely generate a new Person object with the Person object passed as a parameter. Apparently one of the members of this Person object is a pointer to a struct/class that probably looks something like this...

class Date
{
 public:
 void set_year(short year);
 void set_month(short month);
 void set_day(short day)
 etc...
};

The newly created Person object can't use the memory allocations for the existing object passed as a parameter because if this later object gets destroyed/deleted/goes out of scope, the the new object will have bogus memory and will cause a crash. So every dynamically allocated member of the parameter class (the Person& parameter) will have to be duplicated by a memory allocation for the yet to be created object. That's what the 'new' is about. Then you'll have a completely independent object from the one passed in. So yes, you've answered your own question. It needs a new date memory block, otherwise, its sharing the other's, and won't be independent of it. Hope this helps.

Edited by Frederick2 because: n/a
Member Avatar for mrnutty

When you don't specify a copy constructor the compiler does a shallow
copy. Which means that it just copies the value from one class to another.

MyClass obj ( anotherMyClassObj ); //shallow copy

Now if inside MyClass contains a pointer variable( allocated by new).
Then that copy constructor does not copy the pointers value, but
they both point at the same location. This causes many troubles.

Member Avatar for ihatestarch

As I was typing out my question, I realized my answer, but decided to post anyway for a clear cut explanation. Thanks for organizing my thoughts! That was very helpful and clear.

A Copy Constructor has to completely generate a new Person object with the Person object passed as a parameter. Apparently one of the members of this Person object is a pointer to a struct/class that probably looks something like this...

class Date
{
 public:
 void set_year(short year);
 void set_month(short month);
 void set_day(short day)
 etc...
};

The newly created Person object can't use the memory allocations for the existing object passed as a parameter because if this later object gets destroyed/deleted/goes out of scope, the the new object will have bogus memory and will cause a crash. So every dynamically allocated member of the parameter class (the Person& parameter) will have to be duplicated by a memory allocation for the yet to be created object. That's what the 'new' is about. Then you'll have a completely independent object from the one passed in. So yes, you've answered your own question. It needs a new date memory block, otherwise, its sharing the other's, and won't be independent of it. Hope this helps.

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