When would you need to use a copy constructor

Hello,

I am doing some reading on copy constructors. I know that the compiler will create a default copy constructor.

But what is the real reason that people use copy constructors? This is the code I have been working with. I understand the coding ok.

Code:

class Mammal
{
 public:
 Mammal():itsAge(1) { cout << “Mammal constructor...\n”; }
virtual ~Mammal() { cout << “Mammal destructor...\n”; }
 Mammal (const Mammal & rhs);
 virtual void Speak() const { cout << “Mammal speak!\n”; }
 virtual Mammal* Clone() { return new Mammal(*this); }
 int GetAge()const { return itsAge; }
 protected:
 int itsAge;
};

Many thanks,

For classes that have dynamic memory.

Code:

class string
{
private:
  char* data;
  size_t len;
};

If you let the compiler create a default copy constructor then it will copy the pointer for data over. You don't want this. The default copy constructor might look like this:

Code:

string::string(const string& str) {
  data = str.data;
  len = str.len;
}

They would be sharing the same block of memory.
What you want is something like this:

Code:

class string
{
private:
  char* data;
  size_t len;

public:
  string(const string& str) {
    len = str.len;
    data = new char[len];
    memcpy(data, str.data, len);
  }
};

When you want to override the default behavior for the copy constructor or when the compiler cannot create a default copy constructor for you (for example, you have constant member data in your class).

or example, you have constant member data in your class

Constant members prevent generation of a copy assignment operator, not of the copy constructor. Same for references.

Right, didn't think of that, but the principle applies - sometimes the compiler cannot make a copy constructor or other constructors / operators.

I'd say the most prominent example is a member of std::auto_ptr, which is non-copyable.

Remember that there is a default copy constructor so it should be fine for most classes unless yours has a lot of different variables and structure.