Use of default constructor over assignment operator

Question

Use of default constructor over assignment operator

srk24 (Programmer)

(OP)

6 May 12 19:46

Hi all,

Could some one please tell me what is the advantage of Default constructor over default assingment operator.
Is there something that can be accomplished only by constructor?

Regards,
Srk

xwb (Programmer) · Answer 1 · 2012-05-07 01:19:04

RE: Use of default constructor over assignment operator

xwb (Programmer) 7 May 12 01:19

Initialization of refrences.

xwb (Programmer) · Answer 2 · 2012-05-07 05:05:12

For example

CODE

int counter;
class Simple
{
public:
   Simple ()
   :  m_counter(counter)
   ,  m_lemon(0)
   {
   }
   Simple (const Simple& that)
   :   m_counter(that.m_counter)
   ,   m_lemon (that.m_lemon)
   {
   }
   Simple& operator =(const Simple& that)
   {
      // m_counter cannot be initialized here
      this->m_lemon = that.m_lemon;
      return *this;
   }
private:
   int& m_counter;
   int  m_lemon;
};

ArkM (IS/IT--Management) · Answer 3 · 2012-05-07 15:50:34

>what is the advantage of Default constructor over default assingment operator...
What's the advantage of motor plants over automobile sales centres?

You need contructors to create class objects from the scratch.
You need assignment operators to assign existent (created by constructors) objects.

In particular you need default constructor (not assignment operator) to define arrays of class objects (and STL containers too).

An example of a class which can't have correct assignment operator w/o a proper default constructor (othewise you can't declare arrays of this class objects or it's possible to create never-to-assign objects):

CODE

class WithDynArray
{
public:

private:
double* pData_; // Pointer to dynamic memory chunk
size_t dSize_; // This chunk number of elements
};
...
WithDynArray donttreadonme;
...

Let's suppose that WithDynArray has not default constructor. Well, the donttreadonme.pData_ member is not initialized (has garbage value - pointer to nowhere). You can't assign any WithDynArray value to this variable! An assignment operator must deallocate old (target) pData memory chunk but this pointer is undefined in donttreadonme!

Add default constructor:

CODE

...
WithDynArray(): pData_(0), dSize_(0) {}
...

That's OK: now you have a good object donttreadonme (don't forget to change its name now;) and can define correct assignment operator(s) for this class.

Moral: default constructors and assignment operators are not competitors.

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Use of default constructor over assignment operator