Assignment operator on class objects

hi,
Generally,
There are three general cases where the copy constructor is called instead of the assignment operator:

1).When instantiating one object and initializing it with values from another object .
2).When passing an object by value.
3).When an object is returned from a function by value.

The assignment operator is used to copy the values from one object to another already existing object.

MyClass myObject1(someValueHere); // calls MyClass constructor
MyClass myObject2; // calls MyClass default constructor
myObject2= myObject1; // calls MyClass assignment operator

In this case, myObject2 has already been created by the time the assignment is executed. Consequently, the MyClass assignment operator is called. The assignment operator needs to be overloaded as a member function.

A copy constructor is called if the object being copied into does not already exist.

MyClass myObject1(someValueHere); // calls Cents constructor
MyClass myObject2= myObject1; // calls Cents copy constructor!

Because the second statement uses an equals symbol in it, you might expect that it calls the assignment operator. However, it doesn’t! It actually calls a special type of constructor called a copy constructor. A copy constructor is a special constructor that initializes a new object from an existing object.

The purpose of the copy constructor and the assignment operator are almost equivalent — both copy one object to another. However, the assignment operator copies to existing objects, and the copy constructor copies to newly created objects.

do google for more
hope this helps!

For a) and b), it is already answered in other solutions...

For c), you should generally prefer Case 1 as it is a good pratice to initialize an object when it is declared. But you should consider using the follwing form instead:

Calculator obj2(obj1);

It is clear that you want to use the copy constructor and as far as I know event though the compiler optimize away the copy in case 1, you still need a default constructor and assignment operator for code to compile. Same with case 2. Thus the above syntax might works in some case where the other syntax is not allowed (for example if default constructor is not defined).


As a side note for parameter-less constructor, you effectively want to write it without empty ().

Calculator fn(); // Declare a function fn that return a Calculator by value.
Calculator obj; // Create a calculator object

The reason is essentially historical since function prototype were created before classes... and changing that (by adding a required function keyword for déclarations for example) would have break existing code.

For a) and b), it is already answered in other solutions...

For c), you should generally prefer Case 1 as it is a good pratice to initialize an object when it is declared. But you should consider using the follwoing form instead:

Calculator obj2(obj1);

It is clear that you want to use the copy constructor and as far as I know event though the compiler optimize away the copy in case 1, you still need a default constructor and assignment operator for code to compile. Same with case 2. Thus the above syntax might works in some case where the other syntax is not allowed (for example if default constructor is not defined).


As a side note for parameter-less constructor, you effectively want to write it without empty ().

Calculator fn(); // Declare a function fn that return a Calculator by value.
Calculator obj; // Create a calculator object

The reason is essentially historical since function prototype were created before classes... and changing would have break existing code.