C++ Programming – Understanding Copy Constructor and Assignment Operator

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If I override operator= will the copy constructor automatically use the new operator? Similarly, if I define a copy constructor, will operator= automatically 'inherit' the behavior from the copy constructor?

Best Answer

No, they are different operators.

The copy constructor is for creating a new object. It copies an existing object to a newly constructed object.The copy constructor is used to initialize a new instance from an old instance. It is not necessarily called when passing variables by value into functions or as return values out of functions.

The assignment operator is to deal with an already existing object. The assignment operator is used to change an existing instance to have the same values as the rvalue, which means that the instance has to be destroyed and re-initialized if it has internal dynamic memory.

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C++ Copy Constructors and Assignment Operators Explained

No assignment operator is used in the first test-case. It just uses the initialization form called "copy initialization". Copy initialization does not consider explicit constructors when initializing the object.

struct A {
  A();

  // explicit copy constructor
  explicit A(A const&);

  // explicit constructor
  explicit A(int);

  // non-explicit "converting" constructor
  A(char const*c);
};

A a;
A b = a; // fail
A b1(a); // succeeds, "direct initialization"

A c = 1; // fail, no converting constructor found
A d(1); // succeeds

A e = "hello"; // succeeds, converting constructor used

Copy initialization is used in those cases that correspond to implicit conversions, where one does not explicitly kick off a conversion, as in function argument passing, and returning from a function.

C++ – Implementing Copy Constructor and Copy Assignment Operator

Your copy assignment operator is implemented incorrectly. The object being assigned to leaks the object its base points to.

Your default constructor is also incorrect: it leaves both base and var uninitialized, so there is no way to know whether either is valid and in the destructor, when you call delete base;, Bad Things Happen.

The easiest way to implement the copy constructor and copy assignment operator and to know that you have done so correctly is to use the Copy-and-Swap idiom.

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C++ Copy Constructors and Assignment Operators Explained

C++ – Implementing Copy Constructor and Copy Assignment Operator

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