A copy constructor is a type of constructor which constructs the object of the class from another object of the same class. The copy constructor accepts a reference to its own class as a parameter. A valid copy constructor can be specified as, class A { ...... ...... public: A(&A); }
The purpose of the copy constructor is to construct a new object from an existing object of the same class. When passing objects to functions by value, the copy constructor is also called because a temporary object must be created (passing by reference or pointer does not call any constructor). The copy constructor is also used when assigning an existing object to a new object. Note that the assignment operator is only used when assigning an existing object to another existing object.
Consider the following example which demonstrates where and when the copy constructor is called:
#include
class Object
{
public:
Object():data(0){ printf("Default constructed at 0x%.8x with data %d\n",this,data); };
Object(const Object& object):data(object.data){ printf("Copy constructed at 0x%.8x with data %d\n",this,data); }
Object& operator=(const Object& object){ printf("Assign at 0x%.8x with data %d from 0x%.8x with data %d\n",this,data,&object,object.data); data=object.data; return(*this);}
~Object(){printf("Destroy 0x%.8x with data %d\n",this,data);}
void SetData(int i){data=i; printf("Mutate 0x%.8x with data %d\n",this,data);}
private:
int data;
};
void ByVal(const Object object )
{
return;
}
void ByRef(const Object& object )
{
return;
}
void ByPtr(const Object* object )
{
return;
}
int main()
{
printf("Calling default constructor:\n");
Object object1;
object1.SetData(1);
printf("Calling copy constructor:\n");
Object object2(object1);
object2.SetData(2);
printf("Calling copy constructor:\n");
Object object3 = object2;
object3.SetData(3);
printf("Calling assignment operator:\n");
object2 = object1;
printf("Calling ByVal:\n");
ByVal(object1);
printf("Calling ByRef:\n");
ByRef(object2);
printf("Calling ByPtr:\n");
ByPtr(&object3);
printf("Exiting:\n");
return(0);
}
Output:
Calling default constructor:
Default constructed at 0x0022fd60 with data 0
Mutate 0x0022fd60 with data 1
Calling copy constructor:
Copy constructed at 0x0022fd54 with data 1
Mutate 0x0022fd54 with data 2
Calling copy constructor:
Copy constructed at 0x0022fd48 with data 2
Mutate 0x0022fd48 with data 3
Calling assignment operator:
Assign at 0x0022fd54 with data 2 from 0x0022fd60 with data 1
Calling ByVal:
Copy constructed at 0x0022fc54 with data 1
Destroy 0x0022fc54 with data 1
Calling ByRef:
Calling ByPtr:
Exiting:
Destroy 0x0022fd48 with data 3
Destroy 0x0022fd54 with data 1
Destroy 0x0022fd60 with data 1
Note that all classes have a default constructor, a copy constructor, an assignment operator and a destructor, even if you do not declare them (the compiler generates them for you). However, if you declare any constructor, including the copy constructor, the default constructor is not generated for you unless you declare one yourself. But you cannot have a class that only has a copy constructor as it would be impossible to instantiate the objects you wished to copy from.
No. Java does not support copy constructor
A copy constructor usually refers to a constructor which takes an object, and returns a copy of that object. I can think of no way to overload the constructor without changing its functionality.
What is the advantage of user-defined copy constructor
More or less as you would any other function, except there is no return type (not even void) and an initialisation list can be placed between the declaration and the definition that follows it. The initialisation list allows your constructor to call base class constructors besides the default constructor as well as initialise member variables according to the parameters passed to your constructor. The constructor's name must be the name of the class. Note that if you don't declare any constructors, the compiler generates both a default and copy constructor. If any constructor is declared you lose the default constructor unless you declare one yourself. The copy constructor is always present but must be overridden if your class contains pointers to memory allocated to the class itself. If you don't, the compiler generated copy constructor will perform a member-wise copy of the member variables, resulting in a shallow copy of the pointers themselves, rather than a deep copy of the memory they point to. The copy constructor must accept a constant reference to the same class of object.
class X { public: X(); // default constructor X(const X& x); // copy constructor // ... }; int main(void) { X objx1; //normal ctor X objx2 = x1; // copy ctor X x3(x2); // copy ctor }
Java, unlike C++ does not support copy constructors.
The meaning of copy constructor is a special programmer of the C++ language that copys existing coding projects and enhances or makes them better projects.
Overloading a function simply means providing the same function name with different argument types. Class constructors are no different. In fact, even if you declare no constructors in a class, there will be two compiler-generated constructor overloads provided for you: a default constructor; and a copy constructor. If you declare any other constructors, the compiler-generated default constructor will no longer be generated. You must declare your own default constructor if you require one. The copy constructor is always generated, however the default implementation only performs a member-wise copy of the class members. If your class contains a pointer to allocated memory you must provide your own copy constructor to perform a deep-copy of those pointers, so each instances "owns" its own copy of the memory.
No. The parameter of the copy constructor is always passed by reference, specifically, a const reference to the class.Think about it. If it were passed by value, then the compiler would already know how to copy the object into the formal parameter, but the purpose of the copy constructor is to provide the code to copy the object, so its kind of a "cart before the horse" thing to think about call by value here.
A copy constructor gets called any time an object needs to be copied. Unlike in some of the newer languages like Java, you can chose to pass objects either by reference or by value. When you pass by reference, only the address of the function is copied. However, if you pass by value, the whole object must be copied. In order to copy the object, the copy constructor will get called. If the copy constructor's parameter is not a reference, then the object will get passed by value. When the object gets passed by value, it needs to get copied so it will call the same copy constructor. Because the object is still being passed by value it'll call itself again to create a copy of itself. The copy constructor will continue to call itself until the stack overflows.
Constructors are not a requirement of CPP. A default constructor and copy constructor are automatically generated by the compiler for every class you create when no constructors are declared. The only time you need to declare a constructor is when you wish to override the default behaviour of the generated constructors, to ensure the class is correctly initialised. When any constructor is declared, the default constructor is no longer generated by the compiler -- you must define your own default constructor (one that has no parameters, or where all the parameters have default values). The copy constructor is always generated for you regardless of how many other constructors you declare. But if the class contains pointers to allocated memory that is "owned" by the class then you must override the generated copy constructor with your own copy constructor. This is to ensure the memory is deep copied (the generated copy constructor only performs a shallow, member-wise copy of the members). Otherwise two objects of the same class will end up pointing at the same memory, which would be disastrous if either one were to be deleted. The other instance would be automatically invalidated because it would point to memory that was released by the other instance's destructor.
Default constructor: X()Copy constructor: X(const X&)Copy assignment operator: X& operator=(const X&)Move constructor: X(X&&)Move assignment operator: X& operator=(XX&)Destructor: ~X()By default, the compiler will generate each of these operations if a program uses it. However, if the programmer declares any constructor for a class, the default constructor for that class is not generated. If the programmer declares a copy or move operation, no copy, move or destructor is generated. If the programmer declares a destructor, no move operation is generated (a copy constructor is generated for backward compatibility).We can also suppress generation of specific operations with the =delete pseudo-initialiser:class X {public:X (const X&) =delete; // suppress the compiler-generated copy operationsX& operator=(const X&) =delete;// ...};