How to prove that Copy Constructor is mandatory - c++

Question

I Have just created a class with an integer variable and a pointer variable. After creating its object , I passed it to a function. Even after returning the function the program is not throwing the exception
#include"iostream"
using namespace std;
class A
{
public :
int i;
char *c;
void show();
};
void func(A obj);
int main()
{
A a;
a.i = 10;
a.c = "string";
cout << " Before Fun " << endl;
a.show();
cout << " Going To Call func " << endl;
func(a);
cout << " After func " << endl;
a.show();
return 0;
}
void A::show()
{
cout << " The valuses in Object are " << i << '\t' << c << endl;
}
void func(A aa)
{
cout << " The valuses in Object are " << aa.i << '\t' << aa.c << endl;
}
In The Func I am passing the object a (from main) and it would get copied in aa (stack of func). so after returning from the func if i call show ( the pointer c would be null of a), It would give me exception
But it is not happening . please help me to prove the requirement of copy constructor

Answer 1

Hide the copy constructor. That will cause a compilation error everywhere it is called implicitly.
class A
{
public :
int i;
char *c;
private:
A(const A& _other);
};

Answer 2

If no copy constructor is declared for an object, one is implicitly defined. This copy constructor copies each element of the object.
In your example, the call to func(a) will call this copy constructor, and so aa will be a copy of a (aa.i will be 10 and aa.c will point to the first element of "string").

Answer 3

If you are using C++11 you can do the following to remove the copy constructor
class A
{
public :
int i;
char *c;
void operator=(const A& _other) = delete;
A(const A& _other) = delete;
};
or even better:
class A : public NonCopyable // perhaps std:: or if you prefer boost, boost::
{
public :
int i;
char *c;
};
http://en.wikipedia.org/wiki/C++11#Explicitly_defaulted_and_deleted_special_member_functions

Answer 4

When you make a class, a null constructor and copy constructor exist implicitly.
So that is why it does not throw an exception.
However, if you define ANY constructor, you will then need to define others otherwise the rest of the constructors will be overwritten.
For example, you define a null constructor only. Then it will throw an exception. Because the implicitly defined copy constructor will be overridden.
This is one way of proving the need of a copy constructor.

Answer 5

An implicit copy constructor does exist if you do not define one explicitly. It is called in your func() function. A object is copied and assigned a value for its member i.
Result will be: that you do get with show() the value for i that you had before when calling func. This is with no surprise because you are not doing any assignment to members of A inside A.
Implicit copy constructor by the compiler provide member-wise copy. This is what you may need in most cases.
When is default constructor not sufficient ? you have pointer like members, and need a deep-copy of it. You don't want to copy the pointer (what copy constructor would do), but rather to have the object pointed to copied. But why do you need deep copy ? You don't want to copy the pointer (which is owned by any instance of the class), and end up calling twice destructor on the same pointer. From the second call to delete on this pointer, heap corruption ensues. You may also need copy constructor made explicit for shared_pointers.
Note that, for performance reasons, it is best pass object by const reference than by value.

Answer 6

In your case the copy constructor provided by compiler is present.
Modify your code and write a copy constructor as:
//Default and copy constructor
A()
{
i = 0;
c = NULL;
}
A(const A& _other)
{
cout<<"In copy cons"<<endl;
}
When you will modify the code as per above when func gets called then your own copy constructor gets called in this case you will get the junk values(can not predict)
And if you modify your copy constructor code as
A(const A& _other)
{
cout<<"In copy cons"<<endl;
i = _other.i;
c = new char [(strlen(_other.c) +1)];
strcpy(c,_other.c);
}
Now you can see the differnce in both outputs and the use of Copy constructor how and where it's used.