constructor-destructor issue

[Micko]

I have a question about constructor. When is constructor called and when is not?
Look at this sample code

#include <iostream.h>

class SimpleClass
{
public:
	SimpleClass();
	~SimpleClass();
	friend ostream & operator<<(ostream &,SimpleClass&);
	int GetData();
	void SetData(int);
	void testfunction(SimpleClass);
private:
	int itsData;
};

SimpleClass::SimpleClass():itsData(0){cout<<"Default constructor called!"<<endl;}
SimpleClass::~SimpleClass(){cout<<"Destructor called!"<<endl;}

int SimpleClass::GetData(){return itsData;}
 
ostream& operator<<(ostream &o, SimpleClass &rhs)
{
	o<<rhs.GetData();
	return o;
}

void SimpleClass::testfunction(SimpleClass obj)
{	
	//why at this place default constructor is not called
	obj.SetData(2);
}//Destructor is called when obj goes out of scope

void SimpleClass::SetData(int i)
{
	itsData=i;
}

int main()
{
	SimpleClass obj; //this will cause message "Default constructor called"
	cout<<obj<<endl;
	obj.testfunction(obj);
	return 0;
}

When this is executed I get messages

Default constructor called!
0
Destructor called!
Destructor called!

Default constructor is called when I explicitly create object at line SimpleClass obj;
destructor is called when going out of function and at the end of the program, But why constructor is not called in function to create object obj or it is?
How can destructor be called and constructor cannot or some other constructor is called.
This is a bit confusing for me, so please help.

[Benzakhar]

Iam not good with con/destructors here. But if you do something like this in your main function;

int main()
{
                char s[20];
                cout << "push a button then enter to create object";
                cin >> s;
	SimpleClass obj; //this will cause message "Default 
                cout << "push a button to do the next line";
                cin >> s;
	cout<<obj<<endl;
                cout << "push a button then enter again to execute testfunction";
                cin >> s;
	obj.testfunction(obj);
                cout << "all done";
               cin >> s;
	return 0;
}

I added cin >> s; to pause so you can take your code stepbystep to help you understand more.

Hope that helps!

[Fordy]

>>obj.testfunction(obj);

You are passing by value and so are invoking the Copy Constructor...This is implicitly provided by the compiler, but you can add it to your class and track the object's creation when you create a copy

[ripper079]

Here is the "problem" why constructor/destructor isn´t called equally times. Because when user defined object are passed as argument

obj.testfunction(obj);

the parameter (obj in SimpleClass::testfunction)

void SimpleClass::testfunction(SimpleClass obj)
{	
	//why at this place default constructor is not called
	obj.SetData(2);
}//Destructor is called when obj goes out of scope

is calling the copy constructor. When you define and initialize an user defined object
e.i.

SimpleClass obj1;
SimpleClass obj2(obj1);//<-This line invokes the copyconstructor

the copy constructor is invoked. The same mechnism occur when passing user defined objects to functions.
Also be aware that some compiler optimization can reduce some constuctor- and destuctorcalls, e.i Named Return Value.

[Micko]

Thanks guys, I understand now