Constructors and class members

[Filip Ruszkowski]

Hi!

Take a quick look at the following code:

class Test {
private:
int val;
public:
Test(char *s)
{
val=1;
}
Test(int k)
{
val=2;
Test("aaa");
}
void print()
{
printf("%d\n", val);
}
};

int main(int argc, char* argv[])
{
Test *t;

t=new Test(2);
t->print();
delete t;
return 0;
}

The program stubbornly prints 2 every time I run it. I would have
thought that the Test(int) constructor should set val=2 and then the
Test(char *) constructor should set the value of val equal to 1. This
program should print 1. What is wrong?

Thanks,
Filip

[Jakob Bieling]

Re: Constructors and class members

"Filip Ruszkowski" <filiprus@hotma il.com> wrote in message
news:3ed27577.0 307240102.38767 dd9@posting.goo gle.com...[color=blue]
> Hi!
>
> Take a quick look at the following code:
>
> class Test {
> private:
> int val;
> public:
> Test(char *s)
> {
> val=1;
> }
> Test(int k)
> {
> val=2;
> Test("aaa");[/color]

This does not do what you think it does. See below.
[color=blue]
> }
> void print()
> {
> printf("%d\n", val);
> }
> };
>
> int main(int argc, char* argv[])
> {
> Test *t;
>
> t=new Test(2);
> t->print();
> delete t;
> return 0;
> }
>
> The program stubbornly prints 2 every time I run it. I would have
> thought that the Test(int) constructor should set val=2 and then the
> Test(char *) constructor should set the value of val equal to 1. This
> program should print 1. What is wrong?[/color]

You cannot 'call' a constructor. Every 'call' to a constructor creates a
new object. Above, you create a temporary object of type Test using the
Test(char*) c'tor. It is just a temporary and does not affect the current
object. This is why you will always see a '2' printed.

hth
--
jb

(replace y with x if you want to reply by e-mail)

[Robert Bauck Hamar]

Re: Constructors and class members

*Filip Ruszkowski wrote:[color=blue]
> class Test {
> private:
> int val;
> public:
> Test(char *s)
> {
> val=1;
> }
> Test(int k)
> {
> val=2;
> Test("aaa");
> }
> void print()
> {
> printf("%d\n", val);
> }
> };
>
> int main(int argc, char* argv[])
> {
> Test *t;
>
> t=new Test(2);
> t->print();
> delete t;
> return 0;
> }
>
> The program stubbornly prints 2 every time I run it. I would have
> thought that the Test(int) constructor should set val=2 and then the
> Test(char *) constructor should set the value of val equal to 1. This
> program should print 1. What is wrong?[/color]

You are wrong. Test::Test(int) first assigns the value 2 to val, and
then it creates a temporary Test, which is not used.

Try this, and see that the pointers are not equal.
--------------
#include <iostream>

class Test {
public:
Test(char*)
{ std::cout << "char*: " << this << '\n'; }

Test(int)
{ std::cout << "int : " << this << '\n'; Test("a"); }
};

int main()
{
Test t(5);
}

--------------

--
Robert Bauck Hamar

[Ron Natalie]

Re: Constructors and class members


"E. Robert Tisdale" <E.Robert.Tisda le@jpl.nasa.gov > wrote in message news:3F205FAE.2 060403@jpl.nasa .gov...
[color=blue]
> You *can* call your copy constructor like any other function
> that returns an object of type Test[/color]

You can't call constructors.
Constructors don't have return values.