Call constructor in another

[Blair Craft]

hi,

I got a class have 2 constructors:

static int g_idx_counter = 0;

Object::Object():{
counter = g_idx_counter++;
created_at = last_used = time(NULL);
destroyed = false;
id = -1;
}
 
Object::Object(int _id){
Object();
id = _id;
}
 

when I use:

Object *o = new Object();

everything is OK, but when using the second constructor:

Object *o = new Object(32);

member variable "counter" will remain untouched, I did a gdb trace,
Object::Object() was invoked and inside that function "counter" was
initialized, when function returns "counter" went back to 0
again. Behavior is like a local variable inside a code chunk, but here
counter is a class member variable, anyone can shed some light?

thanks

[Kai-Uwe Bux]

Blair Craft wrote:

hi,

I got a class have 2 constructors:

static int g_idx_counter = 0;

Object::Object():{
   counter = g_idx_counter++;
   created_at = last_used = time(NULL);
   destroyed = false;
   id = -1;
}

Object::Object(int _id){
    Object();

 
The line Object() creates a temporary object of type Object and initializes
it using the constructor Object::Object(). It does not call the constructor
Object::Object() for the object currently being initialized by
Object::Object(int).
 
        
                    id = _id;
}
 
 
 

>
when I use:

Object *o = new Object();

everything is OK, but when using the second constructor:

Object *o = new Object(32);

member variable "counter" will remain untouched, I did a gdb trace,
Object::Object() was invoked and inside that function "counter" was
initialized, when function returns "counter" went back to 0
again. Behavior is like a local variable inside a code chunk, but here
counter is a class member variable, anyone can shed some light?

Constructors do not work like ordinary member functions. That's why they
are "special".
Best

Kai-Uwe Bux

[zhangyw80]

uh-huh, it's a common mistake for beginners

[Salt_Peter]

On Apr 28, 12:58 am, Blair Craft <[e-mail]:

hi,

I got a class have 2 constructors:

You actually have 3, if you don't declare a copy ctor, the compiler
generates one for you ( thats a hint ).

>
static int g_idx_counter = 0;

Object::Object():{
   counter = g_idx_counter++;
   created_at = last_used = time(NULL);
   destroyed = false;
   id = -1;

}

 
Use init lists.
// type int is a wild guess, whats type is id?
 
Object::Object( int n = -1 )
: counter(++g_idx_counter),
created_at(time(NULL)),
last_used(time(NULL)),
destroyed(false),
id(n)
{
}
 
        
                >
Object::Object(int _id){
    Object();
    id = _id;}
 
 
The above ctor is not needed, Object() is a local temporary, a
competant compiler would probably optimize it away with no observeable
affect to the result. The above only assigns id.
 
        
                >
 
 
 

>
when I use:

Object *o = new Object();

Object* p_obj = new Object;

>
everything is OK, but when using the second constructor:

Object *o = new Object(32);

member variable "counter" will remain untouched, I did a gdb trace,
Object::Object() was invoked and inside that function "counter" was
initialized, when function returns "counter" went back to 0
again. Behavior is like a local variable inside a code chunk, but here
counter is a class member variable, anyone can shed some light?

thanks

What you saw is expected, Why should member counter be modified by a
local Object() in any way?
Next time, please reproduce a basic reconstruction of the Object
class, since the order of its member declarations can affect the
inititialization list. Its also a pain to 'assume' what id might be.

[Mitesh]

On Apr 29, 8:27 am, Salt_Peter <[e-mail]:

On Apr 28, 12:58 am, Blair Craft <[e-mail]:

hi,

I got a class have 2 constructors:

You actually have 3, if you don't declare a copy ctor, the compiler
generates one for you ( thats a hint ).

static int g_idx_counter = 0;

 Object::Object():{
    counter = g_idx_counter++;
    created_at = last_used = time(NULL);
    destroyed = false;
    id = -1;

        
                 }
 

 

Use init lists.

// type int is a wild guess, whats type is id?

Object::Object( int n = -1 )

                        : counter(++g_idx_counter),

                          created_at(time(NULL)),

                          last_used(time(NULL)),

                          destroyed(false),

                          id(n)

{

}

        

                 Object::Object(int _id){

     Object();

     id = _id;}

 

The above ctor is not needed, Object() is a local temporary, a

competant compiler would probably optimize it away with no observeable

affect to the result. The above only assigns id.