pointer

[fheyn]

hi,

void f(MyStruct* p)
{
p = new MyStruct;
}

is called like this : f(ptr1);

with ptr1 declared as MyStruct* ptr1;

I think passing the pointer as above is incorrect 'cause

p has a correct value but ptr1 is 0 after the function

call.

so what's wrong ?

thanks in advance

 

[trollacious]

The new MyStruct is local to f(), so the structure is lost when the function returns. However, since the pointer isn't deleted inside the function, you'll have a memory leak.

If you want to use the new MyStruct, you'll need to write something like this:

MyStruct* f(MyStruct* p)
{
   p = new MyStruct;
   return p;
}

Lee

 

[SDowd]

you could also pass in a reference of the pointer:

void f(MyStruct &p)
{
   p = new MyStruct;
}
[\code]

either way works.

 

[cpjust]

When you pass a pointer, you're actually passing a copy of a pointer, unless you pass the pointer as a reference like this:

void f(MyStruct*[COLOR=red]&[/color] p)
{
   p = new MyStruct;
}

but some people find that syntax a bit confusing or they prefer a more standard C way of doing it, which is to pass a pointer to the pointer you want to modify:

void f(MyStruct*[COLOR=red]*[/color] p)
{
   [COLOR=red]*[/color]p = new MyStruct;
}

 

[fheyn]

thank you guys,

I tried it, it works !

 

[SDowd]

cpjust, so then

void f(MyStruct &p);

creates a new "version" of the pointer by reference?

and

void f(MyStruct *&p);

actually references the same memory of the original pointer without creating a new object pointer?

 

[trollacious]

Your first example is to a structure named p, and is NOT a pointer to the structure. If you pass that function a pointer, you'll probably get an error when you compile unless you dereference the pointer when you call the function.

Lee

 

[SDowd]

troll,
is that directed to me?

 

[trollacious]

Yes, SDowd. Your first example passes the reference to a MyStruct structure, where the second pases a reference to a MyStruct pointer.

Lee