Pointer Question

[Robert.C.Green@gmail.com]

I am currently passing multiple pointers to a function. Inside the
function I allocate the memory for these pointers. Once the function
completes and I return to the calling function, the pointers act as
though they have not been created. Can anyone tell me why this is
occuring? Thanks.

[=?iso-8859-1?q?Erik_Wikstr=F6m?=]

Re: Pointer Question

On 20 Mar, 14:06, Robert.C.Gr...@ gmail.com wrote:

I am currently passing multiple pointers to a function. Inside the
function I allocate the memory for these pointers. Once the function
completes and I return to the calling function, the pointers act as
though they have not been created. Can anyone tell me why this is
occuring? Thanks.

Let me guess, you have something like this:

void foo(MyClass1* ptr1, MyClass2* ptr2)
{
ptr1 = new MyClass1();
ptr2 = new MyClass2();
}

It's because you pass the pointers by value, so when you change them
inside the function (with new) it's only local copies of the pointers
you passed as parameters. You need to pass the pointers as references,
I think it's like this:

void foo(MyClass1*& ptr1, MyClass2*& ptr2);

--
Erik Wikström

[tragomaskhalos]

Re: Pointer Question

On Mar 20, 1:06 pm, Robert.C.Gr...@ gmail.com wrote:

I am currently passing multiple pointers to a function. Inside the
function I allocate the memory for these pointers. Once the function
completes and I return to the calling function, the pointers act as
though they have not been created. Can anyone tell me why this is
occuring? Thanks.

I'm guessing you're getting confused between pointers and pointers-to-
pointers and doing something like this:

void alloc(Thing* p)
{
p = new Thing; // WRONG
}
void f()
{
Thing* p;
alloc(p); // WRONG
p->use();
}

Whereas you need to do this:
void alloc(Thing** p)
{
*p = new Thing;
}
void f()
{
Thing* p;
alloc(&p);
p->use();
}

or this:
void alloc(Thing*& p)
{
p = new Thing;
}
void f()
{
Thing* p;
alloc(p);
p->use();
}

[Patrick]

Re: Pointer Question

Does this code help

void foo();

void foo(int *Test,int * Test2)
{
Test = new int; // Create new int
Test2 = new int;
*Test2 =22;
}

int main(int argc, char* argv[])
{
int *Test;
int *Test2;

foo(Test,Test);

*Test = *Test2;

cout << Test;

return 0;
}

Pat

[Fred Kleinschmidt]

Re: Pointer Question


"Patrick" <hornepat2@gmai l.comwrote in message
news:1174396733 .051827.80520@e 1g2000hsg.googl egroups.com...

Does this code help
>
void foo();
>
void foo(int *Test,int * Test2)
{
Test = new int; // Create new int
Test2 = new int;
*Test2 =22;
}
>
int main(int argc, char* argv[])
{
int *Test;
int *Test2;
>
foo(Test,Test);
>
*Test = *Test2;

No, it doesn't help. Test2 is uninitialized.

>
cout << Test;
>
return 0;
}
>

--
Fred L. Kleinschmidt
Boeing Associate Technical Fellow
Aero Stability and Controls Computing

[C C++ C++]

Re: Pointer Question

On Mar 20, 9:06 am, Robert.C.Gr...@ gmail.com wrote:

I am currently passing multiple pointers to a function. Inside the
function I allocate the memory for these pointers. Once the function
completes and I return to the calling function, the pointers act as
though they have not been created. Can anyone tell me why this is
occuring? Thanks.

Are you passing the reference of variable which need to be allocated ?

Regards,
M.Azmath

[Lionel B]

Re: Pointer Question

On Tue, 20 Mar 2007 06:18:53 -0700, Patrick wrote:

Does this code help

No, it's wrong (you obviously never tried even to compile, let alone run
it)

#include <iostream// for std::cout

void foo();

This is unnecessary

void foo(int *Test,int * Test2)

void foo(int*& Test,int*& Test2) // must pass by reference

{
Test = new int; // Create new int
Test2 = new int;
*Test2 =22;
}
>
int main(int argc, char* argv[])
{
int *Test;
int *Test2;
>
foo(Test,Test);

foo(Test,Test2) ; // I think this is what you intended

*Test = *Test2;
>
cout << Test;

std::cout << *Test; // assuming you wanted to print the value

return 0;
}

--
Lionel B