c++ Make pointer point to another inside a function - c++

Question

I'm working on something which lead me to a situation similar to the example below. I want b to point to a after on test(). Is it possible to this or creating a copy is the only solution?
Thanks.
void test(Object* a, Object* b)
{
std::cout << "test a: " << a << std::endl;
std::cout << "test b: " << b << std::endl;
b = a;
std::cout << "test b assigned: " << b << std::endl;
}
int main()
{
Object* a = new Object();
Object* b = nullptr;
test(a, b);
std::cout << "a: " << a << std::endl;
std::cout << "b: " << b << std::endl;
delete a;
delete b;
}
test a: 0x1aa8930
test b: 0
test b assigned: 0x1aa8930
a: 0x1aa8930
b: 0

Answer 1

In order to modify a pointer, you need to either:
Pass a pointer to your pointer
void test(Object **a, Object **b)
test(&a,&b);
After which you need to dereference that if you want to change the pointer.
Pass a reference to your pointer
void test(Object *&a, Object *&b)
test(a,b);
Here you won't have to dereference anything if you want to change the address.
For a, you don't necessarily have to pass a pointer to pointer or a reference to pointer (unless you plan to modify it too in the future), but for b you have to in order to modify it.
P.S. change your void main() to int main()

Answer 2

You can do like this:
void test(Object* a, Object*& b)
{
std::cout << "test a: " << a << std::endl;
std::cout << "test b: " << b << std::endl;
b = a;
std::cout << "test b assigned: " << b << std::endl;
}
int main()
{
Object* a = new Object();
Object* b = NULL;
test(a, b);
std::cout << "a: " << a << std::endl;
std::cout << "b: " << b << std::endl;
delete a, b;
}
Output will be:
test a: 0x7a0d28
test b: 0
test b assigned: 0x7a0d28
a: 0x7a0d28
b: 0x7a0d28
Its because in question, you have passed the value of b, not a reference to it, which is required to modify it. Variables can only be modified by a function if that function receives the address of those variables, and that can only can be done via either a pointer (Object**) or a reference (Object* &)

Answer 3

Nowadays we have something called shared_ptr, which does exacly what you want. Plus you don't have to clean it up after you're finished with it!
Example:
#include <memory>
#include <iostream>
using Object = int;
void test(const std::shared_ptr<Object>& a, std::shared_ptr<Object>& b)
{
b = a;
std::cout << "test b assigned: " << b.get() << "\n";
}
int main()
{
auto a = std::make_shared<Object>();
std::shared_ptr<Object> b;
std::cout << "test a: " << a.get() << "\n";
std::cout << "test b: " << b.get() << "\n";
test(a, b);
std::cout << "test a: " << a.get() << "\n";
std::cout << "test b: " << b.get() << "\n";
return 0;
}
output (g++ 5.4.0):
test a: 0x21e2c30
test b: 0
test b assigned: 0x21e2c30
test a: 0x21e2c30
test b: 0x21e2c30
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