C++ passing array of pointers to function - c++

Question

I have problem with passing array of pointers to a function.
Class CTree01 is child of CDecorationObj.
When I use for loop, it's working fine.
Here's my code:
CTree01 *trees01;
int numTree01 = ...;
trees01 = new CTree01[numTree01];
//main loop
for(int i=0;i<numTree01;i++)
{
{
//Everything is working okay here, renders all trees
glPushMatrix();
trees01[i].DrawOnTerrain(camera);
glPopMatrix();
}
}
but since i replaced for with a function, it's not working anymore:
void DrawDecorationType(CDecorationObj* objs, int number, CCamera *camera)
{
int x,z;
for(int i=0;i<number;i++)
{
{
glPushMatrix();
objs[i].DrawOnTerrain(camera);
glPopMatrix();
}
}
}
//main loop
DrawDecoration(trees01, numTree01, camera);
When I do in that function:
objs[0].DrawOnTerrain(camera);
it works, and it crashes only when I render objects with index > 0 so I think it must be problem with parameter passed to the function.

Answer 1

Let me see if I can explain the problem using couple of simple classes.
Let's say you have:
struct Foo
{
char a;
};
struct Bar : Foo
{
char b;
};
sizeof(Foo) is 1 and sizeof(Bar) is 2.
And you create an array of Bar object using:
Bar* barPtr = new Bar[2];
The layout of memory that barPtr points to looks like:
Bar(0) Bar(1)
| |
v v
+---+---+---+---+
| a | b | a | b |
+---+---+---+---+
If you pass that pointer to a function a Foo*, that function will interpret the memory as (since sizeof(Foo) == 1):
Foo(0)
| Foo(1)
| |
v v
+---+---+
| a | a |
+---+---+
As you can see, Foo(1) is not really a Foo object. It is really the Bar sub-object of Bar(0). This can easily lead to undefined behavior. Depending on the kinds of data a base class and a derived class have, it could easily crash your program.
The key point is that a pointer that points to an array of derived class objects cannot be treated as a pointer that points to an array of base class objects.

Answer 2

Very good explanation from R Sahu. It clearly describes why your code isn't working and deserves to be accepted.
So what can you do instead?
As far as I understand you want to pass all your CTree01 to a function in one call and have that function iterate all your CTree01s.
The only way I know is to make the container (i.e. array, vector) holding the base class pointers instead of CTree01.
Something like this:
class A
{
public:
A() { cout << "A constructor" << endl;}
virtual ~A() { cout << "A destructor" << endl;}
void hello() { cout << "Hello from A" << endl;}
};
class B : public A
{
public:
B() { cout << "B constructor" << endl;}
~B() override { cout << "B destructor" << endl;}
};
void f(array<A*,2>& t)
{
for(auto e : t)
{
e->hello();
}
}
int main()
{
array<A*,2> z; // Base class pointer array
z[0]=new B; // but it can still hold pointers to B
z[1]=new B;
f(z);
delete z[0];
delete z[1];
return 0;
}
or using vector instead of array
class A
{
public:
A() { cout << "A constructor" << endl;}
virtual ~A() { cout << "A destructor" << endl;}
void hello() { cout << "Hello from A" << endl;}
};
class B : public A
{
public:
B() { cout << "B constructor" << endl;}
~B() override { cout << "B destructor" << endl;}
};
void f(vector<A*>& t)
{
for(auto e : t)
{
e->hello();
}
}
int main()
{
vector<A*> z;
z.push_back(new B);
z.push_back(new B);
f(z);
delete z[0];
delete z[1];
return 0;
}