best way to pass value through constructors?

[mzimmers]

Hi -

Assume you define a class to contain another class, which in turn contains *another* class. During construction of the top-level class (if possible) you'd like to set an attribute of the lowest class.

So:

class Reg {int i1, i2, i3;}
class Cell (Reg a, b, c, d;}
class Filter (Cell[32]}

When I create an instance of filter, I'd like to pass an argument that will set i1, for example. How best to do this?

Thanks.

[Bazzy]

Constructors and initializer lists
eg:

class Reg 
{
    int i1, i2, i3;
    Reg ( int a = 0, int b = 0, int c = 0 ) : i1 ( a ), i2 ( b ), i3 ( c ) {} // notice: you can use the same name for members and constructor parameters 
};

class Cell
{
     Cell () : a ( 1,2,3 ), b ( 4, 5 ), c ( 6 ) {}
};

[guestgulkan]

But when he create a filter object, the cell member array will be initialised using the default
constructor.

[Bazzy]

He can modify the array inside the constructor.

[mzimmers]

He can modify the array inside the constructor.

So, I create a Filter, passing the Reg value as a parameter. In the Filter constructor, since the Cells (and Regs) are created implicitly, how do I pass the Reg value down?

[guestgulkan]

We could start like this:

Filter::Filter(int i)
{
    for (int count =0; count < 32; count ++)
    {
        cell[count].a.i1 =i;
        cell[count].b.i1 =i;
        cell[count].c.i1 =i;
        cell[count].d.i1 =i;

    }

}

But that would mean that if i1 was a private member of class Reg, then the Filter class would have to be made a friend of Reg

[mzimmers]

Right, and I'd prefer to not do that. I guess I need some routines at the Cell and Reg level for this, don't I?

[Bazzy]

you can change the default values for Cell and Reg constructors

[mzimmers]

I understand that, but I don't see how a value determined at the filter level gets passed through the cell to the reg at construction time.

[Bazzy]

If you use a vector instead of an array, you'll have all your problem solved since you can call the vector constructor with a specified value.
Built-in array are not very flexible

[mzimmers]

I'm not familiar with vectors, but I did a little reading, and I don't see how this addresses the problem.

Again, if I have an object a which contains another object b, which contains another object c, and I want to set a value within c, but do so from the scope of a...is this possible at the time of construction? Or, do I need a set function?

Thanks.

[Bazzy]

If you have a vector, you can construct it like so: cell ( 32, Cell ( foo ) )
where
cell is the name of the vector,
32 is it's size
Cell ( foo ) is the Cell constructor
foo would be an appropriate list of parameters to pass to the constructor, which depends on how you implemented the constructor

Cell(foo) will call the constructors of a,b,c

[mzimmers]

At the risk of drifting slightly off-topic, I've tried to experiment with vectors in my code, and the compiler objected. Here's what I did in a header file:

#include <vector>
class Filter {
   vector<Cell> cellArray(NBR_CELLS);
   etc.
}

The compiler said:

error: ISO C++ forbids declaration of 'vector' with no type
error: expected ';' before '<' token

What did I do wrong?

[guestgulkan]

std::vector

[mzimmers]

Thanks. I put "using namespace std;" earlier in the file, and I seem to have gotten past that problem.

NOW I get an error: NBR_CELLS is not a type. I have NBR_CELLS defined as a const int. This has me confused: doesn't the compiler expect the size of the vector here, and not a type?

[guestgulkan]

mzimmers wrote:

This has me confused: doesn't the compiler expect the size of the vector here, and not a type?

No- not in the class declaration - it just expect a variable name (not something that looks like initialisation code and such )

So just:
vector<Cell> cellArray;

You initialize the vector in the Filter constructor (in the initialization list)

[mzimmers]

Ahh...so the constructor should look like this?

Filter::Filter(int rv) : cellArray(NBR_CELLS, Cell(rv) {}

This compiles and runs, but my initialization isn't working right. If this looks right to you, though, I'll just keep inspecting the lower-level constructors.

[guestgulkan]

Yes.

If you have other issues elsewhere you can post the code.

[mzimmers]

OK, I have another question regarding the constructor. if NBR_CELLS = 32, for example, do I need 32 instances of the Cell(rv) in order to initialize each element in the vector? In other words:

Filter::Filter(int rv) : cellArray(NBR_CELLS, Cell(rv), Cell(rv), Cell(rv)... {}

Or, if not, how do I go about initializing all elements? Currently only the first one seems to get initialized.

[guestgulkan]

The vector constructor cellArray(NBR_CELLS, Cell(rv)) will
create NBR_CELLS amount of elements - all of these cells will be constructed identically
(in this case using the Cell(rv) constructor)
You don't write a seperate constructor for each element - (you can't anyway because the compiler will throw it out)