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BCB : how to iterate over controls on a form?

Answer 1

+2 A:

You are somewhat correct in your assumption that casting would be a good idea and that using dynamic_cast is the best option here.

If you want to iterate over the controls of a form (or any other container). Unfortunately I don't have my C++ Builder on this computer so I am unable to test the code I give you, should be a trivial task to create though.

// You wanna start my iterating through the controls of the container
for (int i = 0; i < container->ControlCount; i++)
{
    // Let us extract the control at index i
    TControl *control = container->Controls[i];

    // Now we want to test whether this is any specific type, for instance a
    // TEdit or any TEdit descendant.
    TEdit *edit = dynamic_cast<TEdit *>(control);

    // If this control is a TEdit or any control deriving from it, we will have 
    // a pointer to it, if this is any other type, we will be left with a zero
    // pointer.
    if (edit)
    {
        // This is a TEdit control... we can now access TEdit only properties.
        edit->Text = "This is an edit control.";
    }

    // We do the same if we want to test for a TButton...
    TButton *button = dynamic_cast<TButton *>(control);

    // Again we test whether this was actually a button
    if (button)
    {
        // It is a button, again we can access TButton only properties...
        button->Caption = "This is a button"; 
        // Yeah ok not a TButton only one, but couldn't think of any.
    }

    // And so on...
}

You do not need to make the function recursive for child controls of child controls, these are included here as well by the VCL. This is in my opinion by far the simplest solution for testing for specific types. I also try to avoid the RTTI features as much as possible, but that is just me.

As you can see my example also shows how you can access properties and functions of the control, even those specific to a certain type. However the ones you mention: Name, Width and Height are all common to TControl and it should not be necessary to cast to access these.

Hope this has helped you out, it should be clear from my example that the variable container can be replaced by myForm which you use in yours.

TommyA 2010-03-07 00:02:31

Thanks, Tommy. I had given up and coded it is Pascal using the `is` operator. It's great to be able to do it in c++.YOu say '"You do not need to make the function recursive for child controls of child controls, these are included here as well by the VCL"` - but what would happen if I had, say a TRadioGroup, or some controls on a TPanel? Are they picked up by your code?

Mawg 2010-03-07 00:21:32

btw, I did have to recurse for TPanel and TGroupBox

Mawg 2010-03-07 09:56:02

The Controls[] list is specific to the Parent that you are accessing. Nested child controls must be handled with recursion.

Remy Lebeau - TeamB 2010-03-10 02:13:14

Depending on whether third-party controls need to be handled, it ay be more beneficial to cast to TCustom... classes instead, such as TCustomEdit, TCustomButton, etc.

Remy Lebeau - TeamB 2010-03-10 02:14:31

Also, using C++'s typid() on VCL classes does not usually work. If you want to get the string representation of a class's name, use the TObject::ClassName() method, which will return the derived class name as expected, ie: Control->ClassName()

Remy Lebeau - TeamB 2010-03-10 02:15:55

Although I agree that recursion is needed, the lack of it in my example is from the situation of not having a C++Builder on the machine I wrote the post from, so had to base my answer purely on what I could remember (or not remember). However I fail to see why `typeid` comment is made to my post and not the authors. Since I am deliberately avoiding the `typeid` and stick to the `dynamic_cast` to identify the class.

TommyA 2010-03-10 11:02:04

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BCB : how to iterate over controls on a form?

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