CppCreateFormsDynamically.md

 

 

 

 

 

(C++) Create Forms Dynamically

 

The C++ Builder (and Turbo C++) TForm class ('Form' from now on) can be created either statically or dynamically.

 

• View the code of 'Create Forms Dynamically' in plain text

 

 

 

 

 

When to consider creating Forms dynamically

 

Consider creating Forms dynamically when:

 

• you want to create multiple instances of the same Form
• you've added additional constructor parameters
• you want to have a hierarchy in your Forms
• you want to create Forms one by one
• you want to create Forms in idle time
• you want Forms to be created by a seperate thread

 

 

 

 

 

Do's and don'ts when creating Forms dynamically

 

When you want to dynamically create a Form

 

• do make it an 'Available Form' ('Project | Options | Forms'), instead of an automatically created Form
• do make a Form refer to itself as 'this', instead of using, for example 'Form1'

 

 

 

 

 

How to create a Form dynamically

 

A Form can be created dynamically as follows:

 

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//Put this line among the other #includes #include <memory> //Your method void __fastcall TFormMain::YourMethod() {   std::auto_ptr<TFormDynamic> f(new TFormDynamic(this));   f->ShowModal(); }

 

Sure you can use a plain pointer, but then don't forget to delete it. And if you don't want to forget to delete this pointer (among others), use a std::auto_ptr.

 

Instead of passing 'this', you might sometimes consider passing '0' as an argument to the constructor of TFormDynamic.

 

 

 

 

 

Communicate with the creator of a Form

 

In the code snippet above, the Form called TFormDynamic is constructed using a pointer to the TFormMain Form. This enables the newly created TFormDynamic to be potentially able to communicate with the TFormMain.

 

 

 

 

 

Communicate with the creator of a Form in the constructor only

 

The constructor of a Form takes as an argument the TComponent that has created it. Therefore, you can easily use it locally:

 

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//Put this line among the other #includes #include <cassert> __fastcall TFormDynamic::TFormDynamic(TComponent* Owner) : TForm(Owner) {   TFormMain * const formMain = dynamic_cast<TFormMain>(Owner);   assert(formMain!=0); //Assume cast succeeded   //Use formMain }

 

 

 

 

 

Communicate with the creator of a Form using a member variable

 

You can also make the TFormMain pointer a member variable of TFormDynamic:

 

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//UnitFormDynamic.h //A forward declaration //Put it after the #includes and before the next line class TFormMain; class TFormDynamic : public TForm { __published: // IDE-managed Components //Stuff private: // User declarations TFormMain * const mFormMain; public: // User declarations __fastcall TFormDynamic(TComponent* Owner); }; //UnitFormDynamic.cpp //Put this line among the other #includes #include <cassert> __fastcall TFormDynamic::TFormDynamic(TComponent* Owner) : TForm(Owner), mFormMain(dynamic_cast<TFormMain>(Owner) { assert(mFormMain!=0); //Assume cast succeeded //Use mFormMain }