I was creating a textEdit by code not in designer form ,like this
It's not about how you create your widget, read again what SixDegrees written, compiler is confused, because you do something like this:
Qt Code: 
  1. class A{
  2. public:
  3. 	void show(){
  4. 	}
  5. };
  6.  
  7. class B{
  8. public:
  9. 	void show(){
  10. 	}
  11. };
  12.  
  13.  
  14. class C : public A, public B{
  15. };
  16.  
  17. int main(){
  18. //...
  19. C c;
  20. c.show(); //!< and what is supposed to be called here : A::show() or B::show() ?
  21. }
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I just want to know how to promote the custom MyTextEdit(which inherits QTextEdit) class to texEdit area....
Stef has already given you a nice tutorial on promoting widgets in designer. Which part of it is not clear ?