Creating UI elements with Codes

[zellwwf]

I have read and watch numerous, ALOT, of tutorial certified qt videos.
i have a general idea of how to create UI elements with code, but the ONE and i believe most important thing they don't mention, is where do you place that piece of code...
Let's say i want to create a LineEdit in a form called LOLApp.UI
where DO i place this code

//this is an example not a real code... writing it on the fly
void LOLapp::createAButton() {
QPushButton *newButton ...... etc etc
}

//this is an example not a real code... writing it on the fly
void LOLapp::createAButton() {
QPushButton *newButton ...... etc etc
}

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now where do i place this code? LOLApp.cpp, LOLApp.h, main.cpp?
if u need more info tell me...