QDialog problem with exec()

[nomadscarecrow]

...
	form_login l;
	if (l.exec() == QDialog::Accepted)
	{
		tmain_form main_form;
		main_form.show();
		return a.exec();
	}      
...

...
	form_login l;
	if (l.exec() == QDialog::Accepted)
	{
		tmain_form main_form;
		main_form.show();
		return a.exec();
	}      
...

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Hi I'm using this code to show a login form on the main.cpp file.

When used in this way before the a.exec(), everything works fine.
But when I try this, the exec() is not executed in the usual way, the dialog is closed not waiting for user action.

tcontrol *control = new tcontrol();
 
	if (control->login())
	{
		tmain_form main_form;
		main_form.show();
		return a.exec();
	}

tcontrol *control = new tcontrol();

	if (control->login())
	{
		tmain_form main_form;
		main_form.show();
		return a.exec();
	}

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and login is just a method wrapping the first code, returning (l.exec() == QDialog::Accepted)

What is the difference?

I'm missing something?

Thanks in advance.