Qt: does "new without delete" cause memory leaks with controls?

No, the addWidget() function will keep ownership of the widget. It will then destroy the widgets it owns.

Additionally you can read here that:

As with QObjects, QWidgets can be created with parent objects to indicate ownership, ensuring that objects are deleted when they are no longer used. With widgets, these parent-child relationships have an additional meaning: Each child widget is displayed within the screen area occupied by its parent widget. This means that when you delete a window widget, all the child widgets it contains are also deleted.

In addition to Klaim's correct answer:

I would store those pointers in a std::auto_ptr, meanwhile you pass them to their parent.

std::auto_ptr<QHBoxLayout> buttonLayout( new QHBoxLayout() );
// make things which could throw...
layout->addLayout(buttonLayout.release());

This way you are sure not to have leaks.

As of C++14 you can use the std::make_unique() convenience function template for creating an std::unique_ptr<> that has exclusive ownership of the widget. Then, at the moment of passing the widget to addLayout(), you make the smart pointer give up ownership by calling release():

auto buttonLayout = std::make_unique<QHBoxLayout>(); 
// ...
// an exception could be thrown here
// ...
layout->addLayout(buttonLayout.release());

If there is an exception thrown between new and addWidget then yes there is a memory leak. Otherwise the parent control takes ownership of the memory.

QHBoxLayout *buttonLayout = new QHBoxLayout();  // Memory leak?
//make sure you don't throw here
buttonLayout->addWidget(sendButton);