how to center a middle widget and expand its cell - c++

Question

In Qt designer, how do i center a middle widget in a vertical/horizontal/grid layout and expand its cell at the same time, what i want to get is something like this:
what i get:

Answer 1

One possible solution is to place the middle QPushButton in a QWidget through a layout, and then place that QWidget in the second column of the QHBoxLayout:
<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
<class>Form</class>
<widget class="QWidget" name="Form">
<property name="geometry">
<rect>
<x>0</x>
<y>0</y>
<width>400</width>
<height>300</height>
</rect>
</property>
<property name="windowTitle">
<string>Form</string>
</property>
<layout class="QHBoxLayout" name="horizontalLayout_2">
<item>
<widget class="QPushButton" name="pushButton">
<property name="text">
<string>PushButton</string>
</property>
</widget>
</item>
<item>
<widget class="QWidget" name="widget" native="true">
<property name="sizePolicy">
<sizepolicy hsizetype="Preferred" vsizetype="Fixed">
<horstretch>0</horstretch>
<verstretch>0</verstretch>
</sizepolicy>
</property>
<layout class="QHBoxLayout" name="horizontalLayout">
<property name="leftMargin">
<number>0</number>
</property>
<property name="topMargin">
<number>0</number>
</property>
<property name="rightMargin">
<number>0</number>
</property>
<property name="bottomMargin">
<number>0</number>
</property>
<item>
<widget class="QPushButton" name="pushButton_2">
<property name="sizePolicy">
<sizepolicy hsizetype="Maximum" vsizetype="Fixed">
<horstretch>0</horstretch>
<verstretch>0</verstretch>
</sizepolicy>
</property>
<property name="text">
<string>PushButton</string>
</property>
</widget>
</item>
</layout>
</widget>
</item>
<item>
<widget class="QPushButton" name="pushButton_3">
<property name="text">
<string>PushButton</string>
</property>
</widget>
</item>
</layout>
</widget>
<resources/>
<connections/>
</ui>
Output:

Answer 2

You're gonna need to add a horizontal spacer. And then for the spacer you can add properties. Whether it should be fixed or expanding.
So first select and add a horizontal spacer, and then when you select the horizontal spacer, in the properties widget you'll be able to set properties to it:
The way the spacer will be placed in-between two widgets, is by first dragging the spacer and placing it in between the widgets, selecting all of them and then right-click and group horizontally. This will put your widgets and the spacer in a horizontal layout:
If you want to do it in the code manually, look into QSpacerItem.