How to define a “template” with child placeholders in QML?

I found a much nicer answer to this question, suggested in a presentation of the Qt Developer Days 2011 “Qt Quick Best Practices and Design Patterns”.

They use default property alias ... to alias the child items to any property of any item. If you don’t want to alias the children but give the alias property a name, just remove default. (Literal children are per QML definition the value of the default property.)

Item {
    id: button
    default property alias contents: placeholder.children

    anchors.fill: parent

    Rectangle {
        anchors.fill: parent
        radius: 10
        color: "gray"

        Item {
            id: placeholder     <-- where the placeholder should be inserted
        }
    }
}