How to define a "template" with child placeholders in QML?

You can move the item(s) (if you want to support multiple items within the placeholder) using this piece of code:

property list<Item> contents

Component.onCompleted: {
    var contentItems = [];
    for(var i = 0; i < contents.length; ++i)
        contentItems.push(contents[i]);
    placeholder.children = contentItems;
}

Note that you do not have to provide a list of Items for the contents property, as single values will be accepted by the QML engine also for list properties.

Necro answering in case someone else end up here as I did.

In Qt5 somewhere along the line the default property became "data" and not "children". This makes it possible to add other object types than "Item". e.g. Connections can be added as well (to answer my own question above)

So in Qt5 you should do:

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

    anchors.fill: parent

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

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

Note the: placeholder.data instead of placeholder.children Also please note that you don't have to use the alias name contents - this can be anything you like. An example:

Item {
    id: button
    default property alias foo: placeholder.data
    ...

}

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
        }
    }
}

Actually, the correct answer from what I've heard is to use a QML Loader to accomplish what you want.

[that being said; I haven't actually tried it yet but it's on my near-term to-try list and looks fairly straight forward]

Also, search stackoverflow for other "QML Loader" questions as there are a number that will help you get started.