How does the placement operator new work?

Question

Qt

In Schlee's book Qt 5.3 there is an example of a program where an unusual notation of the new operator is used, on which I "hung". They write that this is an allocating operator, as I understand it, they can create and initialize pre-allocated memory. But Schlee's record of this operator in the problem somehow does not fit into this.

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    //Widget w;
    //w.show();
    QTextEdit txt;
    QFont fnt("Lucida Console", 12, QFont::Normal);
    txt.document()->setDefaultFont(fnt);

    new SyntaxHighLigther(txt.document()); // Вот она!!!

    QPalette pal = txt.palette();
    pal.setColor(QPalette::Base, Qt::darkBlue);
    pal.setColor(QPalette::Text, Qt::yellow);
    txt.setPalette(pal);

    txt.show();
    txt.resize(640, 480);

    QFile file("syntaxhighlighter.cpp");
    file.open(QFile::ReadOnly);
    txt.setPlainText(file.readAll());

    return a.exec();

This entry is not clear: new SyntaxHighLigther(txt.document());
SyntaxHighLigther is a class whose object has not been created, that is, a new object is created and initialized, but WHERE is it written? After all, there is nothing on the left side of the new operator.

Answer 1

This is not hosting new. This is a normal object creation. The highlighter's constructor registers itself with the parent and the parent is responsible for removing it, so the result of new-expression is not used.
Placement new looks like this:
To understand how placement new works, you need to understand the difference between new-expression and the new operator. new-expression does things:
The allocating operator new takes as a parameter a pointer to the already allocated memory and simply returns it.

Answer 2

but WHERE does it record?

This is a mistake in the book. Such happen at jambs of publishing house.