[Tex/LaTex] How to linebreak part ToC entries using tocloft/titletoc and the book class

line-breakingpartstable of contentstitletoctocloft

I'm currently writing a LaTeX document in the book documentclass. In my ToC I want the actual titles of certain entries (parts in my case) to come in a newline after a generic name of the section-type ('Part') followed by the number ('I', 'II', …), so it should look like this:

Part I

Title of first part

Part II

Title of second part

I'm using the tocloft package to customize the ToC and its manual actually has an example that excactly describes how to achieve this. Apparently, for whatever reason, the newline command (see MWE) has no effect with the book class. Any suggestions on what is the problem here?

\documentclass{book}
\usepackage{tocloft}
\renewcommand{\cftpartpresnum}{\partname\ }
\renewcommand{\cftpartaftersnumb}{\\ }
\begin{document}
  \tableofcontents
  \part{Title of first part}
  \part{Title of second part}
\end{document}

EDIT 1:
I've followed @locksteps advice and tried to implement a solution with the titletocpackage. However, now I'm stuck with the problem that for part entries for whatever reason the lablenummer is always attached to the actual title, for chapter and so on one can call the label independently

\documentclass{book}
\usepackage{titletoc}
\titlecontents{part}[0pt]{}{}{\Large\bfseries Part\\*}{}[]
\titlecontents{chapter}[0pt]{}{Chapter \thecontentslabel\\*}{}{}[]
\begin{document}
   \tableofcontents
   \part{The first part}
   \chapter{The first chapter}
\end{document}

Best Answer

toclofts \cftXaftersnumb macro family doesn't work for X = part in the book class. Quoting from the tocloft manual, p. 9:

In the standard classes the ToC entry for a \part is just typeset as the number and title, followed by the page number, with the \cftpartpresnum macro being called before typesetting the number and title. When a standard class is used the \cftpartaftersnum and \cftpartaftersnumb macros have no effect, but they may do something if a non-standard class is used.

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