\documentclass{article}
\usepackage{xparse,l3regex}

\ExplSyntaxOn
\NewDocumentCommand{\cppstring}{m}
 {
  \tl_set:Nn \l_spraff_cppstring_tl { #1 }
  % change _ to a printable underscore
  \regex_replace_all:nnN { _ } { \cO\_ } \l_spraff_cppstring_tl
  % change :: to \linebreak[0]::
  \regex_replace_all:nnN { :: } { \c{linebreak}[0]:: } \l_spraff_cppstring_tl
  % print the result
  \texttt{ \tl_use:N \l_spraff_cppstring_tl }
 }
\tl_new:N \l_spraff_cppstring_tl
\ExplSyntaxOff

\begin{document}

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\noindent % just for the example
unrecoverable error occurs during the transformation, then a
\cppstring{stdxxxxx::ostreamxxxxxx::traits_typexxxxx::eqxxxxxxx} lorem ipsum  lorem ipsum  lorem ipsum  lorem ipsum  lorem ipsum  lorem ipsum

\end{document}