Hi.
Why are you going to use C++ for such purpose? Usually this language is used in system level apps.
Javascript, java, perl and pyhon commonly are used for web scraping.
We have done many scraping projects using python and scrapy framework; please see [login to view URL]
It is a mature tool for scraping.
Also we can use PhantomJS and javascript to do scraping.
Parallelism would be straightforward, depending on the bandwidth and nature of the target site though.
Thanks