The simplest way to search for a tag across all Stack Exchange sites is to search Google using the "site" operator and the "*" wildcard. For example, a Google query for all questions involving the "c++" tag would be written as follows: "site:*.stackexchange.com [c++]". Alternatively, use the filtering feature on the main Stack Exchange site.
Use of Google search facilities for Stack Exchange was a deliberate design choice, according to the network co-founder Jeff Atwood, who stated that "web search is our 'homepage'". The Stack Exchange sites provide custom Google search boxes for search and a filtering feature for finding questions by tag.
The Stack Exchange meta site contains an answer that explains how to find a tag across all the meta sites using the filtering functionality on the main site. To search for a tag across all Stack Exchange sites, follow the same instructions, but change the search settings to include "All Sites" instead of just "Meta Sites".
To search within specific sites in the Stack Exchange network, such as Stack Overflow, use the tag search feature on individual sites. Alternatively, use the filtering feature on the main Stack Exchange site, or issue a Google search query similar to the one for the Stack Exchange site.