ggrr questions

• What is the function of include guard in .cpp (not in .h)? [duplicate]
• Is there generic way to mirror a pair (pair<A,B> to pair<B,A>)?
• What is "dumb data" and "dumb data object" mean?
• How to group "unlike" elements when iterating array (e.g.:{1,2,2,1,3,3,3,2,1,1} to {1,2}{2,1,3}{3}{3,2,1}{1})?
• why virtual void test()=00 is valid but virtual void test()=+0 and virtual void test()=-0 is not valid?
• Can I use the address of argc in main as random source?
• Why this sin cos look up table inaccurate when radian is large?
• How to check if a number < 1 is power of 2?
• Why doesn't the compiler automatically add or generate an include guard by default?
• What is the difference between Document and Value in rapidjson?

ggrr answers