Trying to make any sense of the “T&&” notation in C++.

It means “an r-value reference” “a forwarding reference”.

https://stackoverflow.com/questions/4549151/c-double-address-operator

https://stackoverflow.com/questions/5481539/what-does-t-double-ampersand-mean-in-c11 (*)

It’s referenced in the C++11 “range-based for loop” loop construct:

https://en.cppreference.com/w/cpp/language/range-for

for (auto&& [first, second] : mymap) { … }                ← WAT?

The summary of the SO (*) above is:

• rvalue is the result of an expression or any temporary (e.g. type conversion).
• Typically, it would be used as a “copy” using a “const T&” reference and becomes an lvalue. E.g. the user cannot modify the value (since it becomes const).
• T&& is used as a receiving argument type in a function.
• An argument of “T&&” preserves (forwards) the “rvalue” property and passes it down as-is. The user can modify the value (since it is not const).
• When used in a template, “T&&” becomes either “T&” (for an lvalue), or “T” (for an rvalue / temporary).
• As such it’s named a “forwarding reference” and is associated with std::forward<T>.

About std::forward<>, it’s equivalent to performing a static cast of “T&&” to preserve its rvalue:

https://stackoverflow.com/questions/3582001/what-are-the-main-purposes-of-stdforward-and-which-problems-does-it-solve/3582313#3582313

Coming back to that “range-based for loop” construct:

https://en.cppreference.com/w/cpp/language/range-for

• for (auto x : array) {}                → access by value (I guess “copy”)
• for (const T& x : array) {}        → access by const reference
• for (auto&& x : array) {}        → access by “forwarding reference” (not const)

are all possible.