In C++, where is std::cin object defined?

Question

In C++, the istream object std::cin is declared in header <iostream>, but where is it defined?

Answer 1

Where it is defined is not mandated by the standard, it's completely up to the implementation.

Here's where mine is:

$ nm -C /usr/local/lib64/libstdc++.so.6 | grep -w cin
0000000000305900 B std::cin