Why does std::pair expose member variables?

Question

From http://www.cplusplus.com/reference/utility/pair/, we know that std::pair has two member variables, first and second.

Why did

Answer 1

The reason is that no real invariant needs to be imposed on the data structure, as std::pair models a general-purpose container for two elements. In other words, an object of type std::pair is assumed to be valid for any possible first and second element of type T and U, respectively. Similarly, subsequent mutations in the value of its elements cannot really affect the validity of the std::pair per se.

Alex Stepanov (the author of the STL) explicitly presents this general design principle during his course Efficient Programming with Components, when commenting on the singleton container (i.e., a container of one element).

Thus, albeit the principle in itself can be a source of debate, this is the reason behind the shape of std::pair.