Comparing two map::iterators: why does it need the copy constructor of std::pair?

Question

The very simple code below compiles and links without a warning in C++98 but gives an incomprehensible compile error in C++11 mode.

#include 

Answer 1

The copy constructor of std::pair isn't needed in this case, but because it is default defined inline in the declaration of std::pair, it is automatically instantiated along with the instantiation of std::pair itself.

It would be possible for the standard library to provide a non-inline default definition of the copy constructor:

template
  struct pair
  {
// ...
    constexpr pair(const pair&);
// ...
  };
// ...
template
constexpr pair<_T1, _T2>::pair(const pair&) = default;

However this would not accord with the strict letter of the standard (clause 20.3.2), where the copy constructor is default defined inline:

  constexpr pair(const pair&) = default;