Extending namespace std to implement make_unique when using C++11

Question

I came across a codebase that is fixed on C++11 features but implements std::make_unique. That is been done extending namespace std to add the feature if C++14 is not use, i.e. wrapping the implementation around

#if defined(__cplusplus) && __cplusplus < 201402L

namespace std {
  ...
}

#endif

I know that is undefined behavior to extend namespace std (with some exception). Is the case above still acceptable or should it be avoided in any case?

Answer 1

No, this is forbidden—even though, via

#define make_unique ? ? ?

a conforming C++11 program can be quite sure that the library never mentions the name (outside of a stringization) and would thus be unable to detect the extension.