src/kudu/util/make_shared.h

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//
//   http://www.apache.org/licenses/LICENSE-2.0
//
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#ifndef KUDU_UTIL_MAKE_SHARED_H_
#define KUDU_UTIL_MAKE_SHARED_H_

#include <memory>

// It isn't possible to use std::make_shared() with a class that has private
// constructors. Moreover, the standard workarounds are inelegant when said
// class has non-default constructors. As such, we employ a simple solution:
// declare the class as a friend to std::make_shared()'s internal allocator.
// This approach is non-portable and must be implemented separately for each
// supported STL implementation.
//
// Note: due to friendship restrictions on partial template specialization,
// it isn't possible to befriend just the allocation function; the entire
// allocator class must be befriended.
//
// See http://stackoverflow.com/q/8147027 for a longer discussion.

#ifdef __GLIBCXX__
  // In libstdc++, new_allocator is defined as a class (ext/new_allocator.h)
  // but forward declared as a struct (ext/alloc_traits.h). Clang complains
  // about this when -Wmismatched-tags is set, which gcc doesn't support
  // (which probably explains why the discrepancy exists in the first place).
  // We can temporarily disable this warning via pragmas [1], but we must
  // not expose them to gcc due to its poor handling of the _Pragma() C99
  // operator [2].
  //
  // 1. http://clang.llvm.org/docs/UsersManual.html#controlling-diagnostics-via-pragmas
  // 2. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60875
  #ifdef __clang__
    #define ALLOW_MAKE_SHARED(T)                                \
      _Pragma("clang diagnostic push")                          \
      _Pragma("clang diagnostic ignored \"-Wmismatched-tags\"") \
      friend class __gnu_cxx::new_allocator<T>                  \
      _Pragma("clang diagnostic pop")
  #else
    #define ALLOW_MAKE_SHARED(T) \
      friend class __gnu_cxx::new_allocator<T>
  #endif
#elif defined(_LIBCPP_VERSION)
  #define ALLOW_MAKE_SHARED(T) \
    friend class std::__1::__libcpp_compressed_pair_imp<std::__1::allocator<T>, T, 1>
#else
  #error "Need to implement ALLOW_MAKE_SHARED for your platform!"
#endif

#endif // KUDU_UTIL_MAKE_SHARED_H_