add compressed pair example

1 files changed, 101 insertions, 0 deletions

diff --git a/compressed_pair.cc b/compressed_pair.cc
new file mode 100644
index 0000000..873c4ce
--- /dev/null
+++ b/compressed_pair.cc
@@ -0,0 +1,101 @@
+#include <algorithm>
+#if 0
+set -xe
+g++     -std=c++17 -fsyntax-only compressed_pair.cc
+clang++ -std=c++17 -fsyntax-only compressed_pair.cc
+g++     -std=c++20 -fsyntax-only compressed_pair.cc
+clang++ -std=c++20 -fsyntax-only compressed_pair.cc
+exit 0
+#endif
+
+#include <type_traits>
+
+// Minimal compressed pair implementation.
+//
+// In cpp even if a class is zero-sized, adding a member of such a class,
+// allocates 1 byte in the layout of the instantiating class. In generic
+// programming one sometimes wants to store a member of a template argument and
+// in the best case for zero-sized template types one does not want to pay a
+// price.
+//
+// One example for this is std::unique_ptr with a custom state-less deleter:
+//
+#include <memory>
+struct freer {
+  void operator()(void* p) {
+    std::free(p);
+  }
+};
+
+std::unique_ptr<char, freer> ptr{nullptr};
+static_assert(sizeof(ptr) == sizeof(char*), "");
+//
+// Before cpp20 this can be achieved with some template specialization tricks
+// and deriving from the type if it is not empty and not final.
+//
+// Starting with cpp20 the [[no_unique_address]] attribute [1] was added which
+// allows the compiler to not allocate space in the layout of a class for a
+// given member if that is zero-sized.
+//
+// [1] https://en.cppreference.com/w/cpp/language/attributes/no_unique_address
+
+namespace cpp11 {
+template <typename T, int Idx, bool = std::is_empty_v<T> && !std::is_final_v<T>>
+struct entry {
+  T& get() {
+    return val;
+  }
+
+  T val;
+};
+
+template <typename T, int Idx>
+struct entry<T, Idx, true> : T {
+  T& get() {
+    return *static_cast<T*>(this);
+  }
+};
+
+template <typename T, typename U>
+struct pair : entry<T, 0>, entry<U, 1> {
+  T& first() {
+    return static_cast<entry<T, 0>*>(this)->get();
+  }
+
+  U& second() {
+    return static_cast<entry<U, 1>*>(this)->get();
+  }
+};
+}  // namespace cpp11
+
+struct empty {};
+
+#if __cplusplus >= 202002L
+namespace cpp20 {
+template <typename T, typename U>
+struct pair {
+  T& first() {
+    return m_first;
+  }
+
+  U& second() {
+    return m_second;
+  }
+
+  [[no_unique_address]] T m_first;
+  [[no_unique_address]] U m_second;
+};
+}  // namespace cpp20
+#endif
+
+// -- TEST ME ------------------------------------------------------------------
+
+static_assert(sizeof(cpp11::pair<int, int>) == 8);
+static_assert(sizeof(cpp11::pair<int, empty>) == 4);
+static_assert(sizeof(cpp11::pair<empty, empty>) == 2);
+
+#if __cplusplus >= 202002L
+static_assert(sizeof(cpp20::pair<int, int>) == 8);
+static_assert(sizeof(cpp20::pair<int, empty>) == 4);
+static_assert(sizeof(cpp20::pair<empty, empty>) == 2);
+#endif