Added macro for ABI compatibility

Author: cschreib

README.md

@@ -72,4 +72,12 @@ The follownig limitations are features that were not implemented simply because
 
 ## Notes
 
+
+### Alternative implementation
+
 An alternative implementation of an "observable unique pointer" can be found [here](https://www.codeproject.com/articles/1011134/smart-observers-to-use-with-unique-ptr). It does not compile out of the box with gcc unfortunately, but it does contain more features (like creating an observer pointer from a raw `this`). Have a look to check if this better suits your needs.
+
+
+### ABI compatibility
+
+When compiled in C++20 mode, by default the implementation will attempt to optimize empty deleters. This is not ABI-compatible with previous versions of C++, which lack the `[[no_unique_address]]` attribute (introduced in C++20). If ABI compatibility with previous versions of C++ is a concern to you, please define the macro `OUP_CPP17_ABI_COMPAT` before including the header of this library. This will disable the empty deleter optimisation, and enable binary compatibility with older C++ versions.

include/oup/observable_unique_ptr.hpp

@@ -5,6 +5,15 @@
 #include <cstddef>
 #include <utility>
 
+// When compiled in C++20 mode, by default the implementation will
+// attempt to optimize away empty deleters. This is not ABI-compatible
+// with previous versions of C++, which lack the [[no_unique_address]]
+// attribute. If ABI compatibility with previous versions of C++ is a
+// concern to you, please define the macro below.
+#if !defined(OUP_CPP17_ABI_COMPAT)
+#   define OUP_CPP17_ABI_COMPAT
+#endif
+
 namespace oup {
 
 template<typename T>
@@ -61,7 +70,7 @@ class observable_unique_ptr {
     control_block* block = nullptr;
     T* data = nullptr;
 
-    #if __cplusplus == 202002L
+    #if __cplusplus == 202002L && !defined(OUP_CPP17_ABI_COMPAT)
     [[no_unique_address]] Deleter deleter;
     #else
     Deleter deleter;