Creating a library to represent opaque typedefs. Opaque typedefs introduce type-safety that typedef/using does not, because it is not an alias.

Imaging the following:
using coordinate_t = std::pair<int, int>;
using distance_t = std::pair<int, int>;

coordinate_t move( coordinate_t pos, distance_t step ) { ... }

void test_move_coord_dist( ) {
  coordinate_t pos{0,0};
  distance_t step{5,5};
  TEST_ASSERT_EQUALS( coordinate_t(5,5), move(step, pos) ); // Compiles, PASSES
}
// Because coordinate_t and distance_t are simply type aliases,
// move is actually a function 'move :: pair<int, int> -> pair<int, int> -> pair<int, int>'
// so reversing the order that I add them compiles anyway. The test case
// even passes in certain examples, where only commutative operations are performed.
// On the other hand, if I did:
struct coordinate_t : public opaque_typedef< std::pair<int, int> > {
  using opaque_typedef< std::pair<int, int> >::explicit_opaque_typedef;
}; // and the same for distance_t
// Then 'move(step, pos)' will no longer compile because coordinate_t and
// distance_t are different (incompatible) types

TODO: Provide std::hash/std::equal_to/std::less overloads?

opaque_typedef.hpp

@@ -0,0 +1,48 @@
+//
+//  opaque_typedef.hpp
+//  opaque-type
+//
+//  Created by Sam Jaffe on 8/16/16.
+//
+
+#pragma once
+
+#define OPAQUE_TYPE_OPERATOR(name, op) \
+  template <typename R, typename O1 = R, typename O2 = R> struct name; \
+  \
+  template <typename R, typename O1, typename O2> \
+  struct name { \
+    friend R operator op(O1 const & lhs, O2 const & rhs) { \
+      return R(lhs.value() op rhs.value()); \
+    } \
+  }; \
+  \
+  template <typename R, typename O2> \
+  struct name<R, R, O2> { \
+    friend R & operator op##=(R & lhs, O2 const & rhs) { \
+      return lhs = R(lhs.value() op rhs.value()); \
+    } \
+    \
+    friend R operator op(R const & lhs, O2 const & rhs) { \
+      return R(lhs.value() op rhs.value()); \
+    } \
+  };
+
+OPAQUE_TYPE_OPERATOR(addition, +)
+OPAQUE_TYPE_OPERATOR(subtraction, -)
+OPAQUE_TYPE_OPERATOR(multiplication, *)
+OPAQUE_TYPE_OPERATOR(division, /)
+
+#undef OPAQUE_TYPE_OPERATOR
+
+template <typename T>
+class opaque_typedef {
+public:
+  opaque_typedef() : _value() {}
+  explicit opaque_typedef(T val) : _value(val) {}
+  explicit operator T() const { return _value; }
+  
+  T value() const { return _value; }
+private:
+  T _value;
+};

opaque_typedef.xcodeproj/project.pbxproj

@@ -0,0 +1,256 @@
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+			productName = "opaque_typedef";
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+		CD3C80601D63EE0000ACC795 /* Project object */ = {
+			isa = PBXProject;
+			attributes = {
+				LastUpgradeCheck = 0720;
+				ORGANIZATIONNAME = "Sam Jaffe";
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+			);
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+				CLANG_CXX_LIBRARY = "libc++";
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+				CLANG_ENABLE_OBJC_ARC = YES;
+				CLANG_WARN_BOOL_CONVERSION = YES;
+				CLANG_WARN_CONSTANT_CONVERSION = YES;
+				CLANG_WARN_DIRECT_OBJC_ISA_USAGE = YES_ERROR;
+				CLANG_WARN_EMPTY_BODY = YES;
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+				);
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+				MTL_ENABLE_DEBUG_INFO = YES;
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+			buildSettings = {
+				ALWAYS_SEARCH_USER_PATHS = NO;
+				CLANG_CXX_LANGUAGE_STANDARD = "gnu++0x";
+				CLANG_CXX_LIBRARY = "libc++";
+				CLANG_ENABLE_MODULES = YES;
+				CLANG_ENABLE_OBJC_ARC = YES;
+				CLANG_WARN_BOOL_CONVERSION = YES;
+				CLANG_WARN_CONSTANT_CONVERSION = YES;
+				CLANG_WARN_DIRECT_OBJC_ISA_USAGE = YES_ERROR;
+				CLANG_WARN_EMPTY_BODY = YES;
+				CLANG_WARN_ENUM_CONVERSION = YES;
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+				CLANG_WARN_OBJC_ROOT_CLASS = YES_ERROR;
+				CLANG_WARN_UNREACHABLE_CODE = YES;
+				CLANG_WARN__DUPLICATE_METHOD_MATCH = YES;
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+				COPY_PHASE_STRIP = NO;
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+				ENABLE_STRICT_OBJC_MSGSEND = YES;
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+				GCC_NO_COMMON_BLOCKS = YES;
+				GCC_WARN_64_TO_32_BIT_CONVERSION = YES;
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+				GCC_WARN_UNDECLARED_SELECTOR = YES;
+				GCC_WARN_UNINITIALIZED_AUTOS = YES_AGGRESSIVE;
+				GCC_WARN_UNUSED_FUNCTION = YES;
+				GCC_WARN_UNUSED_VARIABLE = YES;
+				MACOSX_DEPLOYMENT_TARGET = 10.10;
+				MTL_ENABLE_DEBUG_INFO = NO;
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+			};
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+		};
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+			buildSettings = {
+				DYLIB_COMPATIBILITY_VERSION = 1;
+				DYLIB_CURRENT_VERSION = 1;
+				EXECUTABLE_PREFIX = lib;
+				GCC_ENABLE_CPP_EXCEPTIONS = YES;
+				GCC_ENABLE_CPP_RTTI = YES;
+				GCC_SYMBOLS_PRIVATE_EXTERN = YES;
+				PRODUCT_NAME = "$(TARGET_NAME)";
+			};
+			name = Debug;
+		};
+		CD3C80751D63EE0000ACC795 /* Release */ = {
+			isa = XCBuildConfiguration;
+			buildSettings = {
+				DYLIB_COMPATIBILITY_VERSION = 1;
+				DYLIB_CURRENT_VERSION = 1;
+				EXECUTABLE_PREFIX = lib;
+				GCC_ENABLE_CPP_EXCEPTIONS = YES;
+				GCC_ENABLE_CPP_RTTI = YES;
+				GCC_SYMBOLS_PRIVATE_EXTERN = YES;
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+}