Add Windows build with clang

namecollision/.gitignore

@@ -1,2 +1,6 @@
 libhello.so
+libhello.dll
+libhello.lib
 main
+main.exe
+libhello.exp

namecollision/README.md

@@ -6,8 +6,8 @@
 `print_obj()`. `main.cpp` defines a `namespace nt` with a free function
 `print()`, and calls both `nt::print()` and `print_obj()`.
 
-`hello.cpp` is compiled into a shared library (`libhello.so`), and `main.cpp` is
-linked against it.
+`hello.cpp` is compiled into a shared library (`libhello.so` on Linux,
+`libhello.dll` on Windows), and `main.cpp` is linked against it.
 
 ## The Problem
 
@@ -39,7 +39,7 @@ With `-O3` (optimizations on):
 
 - The compiler inlines the class `nt::print()` directly into `print_obj()`
 - `_ZN2nt5printEv` is never emitted as a standalone symbol in `libhello.so`
-- No collision occurs — each call resolves correctly
+- No collision occurs - each call resolves correctly
 - Output: correct
 
 This is why the bug was **optimization-dependent** and hard to spot.
@@ -58,7 +58,7 @@ This is why the bug was **optimization-dependent** and hard to spot.
 
 The C++ standard (basic.def.odr) requires that any entity with more than one
 definition across translation units must have **identical** definitions.
-Violating this is **ill-formed, no diagnostic required** — the compiler and
+Violating this is **ill-formed, no diagnostic required** - the compiler and
 linker are not required to warn you.
 
 The consequences are undefined behavior: the program may crash, produce wrong
@@ -77,10 +77,37 @@ Windows (where DLL symbols are explicitly imported/exported via import tables
 and don't collide this way).
 
 This flat namespace behavior means a weak symbol in a `.so` can be silently
-overridden by any strong symbol of the same name anywhere in the process — even
+overridden by any strong symbol of the same name anywhere in the process - even
 from the main executable.
 
-## The Fix
+## Why Windows Is Not Affected
+
+On Windows, this collision does not occur. The DLL symbol model is fundamentally
+different:
+
+- Only symbols explicitly marked `__declspec(dllexport)` are visible outside the
+  DLL - everything else is private by default
+- The linker generates an import library (`.lib`) with stubs that reference the
+  DLL by name, so the loader knows exactly which DLL each symbol comes from
+- There is no flat global symbol namespace - each DLL is its own isolated scope
+
+This means the internal `class nt` in `hello.cpp` was never at risk on Windows
+regardless of optimization level. The bug is Linux (and ELF) specific.
+
+To keep `hello.cpp` portable, the `__declspec(dllexport)` on `print_obj` is
+wrapped in a macro:
+
+```cpp
+#ifdef _WIN32
+#define EXPORT __declspec(dllexport)
+#else
+#define EXPORT
+#endif
+```
+
+On Linux `EXPORT` expands to nothing. On Windows it marks the symbol for export.
+
+## The Linux Fix
 
 Wrap internal-use classes in an **anonymous namespace** in `hello.cpp`:
 
@@ -103,14 +130,20 @@ The `_GLOBAL__N_1` prefix is the ABI encoding for the anonymous namespace,
 making it unique per translation unit and invisible to the dynamic linker. No
 collision is possible.
 
+The Linux and Windows fixes solve the same problem from opposite defaults:
+- Linux: **opt-in to hiding** symbols (anonymous namespace, `-fvisibility=hidden`)
+- Windows: **opt-in to exporting** symbols (`__declspec(dllexport)`)
+
 ## Key Takeaways
 
 - Class and namespace names mangle identically in the Itanium C++ ABI
 - Weak symbols in shared libraries can be silently hijacked by strong symbols in
   the executable
-- ODR violations are UB with no required diagnostic — bugs may only appear at
+- ODR violations are UB with no required diagnostic - bugs may only appear at
   certain optimization levels
+- This is a Linux/ELF-specific problem - Windows DLLs use explicit export tables
+  and are not affected
 - Internal implementation details in shared libraries should use anonymous
-  namespaces to prevent symbol leakage
+  namespaces to prevent symbol leakage on Linux
 - Use `objdump -t` or `nm` to inspect symbol visibility and catch these issues
   early

namecollision/build.ps1

@@ -0,0 +1,15 @@
+param(
+    [string]$Opt = "-O0"
+)
+
+$ErrorActionPreference = "Stop"
+
+# Compile hello.cpp as a shared library (also generates libhello.lib import library)
+clang++ -std=c++17 $Opt -shared -o libhello.dll hello.cpp "-Wl,/IMPLIB:libhello.lib"
+if ($LASTEXITCODE -ne 0) { exit $LASTEXITCODE }
+
+# Compile and link main.cpp with the shared library
+clang++ -std=c++17 $Opt -o main.exe main.cpp libhello.lib
+if ($LASTEXITCODE -ne 0) { exit $LASTEXITCODE }
+
+Write-Host "Build successful"

namecollision/hello.cpp

@@ -1,5 +1,11 @@
 #include <iostream>
 
+#ifdef _WIN32
+#define EXPORT __declspec(dllexport)
+#else
+#define EXPORT
+#endif
+
 using namespace std;
 
 class nt {
@@ -9,7 +15,7 @@ public:
     }
 };
 
-void print_obj() {
+EXPORT void print_obj() {
     cout << "Hello from function\n";
     nt obj;
     obj.print();