Fix #554: Document the difference between add, add_global, and set_global (#649)

Updated the cheatsheet to clearly explain that `add` creates thread-local
scoped variables while `add_global`, `add_global_const`, and `set_global`
create global variables shared between all threads. Added a summary table
comparing scope, thread safety, and conflict behavior. Also added a test
exercising the key behavioral differences between these methods.

Co-authored-by: leftibot <leftibot@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cheatsheet.md

@@ -153,17 +153,39 @@ This allows you to pass a ChaiScript function to a function requiring `std::vect
 
 ## Adding Objects
 
-```
-chai.add(chaiscript::var(somevar), "somevar"); // copied in
-chai.add(chaiscript::var(std::ref(somevar)), "somevar"); // by reference, shared between C++ and chai
+### `add` — Thread-Local Scoped Variables
+
+`add` adds an object to the current thread's local scope. The variable is only visible in the
+thread that added it. If the variable already exists in the current scope, it is overwritten.
+
+```cpp
+chai.add(chaiscript::var(somevar), "somevar");                    // copied in
+chai.add(chaiscript::var(std::ref(somevar)), "somevar");          // by reference, shared between C++ and chai
 auto shareddouble = std::make_shared<double>(4.3);
-chai.add(chaiscript::var(shareddouble), "shareddouble"); // by shared_ptr, shared between c++ and chai
-chai.add(chaiscript::const_var(somevar), "somevar"); // copied in and made const
-chai.add_global_const(chaiscript::const_var(somevar), "somevar"); // global const. Throws if value is non-const, throws if object exists
-chai.add_global(chaiscript::var(somevar), "somevar"); // global non-const, throws if object exists
-chai.set_global(chaiscript::var(somevar), "somevar"); // global non-const, overwrites existing object
+chai.add(chaiscript::var(shareddouble), "shareddouble");          // by shared_ptr, shared between C++ and chai
+chai.add(chaiscript::const_var(somevar), "somevar");              // copied in and made const
 ```
 
+### `add_global` / `add_global_const` / `set_global` — Global Shared Variables
+
+Global variables are shared between all threads and are visible from any scope (including inside
+functions). Use these when you need a variable accessible everywhere.
+
+```cpp
+chai.add_global_const(chaiscript::const_var(somevar), "somevar"); // global const, throws if value is non-const or object already exists
+chai.add_global(chaiscript::var(somevar), "somevar");             // global non-const, throws if object already exists
+chai.set_global(chaiscript::var(somevar), "somevar");             // global non-const, overwrites existing or creates new
+```
+
+### Summary of Differences
+
+| Method | Scope | Thread Safety | If Name Exists |
+|--------|-------|---------------|----------------|
+| `add` | Thread-local, current scope | Not shared between threads | Overwrites |
+| `add_global` | Global, all scopes and threads | Mutex-protected, shared between threads | Throws exception |
+| `add_global_const` | Global, all scopes and threads | Mutex-protected, shared between threads | Throws exception |
+| `set_global` | Global, all scopes and threads | Mutex-protected, shared between threads | Overwrites |
+
 ## Adding Namespaces
 
 Namespaces will not be populated until `import` is called.

unittests/add_vs_add_global.chai

@@ -0,0 +1,43 @@
+// Test demonstrating the difference between add (local scope) and add_global/set_global (global scope)
+// See issue #554
+
+// --- Local variables (var / add) are scoped ---
+// A local variable defined in a block is not visible outside that block
+var local_val = 10
+assert_equal(local_val, 10)
+
+// Local variables can be reassigned in scope
+local_val = 20
+assert_equal(local_val, 20)
+
+// Local variables inside a function are not visible outside
+def set_local() {
+  var func_local = 99
+  assert_equal(func_local, 99)
+}
+set_local()
+
+// --- Globals (add_global) are visible everywhere, including inside functions ---
+var g_val = 42
+add_global(g_val, "my_global")
+
+def check_global() {
+  assert_equal(my_global, 42)
+}
+check_global()
+
+// add_global throws if the global already exists
+assert_throws("Name already exists in current context my_global", fun() { add_global(1, "my_global") })
+
+// --- set_global overwrites an existing global ---
+set_global(100, "my_global")
+assert_equal(my_global, 100)
+
+def check_global_updated() {
+  assert_equal(my_global, 100)
+}
+check_global_updated()
+
+// set_global can also create a new global if it doesn't exist
+set_global(77, "new_global")
+assert_equal(new_global, 77)