str: align invalid cdada_str() contract

- return NULL for invalid/corrupted str handles

- update regression to assert cdada_str(NULL) == NULL

- update changelog v0.6.4 entries

Reported-by: CodexGPT-5.3 <noreply@openai.com>

Author: msune

CHANGELOG.md

@@ -16,10 +16,12 @@ Legend:
 - [B] list/map/queue/set/stack: return `NULL` when constructor `malloc()` fails (avoid null dereference)
 - [B] str: fix `cdada_str_replace*()` to reject empty match and self-replacement issues
 - [B] str: return `NULL` in `cdada_str_create()` when input C string is `NULL`
+- [B] str: align `cdada_str()` invalid-handle behavior with API docs (return `NULL`, not empty string)
 - [O] tests: add coverage for `cdada_strerr()` valid, boundary and high invalid values
 - [O] tests: add `oom_create_test` coverage for create paths (`list`, `map`, `queue`, `set`, `stack`, `str`, `bbitmap`)
 - [O] tests: add coverage for `cdada_str_replace_all()` empty-match invalid input and self-overlap replacement
 - [O] tests: add `cdada_str_create(NULL)` regression coverage
+- [O] tests: align invalid `cdada_str()` assertion with documented `NULL` return
 
 ### v0.6.3 (28th January 2026)
 

src/str.cc

@@ -143,15 +143,15 @@ const char* cdada_str(const cdada_str_t* str){
 	__cdada_str_int_t* m = (__cdada_str_int_t*)str;
 
 	if(!m || m->magic_num != CDADA_MAGIC)
-		return "";
+		return NULL;
 
 	try{
 		return m->str->c_str();
 	}catch(...){
 		CDADA_ASSERT(0);
 	}
 
-	return "";
+	return NULL;
 }
 
 int __cdada_str_find(const cdada_str_t* str, const char* substr, uint32_t* pos,

test/str_test.c

@@ -139,7 +139,7 @@ int test_basics(){
 	TEST_ASSERT(cdada_str_length(NULL) == 0);
 
 	ptr = cdada_str(NULL);
-	TEST_ASSERT(strlen(ptr) == 0);
+	TEST_ASSERT(ptr == NULL);
 
 	rv = cdada_str_find_first(NULL, (const char*)ptr_not_null,
 						(uint32_t*)ptr_not_null);