Fix Issue 10535 - Add initialize function to allocate an empty AA

changelog/aa-init.dd

@@ -0,0 +1,18 @@
+Added `initialize` function for associative arrays
+
+This function allocates memory for an empty associative array (AA). It
+is useful to avoid copying a null AA reference. Then
+when AA elements are added through the original reference, the copied
+reference will still point to the same AA:
+---
+int[int] aa;
+auto b = aa;
+assert(b is null);
+aa.initialize;
+assert(aa !is null);
+auto c = aa;
+
+aa[4] = 4;
+assert(b != aa);
+assert(c is aa);
+---

src/object.d

@@ -3024,6 +3024,7 @@ extern (C)
     inout(void)[] _aaKeys(inout void* p, in size_t keysize, const TypeInfo tiKeyArray) pure nothrow;
     void* _aaRehash(void** pp, in TypeInfo keyti) pure nothrow;
     void _aaClear(void* p) pure nothrow;
+    void _aaInitialize(void* p, const TypeInfo_AssociativeArray ti);
 
     // alias _dg_t = extern(D) int delegate(void*);
     // int _aaApply(void* aa, size_t keysize, _dg_t dg);
@@ -3057,10 +3058,47 @@ void* aaLiteral(Key, Value)(Key[] keys, Value[] values) @trusted pure
 
 alias AssociativeArray(Key, Value) = Value[Key];
 
+/***********************************
+ * Allocate memory for an empty associative array.
+ * This is useful to avoid copying a null reference when each reference
+ * should point to the same associative array data.
+ * Without using this function, an associative array is initialized only
+ * when the first element is added.
+ * Params:
+ *      aa = The associative array.
+ * See_Also: $(LREF clear)
+ */
+void initialize(T : Value[Key], Value, Key)(ref T aa) @trusted
+{
+    _aaInitialize(cast(void*) &aa, typeid(T));
+}
+
+///
+@safe unittest
+{
+    int[int] aa;
+    auto b = aa;
+    assert(b is null);
+    aa.initialize;
+    assert(aa !is null);
+    auto c = aa;
+
+    aa[4] = 4;
+    assert(b != aa);
+    assert(c is aa);
+}
+
+/* ditto */
+void initialize(T : Value[Key], Value, Key)(T* aa) @safe
+{
+    _aaInitialize(cast(void*) aa, typeid(T));
+}
+
 /***********************************
  * Removes all remaining keys and values from an associative array.
  * Params:
  *      aa =     The associative array.
+ * See_Also: $(LREF initialize)
  */
 void clear(T : Value[Key], Value, Key)(T aa)
 {

src/rt/aaA.d

@@ -640,6 +640,12 @@ extern (C) void _aaClear(AA aa) pure nothrow
     }
 }
 
+/// Allocate new AA implementation.
+extern (C) void _aaInitialize(AA* paa, const TypeInfo_AssociativeArray ti)
+{
+    paa.impl = new Impl(ti);
+}
+
 /// Rehash AA
 extern (C) void* _aaRehash(AA* paa, in TypeInfo keyti) pure nothrow
 {