add core.factory (#14888)

druntime/mak/COPY

@@ -15,6 +15,7 @@ COPY=\
 	$(IMPDIR)\core\cpuid.d \
 	$(IMPDIR)\core\demangle.d \
 	$(IMPDIR)\core\exception.d \
+	$(IMPDIR)\core\factory.d \
 	$(IMPDIR)\core\int128.d \
 	$(IMPDIR)\core\lifetime.d \
 	$(IMPDIR)\core\math.d \

druntime/mak/DOCS

@@ -12,6 +12,7 @@ DOCS=\
 	$(DOCDIR)\core_attribute.html \
 	$(DOCDIR)\core_cpuid.html \
 	$(DOCDIR)\core_demangle.html \
+	$(DOCDIR)\core_factory.html \
 	$(DOCDIR)\core_lifetime.html \
 	$(DOCDIR)\core_memory.html \
 	$(DOCDIR)\core_runtime.html \

druntime/mak/SRCS

@@ -9,6 +9,7 @@ SRCS=\
 	src\core\cpuid.d \
 	src\core\demangle.d \
 	src\core\exception.d \
+	src\core\factory.d \
 	src\core\int128.d \
 	src\core\lifetime.d \
 	src\core\math.d \

druntime/src/core/factory.d

@@ -0,0 +1,68 @@
+/* Create classes from their modules and names.
+ *
+ * Copyright: Copyright (C) D Language Foundation 2023
+ * License:   $(LINK2 http://www.boost.org/LICENSE_1_0.txt, Boost License 1.0)
+ * Authors:   Walter Bright, Steven Schveighoffer
+ * Source:    $(DRUNTIMESRC core/_factory.d)
+ */
+
+module core.factory;
+
+/**
+ * Create instance of class specified by the module symbol and a string
+ * representing the name of the class.
+ * The class must either have no constructors or have
+ * a default constructor.
+ * Params:
+ *   mod = symbol representing the module that the class is in
+ *   classname = string representing the name of the class
+ * Returns:
+ *   null if failed
+ * Example:
+ * ---
+ * module foo.bar;
+ *
+ * class C
+ * {
+ *     this() { x = 10; }
+ *     int x;
+ * }
+ *
+ * void main()
+ * {
+ *     auto c = cast(C)factory!(foo.bar)("C");
+ *     assert(c !is null && c.x == 10);
+ * }
+ * ---
+ */
+Object factory(alias mod)(string classname)
+{
+    foreach(cl; _getModuleClasses!mod)
+    {
+        if (cl.stringof == classname)
+            return cl.classinfo.create();
+    }
+    return null;
+}
+
+@system unittest
+{
+    Object valid_obj = factory!object("Object");
+    Object invalid_obj = factory!object("__this_class_doesnt_exist__");
+
+    assert(valid_obj !is null);
+    assert(invalid_obj is null);
+}
+
+/**************************************
+ * Retrieve as a tuple all the types of the top level classes in the module mod.
+ */
+private template _getModuleClasses(alias mod) {
+   alias result = _AliasSeq!();
+   static foreach(m; __traits(allMembers, mod))
+      static if(is(__traits(getMember, mod, m) == class))
+         result = _AliasSeq!(result, __traits(getMember, mod, m));
+   alias _getModuleClasses = result;
+}
+
+private template _AliasSeq(TList...) { alias _AliasSeq = TList; }