Index: mlir/include/mlir/IR/AttrTypeBase.td
===================================================================
--- mlir/include/mlir/IR/AttrTypeBase.td
+++ mlir/include/mlir/IR/AttrTypeBase.td
@@ -215,6 +215,11 @@
 
   // Extra code to include in the class declaration.
   code extraClassDeclaration = [{}];
+
+  // Additional code that will be added to the generated source file. The
+  // generated code is placed inside the attribute's C++ namespace. `$cppClass` is
+  // replaced by the attribute's class name.
+  code extraClassDefinition = [{}];
 }
 
 // Define a new attribute, named `name`, belonging to `dialect` that inherits
Index: mlir/include/mlir/TableGen/AttrOrTypeDef.h
===================================================================
--- mlir/include/mlir/TableGen/AttrOrTypeDef.h
+++ mlir/include/mlir/TableGen/AttrOrTypeDef.h
@@ -198,6 +198,9 @@
   /// Returns the def's extra class declaration code.
   Optional<StringRef> getExtraDecls() const;
 
+  /// Returns the def's extra class definition code.
+  Optional<StringRef> getExtraDefs() const;
+
   /// Get the code location (for error printing).
   ArrayRef<SMLoc> getLoc() const;
 
Index: mlir/lib/TableGen/AttrOrTypeDef.cpp
===================================================================
--- mlir/lib/TableGen/AttrOrTypeDef.cpp
+++ mlir/lib/TableGen/AttrOrTypeDef.cpp
@@ -175,6 +175,11 @@
   return value.empty() ? Optional<StringRef>() : value;
 }
 
+Optional<StringRef> AttrOrTypeDef::getExtraDefs() const {
+  auto value = def->getValueAsString("extraClassDefinition");
+  return value.empty() ? Optional<StringRef>() : value;
+}
+
 ArrayRef<SMLoc> AttrOrTypeDef::getLoc() const { return def->getLoc(); }
 
 bool AttrOrTypeDef::skipDefaultBuilders() const {
Index: mlir/tools/mlir-tblgen/AttrOrTypeDefGen.cpp
===================================================================
--- mlir/tools/mlir-tblgen/AttrOrTypeDefGen.cpp
+++ mlir/tools/mlir-tblgen/AttrOrTypeDefGen.cpp
@@ -97,6 +97,8 @@
   void emitAccessors();
   /// Emit interface methods.
   void emitInterfaceMethods();
+  /// Emit extra class definition, if specified.
+  void emitExtraClassDefinition();
 
   //===--------------------------------------------------------------------===//
   // Builder Emission
@@ -157,6 +159,8 @@
   StringRef valueType;
   /// The prefix/suffix of the TableGen def name, either "Attr" or "Type".
   StringRef defType;
+  /// Hand-written extra class definitions.
+  std::string extraClassDefinition;
 };
 } // namespace
 
@@ -191,6 +195,8 @@
     emitAccessors();
   // Emit trait interface methods
   emitInterfaceMethods();
+  // Emit hand written class definitions
+  emitExtraClassDefinition();
   defCls.finalize();
   // Emit a storage class if one is needed
   if (storageCls && def.genStorageClass())
@@ -214,6 +220,14 @@
   defCls.addParent(std::move(defParent));
 }
 
+void DefGen::emitExtraClassDefinition() {
+ if (Optional<StringRef> extraDef = def.getExtraDefs()) {
+   FmtContext ctx = FmtContext().addSubst("cppClass", def.getCppClassName());
+   extraClassDefinition = tgfmt(*extraDef, &ctx).str();
+   defCls.declare<ExtraClassDeclaration>("", extraClassDefinition);
+ }
+}
+
 void DefGen::emitTopLevelDeclarations() {
   // Inherit constructors from the attribute or type class.
   defCls.declare<VisibilityDeclaration>(Visibility::Public);