diff --git a/.github/workflows/clang-format-check.yml b/.github/workflows/clang-format-check.yml
index 90e94ea..83f25f5 100644
--- a/.github/workflows/clang-format-check.yml
+++ b/.github/workflows/clang-format-check.yml
@@ -15,5 +15,5 @@ jobs:
       - name: Run clang-format style check for C/C++/Protobuf programs.
         uses: jidicula/clang-format-action@v4.13.0
         with:
-          clang-format-version: '16'
+          clang-format-version: '19'
           check-path: ${{ github.workspace }}
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b3d20a8..3480f66 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -62,7 +62,7 @@ include_directories(shared)
 
 
 ##### subdirectories #####
-set(DIRS week-W week-2 week-3 week-4 week-5 week-6 week-7 week-8 week-9 week-10 week-11)
+set(DIRS week-W week-2 week-3 week-4 week-5 week-6 week-7 week-8 week-9 week-10 week-11 week-12)
 foreach(DIR ${DIRS})
     add_subdirectory(${DIR})
 endforeach()
diff --git a/week-12/CMakeLists.txt b/week-12/CMakeLists.txt
new file mode 100644
index 0000000..480dea5
--- /dev/null
+++ b/week-12/CMakeLists.txt
@@ -0,0 +1,4 @@
+set(DIRS task-11-01 task-11-03 task-11-06)
+foreach(DIR ${DIRS})
+    add_subdirectory(${DIR})
+endforeach()
diff --git a/week-12/task-11-01/CMakeLists.txt b/week-12/task-11-01/CMakeLists.txt
new file mode 100644
index 0000000..0857059
--- /dev/null
+++ b/week-12/task-11-01/CMakeLists.txt
@@ -0,0 +1,3 @@
+include_directories(include)
+
+add_subdirectory(test)
diff --git a/week-12/task-11-01/include/self-pointed-function.hpp b/week-12/task-11-01/include/self-pointed-function.hpp
new file mode 100644
index 0000000..39d1085
--- /dev/null
+++ b/week-12/task-11-01/include/self-pointed-function.hpp
@@ -0,0 +1,28 @@
+#pragma once
+
+#include <iostream>
+
+namespace functions {
+
+/**
+ * function returns itself.
+ */
+std::function<void(int& p)> do_it(int& param) {
+    param++;
+    static int counter = 1;
+    std::cout << "do it #" << counter << ": param = " << param << std::endl;
+    counter++;
+    return [](int& p) { do_it(p); };
+}
+
+void* foo();
+template <auto F>
+void* addressF() {
+    return reinterpret_cast<void*>(F);
+}
+/**
+ * function returns pointer to itself.
+ */
+void* foo() { return addressF<foo>(); }
+
+}  // namespace functions
diff --git a/week-12/task-11-01/test/CMakeLists.txt b/week-12/task-11-01/test/CMakeLists.txt
new file mode 100644
index 0000000..a18d496
--- /dev/null
+++ b/week-12/task-11-01/test/CMakeLists.txt
@@ -0,0 +1 @@
+cpp_test(test-11-01.cpp)
diff --git a/week-12/task-11-01/test/test-11-01.cpp b/week-12/task-11-01/test/test-11-01.cpp
new file mode 100644
index 0000000..bb9a0de
--- /dev/null
+++ b/week-12/task-11-01/test/test-11-01.cpp
@@ -0,0 +1,20 @@
+#include "gtest/gtest.h"
+#include "self-pointed-function.hpp"
+
+using functions::do_it;
+using functions::foo;
+
+TEST(FunctionReternsItself, Call) {
+    constexpr int initial = 42;
+    int var = initial;  // NOLINT
+    auto do_it_again = do_it(var);
+    EXPECT_EQ(var, initial + 1);
+    do_it_again(var);
+    EXPECT_EQ(var, initial + 2);
+}
+
+TEST(SelfPointedFunction, Call) {
+    void* pfoo = foo();
+    auto func_ptr = reinterpret_cast<void* (*)()>(pfoo);
+    EXPECT_EQ(func_ptr(), foo());
+}
diff --git a/week-12/task-11-03/CMakeLists.txt b/week-12/task-11-03/CMakeLists.txt
new file mode 100644
index 0000000..0857059
--- /dev/null
+++ b/week-12/task-11-03/CMakeLists.txt
@@ -0,0 +1,3 @@
+include_directories(include)
+
+add_subdirectory(test)
diff --git a/week-12/task-11-03/include/factory.hpp b/week-12/task-11-03/include/factory.hpp
new file mode 100644
index 0000000..cd07b57
--- /dev/null
+++ b/week-12/task-11-03/include/factory.hpp
@@ -0,0 +1,25 @@
+#pragma once
+
+#include <functional>
+#include <map>
+#include <typeindex>
+#include <variant>
+
+namespace patterns {
+
+template <typename... Ts>
+class Factory {
+public:
+    using creator_type = std::variant<std::function<Ts()>...>;
+    explicit Factory(std::function<Ts()>... args) : map_{{typeid(Ts), creator_type(args)}...} {}
+
+    template <typename T>
+    T create() const {
+        return std::get<std::function<T()>>(map_.find(typeid(T))->second)();
+    }
+
+private:
+    std::unordered_map<std::type_index, std::variant<std::function<Ts()>...>> map_;
+};
+
+}  // namespace patterns
diff --git a/week-12/task-11-03/test/CMakeLists.txt b/week-12/task-11-03/test/CMakeLists.txt
new file mode 100644
index 0000000..6645278
--- /dev/null
+++ b/week-12/task-11-03/test/CMakeLists.txt
@@ -0,0 +1 @@
+cpp_test(test-11-03.cpp)
diff --git a/week-12/task-11-03/test/test-11-03.cpp b/week-12/task-11-03/test/test-11-03.cpp
new file mode 100644
index 0000000..f43e819
--- /dev/null
+++ b/week-12/task-11-03/test/test-11-03.cpp
@@ -0,0 +1,88 @@
+#include <memory>
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "factory.hpp"
+#include "gtest/gtest.h"
+
+using patterns::Factory;
+
+struct Point {
+    int x, y;
+    bool operator==(const Point& other) const { return x == other.x && y == other.y; }
+};
+
+class Person {
+public:
+    Person(std::string name, int age) : name_(std::move(name)), age_(age) {}
+    std::string getName() const { return name_; }
+    int getAge() const { return age_; }
+    bool operator==(const Person& other) const {
+        return name_ == other.name_ && age_ == other.age_;
+    }
+
+private:
+    std::string name_;
+    int age_;
+};
+
+TEST(Factory, StandardTypes) {
+    constexpr int ret1 = 42;
+    constexpr double ret2 = 42.42;
+    const Factory<int, double> factory{[]() { return ret1; }, []() { return ret2; }};
+    EXPECT_EQ(factory.create<int>(), ret1);
+    EXPECT_FLOAT_EQ(factory.create<double>(), ret2);
+}
+
+TEST(Factory, CustomTypes) {
+    const Point point{10, 20};
+    const Person person{"Alice", 30};
+    const Factory<Point, Person> factory{[point]() { return point; },
+                                         [person]() { return person; }};
+    EXPECT_EQ(factory.create<Point>(), point);
+    EXPECT_EQ(factory.create<Person>(), person);
+}
+
+// @todo #15:30m/DEV find a way to check if a compilation error has occurred or not.
+#ifdef TEST_COMPILE_ERROR
+TEST(Factory, UnregisteredType) {
+    const Factory<int, double> factory{[]() { return 42; }, []() { return 3.14; }};
+    EXPECT_THROW(factory.create<std::string>(), std::runtime_error);
+}
+
+TEST(Factory, EmptyFactory) {
+    const Factory<int, double> factory{};
+    EXPECT_THROW(factory.create<int>(), std::runtime_error);
+    EXPECT_THROW(factory.create<double>(), std::runtime_error);
+}
+#endif
+
+TEST(Factory, ComplexConstructors) {
+    const Factory<std::vector<int>, std::string> factory{
+        []() { return std::vector<int>{1, 2, 3}; }, []() { return std::string("Hello, World!"); }};
+    const std::vector<int> expected_vector{1, 2, 3};
+    const std::string expected_string = "Hello, World!";
+    EXPECT_EQ(factory.create<std::vector<int>>(), expected_vector);
+    EXPECT_EQ(factory.create<std::string>(), expected_string);
+}
+
+TEST(Factory, SmartPointers) {
+    constexpr int ret1 = 42;
+    constexpr double ret2 = 3.14;
+    const Factory<std::unique_ptr<int>, std::shared_ptr<double>> factory{
+        [ret1]() { return std::make_unique<int>(ret1); },
+        [ret2]() { return std::make_shared<double>(ret2); }};
+    auto unique_ptr = factory.create<std::unique_ptr<int>>();
+    auto shared_ptr = factory.create<std::shared_ptr<double>>();
+    EXPECT_EQ(*unique_ptr, ret1);
+    EXPECT_DOUBLE_EQ(*shared_ptr, ret2);
+}
+
+TEST(Factory, OptionalTypes) {
+    const Factory<std::optional<int>, std::optional<std::string>> factory{
+        []() { return std::optional<int>(42); },  // NOLINT
+        []() { return std::optional<std::string>("Hello"); }};
+    EXPECT_EQ(factory.create<std::optional<int>>().value(), 42);
+    EXPECT_EQ(factory.create<std::optional<std::string>>().value(), "Hello");
+}
diff --git a/week-12/task-11-06/CMakeLists.txt b/week-12/task-11-06/CMakeLists.txt
new file mode 100644
index 0000000..0857059
--- /dev/null
+++ b/week-12/task-11-06/CMakeLists.txt
@@ -0,0 +1,3 @@
+include_directories(include)
+
+add_subdirectory(test)
diff --git a/week-12/task-11-06/include/circle.hpp b/week-12/task-11-06/include/circle.hpp
new file mode 100644
index 0000000..cb10f6a
--- /dev/null
+++ b/week-12/task-11-06/include/circle.hpp
@@ -0,0 +1,31 @@
+#pragma once
+
+#include <numbers>
+
+#include "ivisitor.hpp"
+#include "shape.hpp"
+
+namespace geometry2d {
+
+class Circle : public Shape {
+public:
+    static constexpr double pi = std::numbers::pi;
+
+    explicit Circle(double r) : radius_(r) {
+        if (radius_ <= 0) {
+            throw std::invalid_argument("Radius must be positive.");
+        }
+    }
+
+    double radius() const { return radius_; }
+
+    double perimeter() const override { return 2 * pi * radius_; }
+    double area() const override { return pi * radius_ * radius_; }
+
+    void visit_by(const IVisitor& visitor) const override { visitor.visit(*this); }
+
+private:
+    double radius_;
+};
+
+}  // namespace geometry2d
diff --git a/week-12/task-11-06/include/ivisitor.hpp b/week-12/task-11-06/include/ivisitor.hpp
new file mode 100644
index 0000000..cd264ad
--- /dev/null
+++ b/week-12/task-11-06/include/ivisitor.hpp
@@ -0,0 +1,19 @@
+#pragma once
+
+namespace geometry2d {
+
+class Circle;
+class Square;
+class Triangle;
+class Shape;
+
+class IVisitor {
+public:
+    virtual ~IVisitor() = default;
+
+    virtual void visit(const Circle& c) const = 0;
+    virtual void visit(const Square& s) const = 0;
+    virtual void visit(const Triangle& t) const = 0;
+};
+
+}  // namespace geometry2d
diff --git a/week-12/task-11-06/include/serializer.hpp b/week-12/task-11-06/include/serializer.hpp
new file mode 100644
index 0000000..ca3428d
--- /dev/null
+++ b/week-12/task-11-06/include/serializer.hpp
@@ -0,0 +1,36 @@
+#pragma once
+
+#include <iostream>
+
+#include "circle.hpp"
+#include "ivisitor.hpp"
+#include "square.hpp"
+#include "triangle.hpp"
+
+namespace geometry2d {
+
+class Serializer : public IVisitor {
+public:
+    void visit(const Circle &c) const override {
+        std::cout << "Circle:" << std::endl;
+        std::cout << "\tradius: " << c.radius() << std::endl;
+        std::cout << "\t\tperimeter: " << c.perimeter() << std::endl;
+        std::cout << "\t\tarea: " << c.area() << std::endl;
+    }
+    void visit(const Square &s) const override {
+        std::cout << "Square:" << std::endl;
+        std::cout << "\tside: " << s.side() << std::endl;
+        std::cout << "\t\tperimeter: " << s.perimeter() << std::endl;
+        std::cout << "\t\tarea: " << s.area() << std::endl;
+    }
+    void visit(const Triangle &t) const override {
+        std::cout << "Triangle:" << std::endl;
+        std::cout << "\tside1: " << t.side1() << std::endl;
+        std::cout << "\tside2: " << t.side2() << std::endl;
+        std::cout << "\tside3: " << t.side3() << std::endl;
+        std::cout << "\t\tperimeter: " << t.perimeter() << std::endl;
+        std::cout << "\t\tarea: " << t.area() << std::endl;
+    }
+};
+
+}  // namespace geometry2d
diff --git a/week-12/task-11-06/include/shape.hpp b/week-12/task-11-06/include/shape.hpp
new file mode 100644
index 0000000..6959161
--- /dev/null
+++ b/week-12/task-11-06/include/shape.hpp
@@ -0,0 +1,16 @@
+#pragma once
+
+#include "ivisitor.hpp"
+
+namespace geometry2d {
+
+class Shape {
+public:
+    virtual ~Shape() = default;
+
+    virtual void visit_by(const IVisitor& visitor) const = 0;
+    virtual double perimeter() const = 0;
+    virtual double area() const = 0;
+};
+
+}  // namespace geometry2d
diff --git a/week-12/task-11-06/include/square.hpp b/week-12/task-11-06/include/square.hpp
new file mode 100644
index 0000000..75bf96a
--- /dev/null
+++ b/week-12/task-11-06/include/square.hpp
@@ -0,0 +1,27 @@
+#pragma once
+
+#include "ivisitor.hpp"
+#include "shape.hpp"
+
+namespace geometry2d {
+
+class Square : public Shape {
+public:
+    explicit Square(double s) : side_(s) {
+        if (side_ <= 0) {
+            throw std::invalid_argument("Side must be positive.");
+        }
+    }
+
+    double side() const { return side_; }
+
+    double perimeter() const override { return 4 * side_; }
+    double area() const override { return side_ * side_; }
+
+    void visit_by(const IVisitor& visitor) const override { visitor.visit(*this); }
+
+private:
+    double side_;
+};
+
+}  // namespace geometry2d
diff --git a/week-12/task-11-06/include/triangle.hpp b/week-12/task-11-06/include/triangle.hpp
new file mode 100644
index 0000000..1be0c71
--- /dev/null
+++ b/week-12/task-11-06/include/triangle.hpp
@@ -0,0 +1,36 @@
+#pragma once
+
+#include <cmath>
+
+#include "shape.hpp"
+
+namespace geometry2d {
+
+class Triangle : public Shape {
+public:
+    Triangle(double s1, double s2, double s3) : side1_(s1), side2_(s2), side3_(s3) {
+        if (side1_ <= 0 || side2_ <= 0 || side3_ <= 0) {
+            throw std::invalid_argument("All sides must be positive.");
+        }
+        if (side1_ + side2_ <= side3_ || side1_ + side3_ <= side2_ || side2_ + side3_ <= side1_) {
+            throw std::invalid_argument("Invalid triangle sides.");
+        }
+    }
+
+    double side1() const { return side1_; }
+    double side2() const { return side2_; }
+    double side3() const { return side3_; }
+
+    double perimeter() const override { return side1_ + side2_ + side3_; }
+    double area() const override {
+        const double p = perimeter() / 2;
+        return std::sqrt(p * (p - side1_) * (p - side2_) * (p - side3_));
+    }
+
+    void visit_by(const IVisitor& visitor) const override { visitor.visit(*this); }
+
+private:
+    double side1_, side2_, side3_;
+};
+
+}  // namespace geometry2d
diff --git a/week-12/task-11-06/test/CMakeLists.txt b/week-12/task-11-06/test/CMakeLists.txt
new file mode 100644
index 0000000..6162276
--- /dev/null
+++ b/week-12/task-11-06/test/CMakeLists.txt
@@ -0,0 +1 @@
+cpp_test(test-11-06.cpp)
diff --git a/week-12/task-11-06/test/test-11-06.cpp b/week-12/task-11-06/test/test-11-06.cpp
new file mode 100644
index 0000000..5cffc02
--- /dev/null
+++ b/week-12/task-11-06/test/test-11-06.cpp
@@ -0,0 +1,65 @@
+#include <gtest/gtest.h>
+
+#include <iostream>
+#include <memory>
+#include <sstream>
+
+#include "serializer.hpp"
+
+using geometry2d::Circle;
+using geometry2d::Serializer;
+using geometry2d::Shape;
+using geometry2d::Square;
+using geometry2d::Triangle;
+
+#define EXPECT_STDOUT_EQ(printing_statement, expected_output)              \
+    {                                                                      \
+        testing::internal::CaptureStdout();                                \
+        printing_statement;                                                \
+        const std::string output = testing::internal::GetCapturedStdout(); \
+        EXPECT_EQ(output, expected_output);                                \
+    }
+
+TEST(Serializer, ShapeCircle) {
+    const Serializer serializer{};
+    const std::unique_ptr<Shape> shape = std::make_unique<Circle>(4);
+    const std::string expected_output =
+        "Circle:\n"
+        "\tradius: 4\n"
+        "\t\tperimeter: 25.1327\n"
+        "\t\tarea: 50.2655\n";
+    EXPECT_STDOUT_EQ(shape->visit_by(serializer), expected_output);
+}
+
+TEST(Serializer, ShapeSquare) {
+    const Serializer serializer{};
+    const std::unique_ptr<Shape> shape = std::make_unique<Square>(5);
+    const std::string expected_output =
+        "Square:\n"
+        "\tside: 5\n"
+        "\t\tperimeter: 20\n"
+        "\t\tarea: 25\n";
+    EXPECT_STDOUT_EQ(shape->visit_by(serializer), expected_output);
+}
+
+TEST(Serializer, ShapeTriangle) {
+    const Serializer serializer{};
+    const std::unique_ptr<Shape> shape = std::make_unique<Triangle>(3, 4, 5);
+    const std::string expected_output =
+        "Triangle:\n"
+        "\tside1: 3\n"
+        "\tside2: 4\n"
+        "\tside3: 5\n"
+        "\t\tperimeter: 12\n"
+        "\t\tarea: 6\n";
+    EXPECT_STDOUT_EQ(shape->visit_by(serializer), expected_output);
+}
+
+TEST(Serializer, InvalidCircleRadius) { EXPECT_THROW(Circle(-1), std::invalid_argument); }
+
+TEST(Serializer, InvalidSquareSide) { EXPECT_THROW(Square(0), std::invalid_argument); }
+
+TEST(Serializer, InvalidTriangleSides) {
+    EXPECT_THROW(Triangle(1, 2, 3), std::invalid_argument);
+    EXPECT_THROW(Triangle(-1, 2, 2), std::invalid_argument);
+}