Update: Using C++26 with contracts enabled

README.md

@@ -5,21 +5,33 @@ A hands-on C++ course with **56 Google Test exercises** organized into **14 modu
 ## Requirements
 
 - **CMake** 3.20+
-- **C++ compiler** with C++23 support for later modules
-  - Apple Clang 15+, GCC 13+, or MSVC 19.34+ recommended
+- **C++26-capable compiler**
+  - **GCC 16+** recommended (required for `pre`/`post` contracts)
+  - Apple Clang can build most exercises but **does not support contracts yet**
 - **CLion** (recommended) or any CMake-aware IDE
 - Internet access on first configure (Google Test is fetched automatically)
 
 ## Quick Start (CLion)
 
 1. Open this folder in CLion (`File → Open…`).
-2. CLion detects `CMakeLists.txt` and configures automatically.
-3. Pick an exercise target (e.g. `01_fundamentals_hello_world`) in the run configuration dropdown.
-4. Implement the TODOs in the matching `src/*.cpp` file.
-5. Run the test target until all assertions pass.
+2. Select the **`gcc26-contracts`** CMake profile (see `CMakePresets.json`).
+3. Reload CMake when prompted.
+4. Pick an exercise target (e.g. `02_control_flow_loops`) in the run configuration dropdown.
+5. Implement the TODOs in the matching `src/*.cpp` file.
+6. Run the test target until all assertions pass.
 
 ## Quick Start (Terminal)
 
+**Recommended — GCC 16 with contracts:**
+
+```bash
+cmake --preset gcc26-contracts
+cmake --build build-gcc26
+ctest --test-dir build-gcc26 --output-on-failure
+```
+
+**Default toolchain:**
+
 ```bash
 cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug
 cmake --build build
@@ -29,8 +41,8 @@ ctest --test-dir build --output-on-failure
 Run a single exercise:
 
 ```bash
-cmake --build build --target 01_fundamentals_hello_world
-./build/modules/01_fundamentals/01_fundamentals_hello_world
+cmake --build build-gcc26 --target 02_control_flow_loops
+./build-gcc26/modules/02_control_flow/02_control_flow_loops
 ```
 
 ## Project Structure
@@ -40,7 +52,10 @@ cppc/
 ├── CMakeLists.txt              # Root project + Google Test
 ├── cmake/
 │   ├── CourseExercise.cmake    # add_course_exercise() helper
-│   └── CourseOptions.cmake     # Warnings and standards
+│   ├── CourseOptions.cmake     # Warnings, standards, contracts
+│   └── toolchains/gcc-16.cmake # Optional GCC 16 toolchain file
+├── CMakePresets.json           # CLion/CMake presets (gcc26-contracts)
+├── support/                    # Shared contract violation handler
 └── modules/
     ├── 01_fundamentals/        # Types, operators, I/O
     ├── 02_control_flow/       # if, switch, loops
@@ -75,22 +90,22 @@ Each module contains:
 
 ## Course Map
 
-| Module | Topic | Exercises | C++ Standard |
-|--------|-------|-----------|--------------|
-| 01 | Fundamentals | 4 | C++17 |
-| 02 | Control flow | 4 | C++17 |
-| 03 | Functions | 4 | C++17 |
-| 04 | Arrays & strings | 4 | C++17 |
-| 05 | Pointers & references | 4 | C++17 |
-| 06 | OOP basics | 4 | C++17 |
-| 07 | STL containers | 4 | C++17 |
-| 08 | STL algorithms | 4 | C++17 |
-| 09 | C++11 | 4 | C++11 |
-| 10 | C++14 | 4 | C++14 |
-| 11 | C++17 | 4 | C++17 |
-| 12 | C++20 | 4 | C++20 |
-| 13 | C++23 | 4 | C++23 |
-| 14 | C++26 frontier | 4 | C++23+ |
+| Module | Topic | Exercises | Build standard |
+|--------|-------|-----------|----------------|
+| 01 | Fundamentals | 4 | C++26 |
+| 02 | Control flow | 4 | C++26 |
+| 03 | Functions | 4 | C++26 |
+| 04 | Arrays & strings | 4 | C++26 |
+| 05 | Pointers & references | 4 | C++26 |
+| 06 | OOP basics | 4 | C++26 |
+| 07 | STL containers | 4 | C++26 |
+| 08 | STL algorithms | 4 | C++26 |
+| 09 | C++11 features | 4 | C++26 |
+| 10 | C++14 features | 4 | C++26 |
+| 11 | C++17 features | 4 | C++26 |
+| 12 | C++20 features | 4 | C++26 |
+| 13 | C++23 features | 4 | C++26 |
+| 14 | C++26 frontier | 4 | C++26 |
 
 ## CMake Options
 
@@ -98,11 +113,38 @@ Each module contains:
 |--------|---------|-------------|
 | `COURSE_BUILD_TESTS` | ON | Build Google Test exercise targets |
 | `COURSE_WARNINGS_AS_ERRORS` | OFF | Treat warnings as errors |
+| `COURSE_ENABLE_CONTRACTS` | ON | Enable `-fcontracts` on GCC 16+ targets |
 
 Example:
 
 ```bash
-cmake -S . -B build -DCOURSE_WARNINGS_AS_ERRORS=ON
+cmake --preset gcc26-contracts -DCOURSE_WARNINGS_AS_ERRORS=ON
+```
+
+## C++26 Contracts
+
+Exercises compile as **C++26**. On **GCC 16+**, the build enables:
+
+- `-fcontracts`
+- `-fcontract-semantic=enforce`
+- A shared `handle_contract_violation` handler in `support/contract_handler.cpp`
+
+Use `#include <contracts>` and syntax like:
+
+```cpp
+auto factorial(int n) -> long long
+    pre(n >= 0)
+{
+    // ...
+}
+```
+
+**Apple Clang does not support contracts yet.** Use the `gcc26-contracts` preset in CLion or:
+
+```bash
+cmake -S . -B build-gcc26 \
+  -DCMAKE_CXX_COMPILER=/opt/homebrew/bin/g++-16 \
+  -DCOURSE_ENABLE_CONTRACTS=ON
 ```
 
 ## CLion Tips
@@ -114,6 +156,6 @@ cmake -S . -B build -DCOURSE_WARNINGS_AS_ERRORS=ON
 
 ## Notes on C++26
 
-C++26 is still in active standardization. Module 14 focuses on techniques and library directions that remain relevant—deep `constexpr`, monadic `expected`, ranges pipelines, and `std::formatter`—using C++23 as the practical baseline.
+All exercises now build with **C++26**. Module READMEs still describe the language feature each exercise teaches (C++11, C++17, etc.), but the toolchain baseline is unified at C++26 for a consistent modern environment.
 
 Happy learning!