add critical sections example

Author: Tsche

example/critical_section/CMakeLists.txt

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+cmake_minimum_required(VERSION 3.15)
+project(chroma-examples LANGUAGES CXX)
+
+set(CMAKE_CXX_STANDARD 26)
+set(CMAKE_CXX_STANDARD_REQUIRED True)
+
+find_package(chroma REQUIRED)
+
+add_executable(chroma-critical-section src/main.cpp)
+target_link_libraries(chroma-critical-section PRIVATE chroma::chroma)
+target_include_directories(chroma-critical-section PRIVATE include)
+
+# WORKAROUND:
+# Conan needs the new CMakeConfigDeps generator to handle INTERFACE options etc properly
+target_compile_features(chroma-critical-section PRIVATE cxx_std_26)
+if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+  target_compile_options(chroma-critical-section PRIVATE "-stdlib=libc++" "-freflection-latest")
+  target_link_options(chroma-critical-section PRIVATE "-stdlib=libc++")
+  target_link_libraries(chroma-critical-section PRIVATE "c++abi")
+elseif (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+  target_compile_options(chroma-critical-section PRIVATE "-freflection")
+endif()

example/critical_section/conanfile.py

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+from conan import ConanFile
+from conan.tools.cmake import CMake, cmake_layout
+
+class Consumer(ConanFile):
+    name = "chroma-critical-section"
+    version = "0.1"
+    settings = "os", "compiler", "build_type", "arch"
+    requires = "chroma/0.0.1"
+    generators = "CMakeDeps", "CMakeToolchain"
+
+    def layout(self):
+        cmake_layout(self)
+
+    def build(self):
+        cmake = CMake(self)
+        cmake.configure()
+        cmake.build()

example/critical_section/include/lock.hpp

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+
+#pragma once
+#include <mutex>
+#include <utility>
+#include <chroma>
+
+class locked_context {
+public:
+  explicit locked_context(std::mutex& mtx) : m_mutex(&mtx) { m_mutex->lock(); }
+
+  ~locked_context() {
+    if (m_mutex != nullptr) {
+      m_mutex->unlock();
+    }
+  }
+
+  locked_context(locked_context&& other) noexcept
+      : m_mutex(std::exchange(other.m_mutex, nullptr)) {}
+
+  locked_context& operator=(locked_context&& other) noexcept {
+    if (this != &other) {
+      if (m_mutex != nullptr) {
+        m_mutex->unlock();
+      }
+      m_mutex = std::exchange(other.m_mutex, nullptr);
+    }
+    return *this;
+  }
+
+  locked_context(const locked_context&)            = delete;
+  locked_context& operator=(const locked_context&) = delete;
+
+  [[nodiscard]] chroma::color<locked_context> context() const {
+    if (m_mutex == nullptr) {
+      throw std::runtime_error("Invalid context");
+    }
+    return {};
+  }
+
+  static constexpr auto error_message = "not in locked context";
+
+private:
+  std::mutex* m_mutex;
+};

example/critical_section/src/main.cpp

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+#include <chroma>
+#include <lock.hpp>
+
+void still_in_critical_section(callable_from<locked_context> = {}) {
+  // ...
+}
+
+void must_be_locked(callable_from<locked_context> = {}) {
+  // call can be implicit, lock is still held
+  still_in_critical_section();
+}
+
+int main() {
+  std::mutex mtx;
+  {
+    locked_context lock(mtx); // acquire lock
+
+    // get color for the acquired lock
+    must_be_locked(lock.context());
+  }
+}