Initial commit

.github/workflows/LinuxBuild.yml

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+name: Linux Build
+on:
+  push
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+      - name: Configure CMake
+        run: cmake -B ${{github.workspace}}/build
+      - name: Build
+        run: cmake --build ${{github.workspace}}/build
+      - name: Check that AbstractPlugin::preface is defined in the executable
+        run: nm -g ${{github.workspace}}/build/dynamic_lookup | grep AbstractPlugin.preface | grep T
+      - name: Check that AbstractPlugin::preface is undefined in the module
+        run: nm -g ${{github.workspace}}/build/libModulePlugin.so | grep AbstractPlugin.preface | grep U
+      - name: Run the executable
+        run: cd ${{github.workspace}}/build && ./dynamic_lookup

.github/workflows/MacBuildWithFlags.yml

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+name: Mac Build With Flags
+on:
+  push
+
+jobs:
+  build:
+    runs-on: macos-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+      - name: Configure CMake
+        run: cmake -B ${{github.workspace}}/build -DDYNAMIC_LOOKUP=ON
+      - name: Build
+        run: cmake --build ${{github.workspace}}/build
+      - name: Check that AbstractPlugin::preface is defined in the executable
+        run: nm -g ${{github.workspace}}/build/dynamic_lookup | grep AbstractPlugin.preface | grep T
+      - name: Check that AbstractPlugin::preface is undefined in the module
+        run: nm -g ${{github.workspace}}/build/libModulePlugin.so | grep AbstractPlugin.preface | grep U
+      - name: Run the executable
+        run: cd ${{github.workspace}}/build && ./dynamic_lookup

.github/workflows/MacBuildWithoutFlags.yml

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+name: Mac Build Without Flags
+on:
+  push
+
+jobs:
+  build:
+    runs-on: macos-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+      - name: Configure CMake
+        run: cmake -B ${{github.workspace}}/build
+      - name: Fail to build
+        run: cmake --build ${{github.workspace}}/build
+        continue-on-error: true

AbstractPlugin.cpp

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+#include "AbstractPlugin.hpp"
+#include <iostream>
+
+void AbstractPlugin::preface() {
+  std::cout << "Hello world" << std::endl;
+}

AbstractPlugin.hpp

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+#pragma once
+
+class AbstractPlugin {
+public:
+  /// This symbol must be defined in the module
+  virtual void run() = 0;
+
+  virtual ~AbstractPlugin() = default;
+
+protected:
+  /// This symbol should not be defined in the module
+  void preface();
+};

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.12)
+project(dynamic_lookup)
+set(CMAKE_CXX_STANDARD 17)
+
+option(DYNAMIC_LOOKUP "Adds dynamic lookup linker flag" OFF)
+
+add_library(AbstractPlugin OBJECT AbstractPlugin.cpp)
+add_library(ModulePlugin MODULE ModulePlugin.cpp)
+if (${DYNAMIC_LOOKUP})
+    target_link_options(ModulePlugin PRIVATE -undefined dynamic_lookup)
+endif()
+
+add_executable(dynamic_lookup main.cpp)
+target_link_libraries(dynamic_lookup PUBLIC AbstractPlugin ${CMAKE_DL_LIBS})
+
+set_property(TARGET dynamic_lookup PROPERTY ENABLE_EXPORTS ON)

ModulePlugin.cpp

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+#include "AbstractPlugin.hpp"
+#include <iostream>
+
+class ModulePlugin final : public AbstractPlugin {
+public:
+  void run() override {
+    preface();
+    std::cout << "Bye world" << std::endl;
+  }
+};
+
+extern "C" {
+AbstractPlugin* createPlugin() { return new ModulePlugin(); }
+}

README.md

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+This example shows different behaviour of cmake project build when building MODULEs on Linux and Mac OS
+
+The example consists of three parts:
+- `AbstractPlugin` OBJECT library defines a virtual class AbstractPlugin. It has one defined method `preface` and one pure virtual method `run`. The `preface` method **required** to be defined in the object library 
+- `dynamic_lookup` executable links with the AbstractPlugin library and therefore contains a definition for `AbstractPlugin::preface` method
+- `ModulePlugin` MODULE library defines an implementation of AbstractPlugin. The resulting `.so` file contains the `run` definition and does not contain the `preface` method definition.
+
+The CMakeLists.txt file describes an option DYNAMIC_LOOKUP (off by default) which adds a `-undefined dynamic_lookup` linker flag for the MODULE target.
+
+`dynamic_lookup` executable lists `preface` symbol as defined:
+```shell
+$ nm -g build/dynamic_lookup | grep AbstractPlugin.preface | grep T
+0000000100003010 T __ZN14AbstractPlugin7prefaceEv
+```
+
+and `libModulePlugin` module lists `preface` symbol as undefined:
+```shell
+$ nm -g build/libModulePlugin.so | grep AbstractPlugin.preface | grep T
+                 U __ZN14AbstractPlugin7prefaceEv
+```
+
+Linux build ([Linux Build CI](https://github.com/bugdea1er/dynamic_lookup/actions/workflows/LinuxBuild.yml)) completes just fine: no need for extra flags on top of marking the libs as OBJECT and MODULE where needed:
+```shell
+$ cmake -B build
+$ cmake --build build
+[ 20%] Building CXX object CMakeFiles/AbstractPlugin.dir/AbstractPlugin.cpp.o
+[ 20%] Built target AbstractPlugin
+[ 40%] Building CXX object CMakeFiles/ModulePlugin.dir/ModulePlugin.cpp.o
+[ 60%] Linking CXX shared module libModulePlugin.so
+[ 60%] Built target ModulePlugin
+[ 80%] Building CXX object CMakeFiles/dynamic_lookup.dir/main.cpp.o
+[100%] Linking CXX executable dynamic_lookup
+[100%] Built target dynamic_lookup
+```
+
+Mac OS build ([Mac Build Without Flags CI](https://github.com/bugdea1er/dynamic_lookup/actions/workflows/MacBuildWithoutFlags.yml)) fails under the same conditions:
+```shell
+$ cmake -B build
+$ cmake --build build
+[ 20%] Building CXX object CMakeFiles/AbstractPlugin.dir/AbstractPlugin.cpp.o
+[ 20%] Built target AbstractPlugin
+[ 40%] Building CXX object CMakeFiles/ModulePlugin.dir/ModulePlugin.cpp.o
+[ 60%] Linking CXX shared module libModulePlugin.so
+Undefined symbols for architecture x86_64:
+  "AbstractPlugin::preface()", referenced from:
+      ModulePlugin::run() in ModulePlugin.cpp.o
+ld: symbol(s) not found for architecture x86_64
+clang: error: linker command failed with exit code 1 (use -v to see invocation)
+make[2]: *** [libModulePlugin.so] Error 1
+make[1]: *** [CMakeFiles/ModulePlugin.dir/all] Error 2
+make: *** [all] Error 2
+```
+
+Mac OS build needs a separate linker flag `-undefined dynamic_lookup` in order to achive the same results ([Mac Build With Flags CI](https://github.com/bugdea1er/dynamic_lookup/actions/workflows/MacBuildWithFlags.yml)):
+```shell
+$ cmake -B build -DDYNAMIC_LOOKUP=ON
+$ cmake --build build
+[ 20%] Building CXX object CMakeFiles/AbstractPlugin.dir/AbstractPlugin.cpp.o
+[ 20%] Built target AbstractPlugin
+[ 40%] Building CXX object CMakeFiles/ModulePlugin.dir/ModulePlugin.cpp.o
+[ 60%] Linking CXX shared module libModulePlugin.so
+[ 60%] Built target ModulePlugin
+[ 80%] Building CXX object CMakeFiles/dynamic_lookup.dir/main.cpp.o
+[100%] Linking CXX executable dynamic_lookup
+[100%] Built target dynamic_lookup
+```

main.cpp

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+#include "AbstractPlugin.hpp"
+#include <dlfcn.h>
+#include <filesystem>
+
+int main() {
+  auto module = std::filesystem::absolute("libModulePlugin.so");
+  auto* handle = dlopen(module.c_str(), RTLD_NOW);
+  auto* constructor = (AbstractPlugin*(*)()) dlsym(handle, "createPlugin");
+  auto* plugin = constructor();
+  plugin->run();
+
+  delete plugin;
+  dlclose(handle);
+}