add example with docs

.gitignore

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+build
+.cache

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.15)
+
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
+project(core CXX)
+
+find_package(fmt REQUIRED)
+find_package(range-v3 REQUIRED)
+find_package(cxxopts REQUIRED)
+find_package(Protobuf CONFIG REQUIRED)
+
+set(CMAKE_CXX_STANDARD 17)
+
+add_library(core STATIC
+  "${CMAKE_CURRENT_SOURCE_DIR}/protocol/messenger/messenger.proto"
+  fib.cpp
+)
+
+set(PROTO_BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}/generated")
+target_include_directories(core 
+  PUBLIC 
+    ${CMAKE_CURRENT_SOURCE_DIR}
+    "$<BUILD_INTERFACE:${PROTO_BINARY_DIR}>"
+)
+
+target_link_libraries(core
+  PRIVATE
+    fmt::fmt
+    range-v3::range-v3
+    cxxopts::cxxopts
+  PUBLIC
+    protobuf::libprotobuf
+)
+
+protobuf_generate(
+    TARGET core
+    IMPORT_DIRS "${CMAKE_CURRENT_SOURCE_DIR}/protocol"
+    PROTOC_OUT_DIR "${PROTO_BINARY_DIR}"
+)

README.md

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+# Overview
+This repository serves as a complex example of how C++ tooling and cross-compilation works.
+
+## The challenge
+The variables at play:
+* Protoc used to generate C++ files
+* Compilation of the generated C++ files & other library/source files.
+* Linking the correct protobuf runtime library for the target machine.
+
+`protoc` from any host machine can generate portable C++ files (.cpp and .h). However, the version of protoc that is used must match the _protobuf runtime library_ that is used.
+
+For example, the generated header will have something like this:
+```cpp
+#include "google/protobuf/runtime_version.h"
+#if PROTOBUF_VERSION != 5029003
+#error "Protobuf C++ gencode is built with an incompatible version of"
+#error "Protobuf C++ headers/runtime. See"
+#error "https://protobuf.dev/support/cross-version-runtime-guarantee/#cpp"
+#endif
+```
+
+If we are trying to build an application for arm64 from a x86_64 machine, the protobuf runtime library that we use _must_ be for that target (AND still match the protoc version that we used to generate the C++ files from `.proto`). This is why we use vcpkg.
+
+
+See [here](https://claude.ai/share/c6c6394d-b9b4-41b4-9146-265682223174) for in-depth conversation with claude.
+
+
+## The solution
+```sh
+./build.sh # build for applications meant only for the host machine
+./cross-compile-for-ios.sh # uses ios/arm64 toolchain to compile all protoc generated C++ as well as other library files. More importantly, it also ensures that it compiles (or pulls from vcpkg cache) the correct protobuf runtime version for arm64.
+```
+
+Please look into the scripts to see how it all comes together.
+
+## Troubleshooting
+I had an issue with vcpkg whereby the protobuf was conflicting between host machine one and the target machine one. So I had to add the cmake flag `-DVCPKG_BINARY_SOURCES=clear`.
+
+See [this issue](https://github.com/microsoft/vcpkg/issues/44517#issuecomment-2816462459) that I posted.

build.sh

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+#!/bin/bash -e
+
+rm -rf build
+cmake -Bbuild -S. -GNinja \
+  -DCMAKE_TOOLCHAIN_FILE=$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake \
+  -DVCPKG_BINARY_SOURCES=clear \
+  -DVCPKG_INSTALL_OPTIONS="--debug"
+
+cmake --build build/

cross-compile-for-ios.sh

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+#!/bin/bash -e
+
+echo "THIS SERVES AS AN EXAMPLE OF HOW TO CROSS COMPILE THIS FOR USE ON IOS"
+
+# NOTE: DVCPKG_TARGET_ARCHITECTURE is for a setting within the set DVCPKG_CHAINLOAD_TOOLCHAIN_FILE
+# if using another toolchain or none at all, consider using the following native cmake variables
+# - CMAKE_OSX_SYSROOT
+# - CMAKE_OSX_DEPLOYMENT_TARGET
+# - CMAKE_OSX_ARCHITECTURES
+# - MACOSX_BUNDLE_GUI_IDENTIFIER
+# - ...
+# Qt has toolchain files that take care of these sorts of things, as does `vcpkg` as shown below
+
+rm -rf build
+cmake -Bbuild -S. -GNinja \
+  -DVCPKG_TARGET_TRIPLET=arm64-ios \
+  -DCMAKE_TOOLCHAIN_FILE=$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake \
+  -DVCPKG_CHAINLOAD_TOOLCHAIN_FILE=$VCPKG_ROOT/scripts/toolchains/ios.cmake \
+  -DVCPKG_TARGET_ARCHITECTURE=arm64 \
+  -DVCPKG_BINARY_SOURCES=clear \
+  -DVCPKG_INSTALL_OPTIONS="--debug"
+
+
+cmake --build build/

fib.cpp

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+#include <cxxopts.hpp>
+#include <fmt/format.h>
+#include <range/v3/view.hpp>
+
+namespace view = ranges::views;
+
+int fib(int x) {
+  int a = 0, b = 1;
+
+  for (int it : view::repeat(0) | view::take(x)) {
+    (void)it;
+    int tmp = a;
+    a += b;
+    b = tmp;
+  }
+
+  return a;
+}

fib.h

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+#ifndef FIB_H
+#define FIB_H
+
+int fib(int x);
+
+#endif // FIB_H

protocol/messenger/messenger.proto

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+syntax="proto3";
+package messenger;
+
+service HelloService {
+  rpc SayHello(SayHelloRequest) returns (SayHelloResponse) {}
+}
+
+message SayHelloRequest {
+  string name = 1;
+}
+message SayHelloResponse {
+  string message = 1;
+}

vcpkg.json

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+{
+  "dependencies": [
+    "cxxopts",
+    "fmt",
+    "range-v3",
+    "protobuf"
+  ]
+}