# 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.