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diff --git a/doc/core/moving-to-c++.md b/doc/core/moving-to-c++.md new file mode 100644 index 0000000000..4c745b38a9 --- /dev/null +++ b/doc/core/moving-to-c++.md @@ -0,0 +1,60 @@ +# Moving gRPC core to C++ + +October 2017 + +ctiller, markdroth, vjpai + +## Background and Goal + +gRPC core was originally written in C89 for several reasons +(possibility of kernel integration, ease of wrapping, compiler +support, etc). Over time, this was changed to C99 as all relevant +compilers in active use came to support C99 effectively. +[Now, gRPC core is C++](https://github.com/grpc/proposal/blob/master/L6-allow-c%2B%2B-in-grpc-core.md) +(although the code is still idiomatically C code) with C linkage for +public functions. Throughout all of these transitions, the public +header files are committed to remain in C89. + +The goal now is to make the gRPC core implementation true idiomatic +C++ compatible with +[Google's C++ style guide](https://google.github.io/styleguide/cppguide.html). + +## Constraints + +- No use of standard library + - Standard library makes wrapping difficult/impossible and also reduces platform portability + - This takes precedence over using C++ style guide +- But lambdas are ok +- As are third-party libraries that meet our build requirements (such as many parts of abseil) +- There will be some C++ features that don't work + - `new` and `delete` + - pure virtual functions are not allowed because the message that prints out "Pure Virtual Function called" is part of the standard library + - Make a `#define GRPC_ABSTRACT {GPR_ASSERT(false);}` instead of `= 0;` +- The sanity for making sure that we don't depend on libstdc++ is that at least some tests should explicitly not include it + - Most tests can migrate to use gtest + - There are tremendous # of code paths that can now be exposed to unit tests because of the use of gtest and C++ + - But at least some tests should not use gtest + + +## Roadmap + +- What should be the phases of getting code converted to idiomatic C++ + - Opportunistically do leaf code that other parts don't depend on + - Spend a little time deciding how to do non-leaf stuff that isn't central or polymorphic (e.g., timer, call combiner) + - For big central or polymorphic interfaces, actually do an API review (for things like transport, filter API, endpoint, closure, exec_ctx, ...) . + - Core internal changes don't need a gRFC, but core surface changes do + - But an API review should include at least a PR with the header change and tests to use it before it gets used more broadly + - iomgr polling for POSIX is a gray area whether it's a leaf or central +- What is the schedule? + - In Q4 2017, if some stuff happens opportunistically, great; otherwise ¯\\\_(ツ)\_/¯ + - More updates as team time becomes available and committed to this project + +## Implications for C++ API and wrapped languages + +- For C++ structs, switch to `using` when possible (e.g., Slice, +ByteBuffer, ...) +- The C++ API implementation might directly start using +`grpc_transport_stream_op_batch` rather than the core surface `grpc_op`. +- Can we get wrapped languages to a point where we can statically link C++? This will take a year in probability but that would allow the use of `std::` + - Are there other environments that don't support std library, like maybe Android NDK? + - Probably, that might push things out to 18 months |