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