[C++] Generate DTOs for non-perf-sensitive usecases. #960
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sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppDtoGenerator.java
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In some applications performance is not cricital. Some users would like to use SBE across their whole "estate", but don't want the "sharp edges" associated with using flyweight codecs, e.g., accidental escape. In this commit, I've added a first cut of DTO generation for C++ and a simple test based on the Car Example. The DTOs support encoding and decoding via the generated codecs using `DtoT::encode(CodecT& codec, const DtoT& dto)` and `DtoT::decode(CodecT& codec, Dto& dto)` methods. Generation can be enabled specifying the target code generator class, `uk.co.real_logic.sbe.generation.cpp.CppDtos`, or by passing a system property `-Dsbe.cpp.generate.dtos=true`.
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Changes: - `encode` -> `encodeWith` - `decode` -> `decodeWith`
As we use the generated codecs to create a string representation of our DTOs, we don't use Agrona buffers in C++, and there is no concept of resizing, it is necessary to size a temporary buffer during the construction of the string data. Previously, we were letting the user supply this value, which wasn't a very friendly API. Now, we use the `computeLength` methods on the codec to determine how big of a temporary buffer we need. Perhaps the methods will also be useful for avoiding a buffer copy when used in conjunction with Aeron. For example, a developer could use `dto.computeEncodedLength()` to initialise a buffer claim rather than copying via the `offer(...)` API.
I had incorrectly assumed that the `Flyweight::computeLength` method took _encoded lengths_ of groups etc., but actually it takes a complicated structure of group counts and variable lengths. As it was hard to build this list, I've opted for a simpler approach: do the length calculation within the generated DTO message and its groups. In this commit, I've also added some convenience methods for converting between DTOs and "byte arrays".
It is more-idiomatic to represent variable-length data using `std::string` even when there is no character encoding specified, the the `std::string` API provides useful utilities regardless.
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Combining with C# changes, as both rely on PBT infrastructure. |
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