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[C++] Object API: document custom string type requirements,

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implement better custom string type constructor alternative
for Unpack() and fix bug with vector of custom string types
in Pack().

Squashed commit of the following:

commit e9519c647e
Author: Luca Longinotti <luca.longinotti@inivation.com>
Date:   Tue Mar 5 18:24:49 2019 +0100

    tests: regenerate code, reverts change to CreateVectorOfStrings().

commit 117e3b0679
Author: Luca Longinotti <luca.longinotti@inivation.com>
Date:   Tue Mar 5 18:15:05 2019 +0100

    idl_gen_cpp.cpp: move clang-format on/off outside of declaration, so they are kept properly aligned automatically.

commit 4791923806
Author: Luca Longinotti <luca.longinotti@inivation.com>
Date:   Tue Mar 5 18:11:40 2019 +0100

    idl_gen_cpp.cpp: full clang-format run with provided Google format file, enforce 80 lines width.

commit 2f0402f9ff
Author: Luca Longinotti <luca.longinotti@inivation.com>
Date:   Tue Mar 5 18:09:32 2019 +0100

    CppUsage: address requested changes.
    idl_gen_cpp.cpp: fix formatting, keep CreateVectorOfStrings for normal string cases.

commit 371d4e0b79
Author: Luca Longinotti <luca.longinotti@inivation.com>
Date:   Fri Mar 1 16:35:29 2019 +0100

    Fix compile error with a vector of non-std::strings. CreateVectorOfStrings() expects a vector of std::string types, but that's not always the case.

commit 92b90d7f0f
Author: Luca Longinotti <luca.longinotti@inivation.com>
Date:   Fri Mar 1 16:15:36 2019 +0100

    Document requirement for custom string types to implement empty() and be constructible from std::string.
    Add new option --cpp-str-flex-ctor to construct custom string types not via std::string, but (char * + length).

commit 28cb2e92d5
Author: Luca Longinotti <luca.longinotti@inivation.com>
Date:   Fri Mar 1 14:31:17 2019 +0100

    idl_gen_cpp.cpp: clang-format run, to better separate changes in content from formatting.

Change-Id: I4887ba2f2c632b9e7a8c938659b088cd95690870
This commit is contained in:
Wouter van Oortmerssen
2019-03-11 09:42:02 -07:00
parent 3968d00568
commit eb2a81f73d
5 changed files with 254 additions and 174 deletions
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14
docs/source/CppUsage.md
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@@ -255,14 +255,24 @@ you, so you'll have to manage their lifecycles manually. To reference the
pointer type specified by the `--cpp-ptr-type` argument to `flatc` from a
flatbuffer field set the `cpp_ptr_type` attribute to `default_ptr_type`.
# Using different string type.
By default the object tree is built out of `std::string`, but you can
influence this either globally (using the `--cpp-str-type` argument to
`flatc`) or per field using the `cpp_str_type` attribute.
The type must support T::c_str() and T::length() as member functions.
The type must support T::c_str(), T::length() and T::empty() as member functions.
Further, the type must be constructible from std::string, as by default a
std::string instance is constructed and then used to initialize the custom
string type. This behavior impedes efficient and zero-copy construction of
custom string types; the `--cpp-str-flex-ctor` argument to `flatc` or the
per field attribute `cpp_str_flex_ctor` can be used to change this behavior,
so that the custom string type is constructed by passing the pointer and
length of the FlatBuffers String. The custom string class will require a
constructor in the following format: custom_str_class(const char *, size_t).
Please note that the character array is not guaranteed to be NULL terminated,
you should always use the provided size to determine end of string.
## Reflection (& Resizing)