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

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

docs/source/Compiler.md

@@ -75,7 +75,7 @@ Additional options:
 -   `--allow-non-utf8` : Pass non-UTF-8 input through parser and emit nonstandard
     \x escapes in JSON. (Default is to raise parse error on non-UTF-8 input.)
 
--  `--natural-utf8` : Output strings with UTF-8 as human-readable strings. 
+-  `--natural-utf8` : Output strings with UTF-8 as human-readable strings.
      By default, UTF-8 characters are printed as \uXXXX escapes."
 
 -   `--defaults-json` : Output fields whose value is equal to the default value
@@ -120,7 +120,13 @@ Additional options:
 -   `--cpp-ptr-type T` : Set object API pointer type (default std::unique_ptr)
 
 -   `--cpp-str-type T` : Set object API string type (default std::string)
--   T::c_str() and T::length() must be supported.
+    T::c_str(), T::length() and T::empty() must be supported.
+    The custom type also needs to be constructible from std::string (see the
+	--cpp-str-flex-ctor option to change this behavior).
+
+-   `--cpp-str-flex-ctor` : Don't construct custom string types by passing
+    std::string from Flatbuffers, but (char* + length). This allows efficient
+	construction of custom string types, including zero-copy construction.
 
 -   `--object-prefix` : Customise class prefix for C++ object-based API.
 
@@ -168,7 +174,7 @@ Additional options:
     an evolution of. Gives errors if not. Useful to check if schema
     modifications don't break schema evolution rules.
 
--   `--conform-includes PATH` : Include path for the schema given with 
+-   `--conform-includes PATH` : Include path for the schema given with
     `--conform PATH`.
 
 -   `--include-prefix PATH` : Prefix this path to any generated include

docs/source/CppUsage.md

@@ -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)