C++ Add move constructor for Reflection's SetString

[reed-lau]

Add move constructor for Reflection's SetString
add feature #6399

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[reed-lau]

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[t-rybarski]

Rvalue reference cannot be const

[t-rybarski]

@reed-lau rvalue references cannot be const as they cannot be moved. The overloaded functions rvalue parameter should be SetString(..,..,std::string&& value) without the const!

[t-rybarski]

Also here i believe the value parameter (i.e. the input string) should be moved, so:
SetString(...,..., std::move(value), field)
Additionaly, the string which you are creating in your code should also be moved into place:

bool foo(const char *buf, size_t size,...) { ... std::string temp(buf, size); // actually, we don't need to create a temporary std::string here, if we have // some method like SetStringByArray(..., buf, size); reflection->SetString(...., std::move(temp)); ... }

[reed-lau]

@t-rybarski have fixed it according to your comment.

[reed-lau]

@acozzette @protobuf-kokoro
what should I do to fix the Kokoro build failure for Linux Golang?
it seems that the docker pulling failed due to the networking or environment.

[acozzette]

That Go error looks unrelated, so I wouldn't worry about it for now.

[acozzette]

I believe it's possible to enable this optimization without creating new overloads. Could you try taking the existing methods and changing them to take a std::string (by value) instead of const std::string&? I think that should be backward-compatible with existing callers and still allow us to optimize the implementation.

[reed-lau]

after change the interface to std::string(by value), the following line may using the std::move form, and it could benefit from it, but I didn't change these things at this PR except the python's wrapper SetString.

protobuf/src/google/protobuf/generated_message_reflection.cc

Line 589 in d2d6ff5

SetString(message2, field1, temp_string);

[acozzette]

This looks great but we are having some issues with Docker in our CI setup. Hopefully we will get the test issues sorted out soon and then I plan to merge this.