c++11 building support

[wbpcode]

Signed-off-by: wangbaiping [email protected]

std::make_unique (from c++14) -> absl::make_unique
std::string_view (from c++17) -> absl::string_view
std::optional (from c++17) -> absl::optional
std::nullopt (from c++17) -> absl::nullopt

absl::string_view/absl::optional/absl::nullopt/... will be alias of std::string_view/std::optional/std::nullopt/... after c++17 by default. So it's ok to always use the absl::* in c++11/c++14/c++17 versions.

[wu-sheng]

So, we will have c++11 and c++17 in support at the same time?

[wbpcode]

So, we will have c++11 and c++17 in support at the same time?

yep.

[wu-sheng]

Could you add a CI for C++11 build and update the doc to mention 11 and 17 supported?

[wbpcode]

In fact it's build by c++11 by default for now because I remove the explicit option of --cxxopt=-std:c++17. But it's OK for me to add a new test for it.

[wbpcode]

/wait

[wbpcode]

some warnings need to be resolved.

[wu-sheng]

👍

[wu-sheng]

LGTM.