Add cxx17 string_view operator[] for Json::Value

[ObjatieGroba]

Is it possible to add string_view?
How can we turn it off on older c++ versions?
Can it be another define "JSON_USE_CXX17"? Or compiler defines?

Thank you

[dota17]

It seems the system doesn't support C++17 Standard now, so it cannot open include file string_view
but if you like, you could use the __plusplus marco switch :

#if __cplusplus >= 201703L 
#include <string_view>
#endif 

[dota17]

Only when the __cplusplus >= 201703L, can this semantics work.

[baylesj]

We would have to include abseil or move to C++17 for this, so I'm voting to close for now.

[ObjatieGroba]

Have to include abseil or move to c++17 for this

Why? It is just StringView. There are many cases where we can use it. What difference between const string ref and StringView?

[dota17]

Have to include abseil or move to c++17 for this

Why? It is just StringView. There are many cases where we can use it. What difference between const string ref and StringView?

string_view can avoid unnecessary memory allocation. but it only applied in the new c++17 standard and abseil lib.

[ObjatieGroba]

it only applied in the new c++17 standard and abseil lib

What about people who use current master with c++17 standart? For them it is new standart and std::string_view can be applied because it contains pointer and size. Where the problem is?
For usage in operator[] const reference same as StringView. There are no memory allocation on this operator.
Can we add std::string_view to functions, where we do not care about memory ownership?
It is really necessary for people who use c++17 because them have to do smth like that

std::string_view sv;
Json::Value json;
json[std::string(sv)] = .. // Memory allocation. Why?

Thank you

[ObjatieGroba]

Maybe it is possible to add as default disabled feature?
To enable just define JSON_USE_STRING_VIEW

[dota17]

I guess baylesj temporarily closed this PR because we haven't decided to use abseil C++ library or C++17 Standard library.
Currently, we should focus on how to migrate to C++14, so we should not rush to use new C++ standard, I think?
And there is a link about the detail of std::string_view, hope it can help you.

[BillyDonahue]

supporting std::string_view as a value key should be doable but it would have to be carefully controlled by a #if and we'd have to carefully control the overload set to avoid ambiguities with other string-like types we're using.

I don't think abseil has much to do with the decision though, unless I'm missing something.

[aberaud]

As more projects move to C++17, I believe this becomes increasingly necessary.

Additionally to the proposals in this PR, it would be great to have

bool Json::Value::getString(std::string_view& out) const;

To be able to read a string Json value without unneeded allocations + copies.

[aberaud]

GCC is now moving to C++17 by default, and C++20/22 include additional support for std::string_view.

Other libraries like asio have included support for std::string_view for years now.

Could we please list issues with the current patch so they could be solved, so this could move forward and get merged?