Add json_view type support to avoid excessive copying

[slonm]

Feature name and mission inspired by std::string_view. json_view is immutable json object which refers inside to another outer json object and do not keep data. json will have

Motivation example:

//For illustate how to possible use json_view args reportA and reportB declared differently
//but from caller perspective both cases guarantee constness of passes arguments

void prepareAndSendReport(const json& reportA, json_view reportB) {
 //while combine rpt reportA and reportB will be not copied
	json rpt;
	rpt["reportA"] = json_view{reportA};
	rpt["reportB"] = reportB;
	rpt["reporttime"] = time(NULL);
	serviceClient.post(rpt);
}

struct A {
	json getReport();
}

struct B {
	const json& getReport();
	void fillReport();
  private:
	json report;
}

void main() {
	A a;
	B b;
	b.fillReport();
//a.getReport() returns temporary object but that's ok while stack is not unwind
//b.getReport() returns refecence to B::report
	prepareAndSendReport(a.getReport(), b.getReport());
}

[FrancoisChabot]

I kinda like the idea (usual reservations wrt. dangling references footguns aside) , but not the name. If I see json_view, I'm immediately expecting some kind of lazy evaluated interpretation of a raw string buffer.

[slonm]

I'm immediately expecting some kind of lazy evaluated interpretation of a raw string buffer.

Basically view should looks like immutable json object and could have any underlying storage. It could be json or some raw string or primitive type, etc. Don't see contradiction here

[stale]

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