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Writable references to ValueArray/ValueMap/Value elements [message #43612]

Sat, 13 September 2014 16:31

mirek
Messages: 14256
Registered: November 2005

Ultimate Member

Lately I have found myself doing a lot of work on hierarchical Values (usually representing some JSON). Here, the strict non-mutable value-only features of Value make things a little bit complicated. Thus, after a long period of hesitation, I have introduced possibility to get a reference to "internal" Value contained in ValueArray/ValueMap/Value. It is now possible to write

	Value v;
	for(int i = 0; i < 3; i++) {
		Value& p = v.At(i)("person");
		p("name") = "Name" + AsString(i + 1);
		p("lastname") = "LastName" + AsString(i + 1);
	}
	LOG(AsJSON(v));

and result is

[{"person":{"name":"Name1","lastname":"LastName1"}},{"person":{"name":"Name2","lastname":"LastName2"}},{"person":{"name":"Name3","lastname":"LastName3"}}]

However, there is a strict rule limiting the validity of such reference (caused by general nature of Value):

They are only valid until the next operation on originating Value - including just reading it!

Examples of faulty code:

Value m;
Value& x = m("key");
x = m; // using m as source invalidates x

Value m;
Value& x = m("key");
Value& y = m("key2"); // Invalidates x
x = 123; // undefined

Value m;
Value& x = m.At(1);
Value m2 = m; // Invalidates x
x = "fail"; // undefined

Mirek

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