Rules for references to ValueArray, ValueMap and Value elements
Value is originally designed as non-mutable concrete Value of different concrete types.
However, as Value is commonly used to store complex hierarchies (using ValueArray/ValueMap), it is in the end very useful to be able to reference ValueArray/ValueMap elements directly. Validity of such references is restricted by following rule:
Reference obtained from Value/ValueArray/ValueMap (by Value::At, Value::GetAdd, ValueArray::At, ValueMap::GetAdd and derivative methods) are only valid until the next operation on originating Value - including just reading it.
Examples of invalid 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
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