I believe your JSON file is syntactically invalid. See www.json.org. Your file should contain a single object or array, e.g. in your case it should like this:
[{"A":"something1","B":"something2","C":"something3","D":"something4"},
{"A":"something5","B":"something6","C":"something7","D":"something8"},
{"A":"something9","B":"something10","C":"something11","D":"something12"}]
Then you can access each object of the array in your loop:
for (Json::Value::ArrayIndex i = 0; i != root.size(); i++)
{
std::string A = root[i].get("A", "ASCII").asString();
// etc.
}