例如，如果我正在建立这样的json obj

    json::value obj
    obj["a"];
    obj["b"]=2;
    cout<<obj;
    //It gives me the correct output but what if after this I have to attach an array to an existing json value eg if I try
    obj["b"]append({obj["b"},3,"aa"})
    //obj[b] is placed at right to keep the old value too which is 2

这给出了一个错误。如何将数组或某些值附加到现有值？如何获得现有值的长度？