C++ 类型转换操作符重载

template <typename T>
operator T () const
{
}

template <typename T>
operator T () const
{
    VarHolder* pHolder = content();

    if (!pHolder)
            throw InvalidAccessException("Can not convert empty value.");

    if (typeid(T) == pHolder->type())
        return extract<T>();
    else
    {
        T result;
        pHolder->convert(result);
        return result;
    }
}

// 类型是Var
auto count_var = json->get("count");
// 能够直接"赋值" 给 int 型变量 有点动态语言的感觉了
int count = count_var;