V>struct ClanId {
V>    public int Value { get {
V>        if(HasValue)
V>            return value_;
V>        throw ...
V>    }}

V>    public bool HasValue { get { return Value != 0; }}
V>}

V>ClanInfo GetClanInfo(int clan_id) {}

V>class PlayerInfo {
V>    ClanId clanId;
V>    ...
V>}

V>...
V>PlayerInfo pi = ...
V>ClanInfo ci;

V>if(pi.clanId.HasValue)
V>    ci = GetClanInfo(pi.clanId.Value);
V>