[AGENT++]why I can not log into file?

[OELoo Jerry]

hi all
   I have add the log into my program,here is a snapshoot:
    
    AgentLogImpl* log = new AgentLogImpl( "snmplog.log" );
    DefaultLog::init( log );

    DefaultLog::log()->set_filter(ERROR_LOG, 2);
    DefaultLog::log()->set_filter(WARNING_LOG, 4);
    DefaultLog::log()->set_filter(EVENT_LOG, 4);
    DefaultLog::log()->set_filter(INFO_LOG, 4);
    DefaultLog::log()->set_filter(DEBUG_LOG, 9);
but when i run my program,i can find the snmplog.log in my file, but ,there 

are nothing in it? so why? 
Thanks for any help

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