From: ian-news@this.is.invalid   
      
   Dave Abrahams wrote:   
   > on Fri Dec 28 2012, Ian Collins  wrote:   
   >   
   >> Dave Abrahams wrote:   
   >>>   
   >>> A logger singleton is probably not the greatest example for proving   
   >>> this point, because its actions are usually benign and often   
   >>> untested.  However, the use of a singleton may impede or prevent the   
   >>> logger from being replaced for testing purposes, or prevent   
   >>> different loggers from being used for individual tests.   
   >>   
   >> How?   
   >>   
   >> This is the nub of the argument, but nothing posted so far backs up   
   >> the assertion.  The preceding posts all demonstrate the use of   
   >> different loggers.   
   >   
   > It seems self-evident to me, but I'll try to explain anyway.   
   >   
   > * Most singletons are designed not to allow arbitrary lifetime   
   >    management at all, so you can only have one during the life of the   
   >    executable   
   >   
   > * In any case, only one instance of a singleton can exist at a given   
   >    time, by definition   
   >   
   > * Therefore if you have two tests that need to talk to different loggers   
   >    they either must be in separate executables (in the first case) or at   
   >    least can't run in separate threads (in the second case).   
   >   
   > What am I missing?   
      
   Unit tests normally (in the case of a singleton, nearly always) use   
   mocks for foreign objects required by the tests.  I think that was clear   
   from the examples in this thread using a type called "MyLogger".   
      
   --   
   Ian Collins   
      
      
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