From: daniel.kruegler@googlemail.com   
      
   On 2013-02-14 01:14, TS wrote:   
   > I just noticed that C++0x includes unordered_map/set based on hash table.   
   > They allow users to specify their own hash and compare functions. However,   
   > it seems that they also include some default implementations of those   
   > functions, as compilers do not complain even if I don't provide them.   
   >   
   > My questions are   
   > 1. How are the performance of the default hash functions?   
      
   How do you define "performance"? The standard is silent on this, so this   
   is not specified.   
      
   > 2. If I use a class defined by myself as a key, how does the default   
   > hash function know how to hash my type?   
      
   It cannot know, unless you specialize the std::hash template for that   
   class type. If you don't provide it, the instantiation of   
   std::hash will be ill-formed.   
      
   > 3. Have the default functions been optimized for certain built-in types?   
      
   How do you define "optimized"? The standard is silent on this, so this   
   is not specified.   
      
   Your questions seem to indicate that you have a rather specific   
   understanding on what you would consider as an appropriate hash   
   function. If this is so, you should provide your own hasher.   
      
   HTH & Greetings from Bremen,   
      
   Daniel Krügler   
      
      
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