C C switch case with string

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Possible Duplicate    C C    switch for non-integers       Hi  I need to use a string in switch case  My solution so far was to calculate the hash of the string with my hash function  Problem is I have to manually pre-calculate all my hash values for strings  Is there a better approach    h  myhash  mystring   switch  h    case 66452             case 1342537

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Typically  you would use a hash table and function object  both available in Boost  TR1 and C  0x   void func1       std  unordered map lt std  string  std  function lt void   gt  gt  hash map  hash map  Value1      amp func1          etc hash map mystring       This is a little more overhead at runtime but a bajillion times more maintainable  Hash tables offer O 1  insertion  lookup  and etc  which makes them the same complexity as the assembly-style jump-table

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If you are after performance and don t want to go through all the if clauses each time if there are many or the need to hash the values  you could send some extra information to the function with the help of enum or just add an enum type to your structure

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Just use a if       else if        chain  Using a hash value is going to be a maintenance nightmare  switch is intended to be a low-level statement which would not be appropriate for string comparisons

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You could map the strings to function pointer using a standard collection  executing the function when a match is found   EDIT  Using the example in the article I gave the link to in my comment  you can declare a function pointer type   typedef void   funcPointer  int     and create multiple functions to match the signature   void String1Action int arg   void String2Action int arg     The map would be std  string to funcPointer   std  map lt std  string  funcPointer gt  stringFunctionMap    Then add the strings and function pointers   stringFunctionMap add  string1    amp String1Action     I ve not tested any of the code I have just posted  it s off the top of my head

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You could use the string to index into a hash table of function pointers   Edit  glib has a hash table implementation that supports strings as keys and arbitrary pointers as values  http   library gnome org devel glib stable glib-Hash-Tables html

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You could create a hashtable   The keys can be the string and the value can be and integer   Setup your integers for the values as constants and then you can check for them with the switch

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You can use enumeration and a map  so your string will become the key and enum value is value for that key

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There is no good solution to your problem  so here is an okey solution  -   It keeps your efficiency when assertions are disabled and when assertions are enabled it will raise an assertion error when the hash value is wrong   I suspect that the D programming language could compute the hash value during compile time  thus removing the need to explicitly write down the hash value   template  lt std  size t h gt  struct prehash       const your string type str       static const std  size t hash value   h       pre hash const your string type amp  s    str s                assert  myhash s     hash value                        std  size t h    myhash mystring    static prehash lt 66452 gt  first label    label1    switch  h    case first label hash value                       By the way  consider removing the initial underscore from the declaration of   myhash    sorry but stackoverflow forces me to insert a space between   and myhash   A C   implementation is free to implement macros with names starting with underscore and an uppercase letter  Item 36 of  Exceptional C   Style  by Herb Sutter   so if you get into the habit of giving things names that start underscore  then a beautiful day could come when you give a symbol a name that starts with underscore and an uppercase letter  where the implementation has defined a macro with the same name

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The best way is to use source generation  so that you could use  if  hash str     HASH  some string        in your main source  and an pre-build step would convert the HASH const char   expression to an integer value

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Ruslik s suggestion to use source generation seems like a good thing to me  However  I wouldn t go with the concept of  main  and  generated  source files  I d rather have one file with code almost identical to yours   h  myhash  mystring   switch  h    case 66452       hash  Vasia              case 1342537       hash  Petya                   The next thing I d do  I d write a simple script  Perl is good for such kind of things  but nothing stops you even from writing a simple program in C C   if you don t want to use any other languages  This script  or program  would take the source file  read it line-by-line  find all those case NUMBERS       hash  SOMESTRING   lines  use regular expressions here   replace NUMBERS with the actual hash value and write the modified source into a temporary file  Finally  it would back up the source file and replace it with the temporary file  If you don t want your source file to have a new time stamp each time  the program could check if something was actually changed and if not  skip the file replacement   The last thing to do is to integrate this script into the build system used  so you won t accidentally forget to launch it before building the project

[c++] C/C++ switch case with string

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