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std::ios_ba...er_callback(3)   C++ Standard Libary  std::ios_ba...er_callback(3)

NAME
     std::ios_base::register_callback - std::ios_base::register_callback

Synopsis
	void register_callback( event_callback function, int index );

	Registers a user-defined function which will be called by imbue(),
	std::basic_ios::copyfmt()  and ~ios_base(). Every registered callback is
     called every
	time: the event type (a value of type event) is passed as its first  ar-
     gument, and
	may be used to distinguish between the callers.

	The  callbacks are called in the reverse order of registration (in other
     words,
	register_callback() pushes a callback pair on the callback stack). If
	register_callback() is called from within a callback function to  add  a
     new callback,
	the new callback is only called on the next event.

	The user-defined callback function is not allowed to throw exceptions.

Parameters
	function  -  the  function  which will be called on event, supplied as a
     function
		   pointer of type event_callback
	index	 - custom parameter which will be passed to the function

Return value
	(none)

Notes
	Once registered, a callback cannot be deregistered: it remains a part of
     the stream
	object for the rest of its lifetime. If the behavior of a callback needs
     to change,
	it may be controlled through iword() or pword().

	If the same function is registered multiple times, it is called multiple
     times.

	The integer value that is stored together with the callback is typically
     an index
	obtained from xalloc().

Example
	Demonstrates the use of  register_callback  to	update	locale-dependent
     cached values
	that are used by a custom output operator.

     // Run this code

      #include <functional>
      #include <iostream>
      #include <locale>

      // Cached locale-specific message and its hash
      typedef std::pair<std::string, std::size_t> cache_t;

      // Populate the cached message and its hash from the locale
      void update_cache(cache_t& cache, std::locale loc)
      {
	  auto& fct = std::use_facet< std::messages<char> >(loc);
	  std::messages_base::catalog cat = fct.open("sed", loc);
	  cache.first = cat < 0 ? "" : fct.get(cat, 0, 0, "Memory exhausted");
	  cache.second = std::hash<std::string>()(cache.first);
      }

      // Update the cache if the locale changed
      void true_callback(std::ios_base::event evt, std::ios_base& str, int idx)
      {
	  if (evt == std::ios_base::imbue_event)
	  {
	      cache_t* ptr = static_cast<cache_t*>(str.pword(idx));
	      update_cache(*ptr, str.getloc());
	  }
      }

      // Registers the cache in pword() and sets up the callback
      struct CacheSetup
      {
	  CacheSetup(std::ostream& os, std::ios_base::event_callback f, cache_t*
     cache)
	  {
	      int index = std::ostream::xalloc();
	      os.pword(index) = cache; // Store pointer to cache in the stream
	      os.register_callback(f, index); // Store callback and the index to
     the pointer
	      update_cache(*cache, os.getloc()); // Initialize cache
	  };
      };

      // Some custom class
      struct S {};
      // Some custom class's operator<< that needs fast access to hashed message
      std::ostream& operator<<(std::ostream& os, const S&)
      {
	  static cache_t cache;
	  static CacheSetup setup(os, true_callback, &cache);
	  return os << cache.first << " : " << cache.second;
      }

      int main()
      {
	  std::locale loc("en_US.utf8");

	  S s;
	  std::cout.imbue(loc);
	  std::cout << s << '\n';

	  std::cout.imbue(std::locale(loc,	   new	       std::messages_by-
     name<char>("de_DE.utf8")));
	  std::cout << s << '\n';

	  std::cout.imbue(std::locale(loc,	   new	       std::messages_by-
     name<char>("ja_JP.utf8")));
	  std::cout << s << '\n';

	  std::cout.imbue(std::locale(loc,	   new	       std::messages_by-
     name<char>("ru_RU.utf8")));
	  std::cout << s << '\n';
      }

Output:
      Memory exhausted : 2,295,079,096
      Speicher erschA<paragraph>pft : 3,139,423,551
      a!acaaa1/4ae<paragraph>^3aa3/4aa : 3,837,351,114
      DhDh<degree>Dh1/4NNN     Dh,NNDh<micro>NDh?Dh<degree>Dh1/2Dh<degree>     :
     3,742,732,851

http://cppreference.com 	   2024.06.10	    std::ios_ba...er_callback(3)
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