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Tip of the Day
Language: C++
Expertise: All
Apr 16, 1998



Building the Right Environment to Support AI, Machine Learning and Deep Learning

Uses of an anonymous union

Unions are used to minimize memory waste. For example, consider the following database-transaction class used to update a person's data. The key can be either a unique ID number or a person's last name, but never both at once:
class UpdateDetails {
	enum keytype{ keystring, keyID} key;
	char *name;
	long ID;
	UpdateDetails(const char *n): key(keystring),
						n (new char [strlen)n) +1]

	UpdatePersonalDetails(long id) : ID(id), key(keyID) {}
Clearly, memory is wasted here since only one of the keys can be used in each transaction. An anonymous union (for instance, an embedded union having neither a tag-name nor an instance name) can be used in this case to avoid memory waste:
class UpdateDetails {
	enum keytype{ keystring, keyID} key;
	union {  //anonymous union: 1. has no tag name,
		char *name;
		long ID 
};    // 2. and no instance name.
UpdateDetails(const char *n) : key (keystring), 
						 n (new char [strlen(n) +1] 
						{strcpy(name, n);} 

	UpdateDetails(long id) : ID(id), key(keyID) {};
The advantage over an ordinary union is that in this case, the members of an anonymous union are accessed directly:
void  UpdateDetails::GetID() const 
  if (key == keyID) 
   return ID;//anonymous union member, accessed like ordinary member

  return 0L;  //indicate string key is used 
Danny Kalev
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