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double _dval; };
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If the largest data type among the members of TokenValue is dval, the size of TokenValue is the size of an object of type double The members of a union are public members by default The name of a union can be used in a program wherever a class name can be used For example:
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// object of type TokenValue TokenValue last_token; // pointer to an object of type TokenValue TokenValue *pt = new TokenValue;
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The members of a union are accessed through the class member access operators ( and ->), just like class members, using a union object or a pointer to a union before the operator For example:
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last_token_ival = 97; char ch = pt->_cval;
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Union members can be declared as either public, protected, or private:
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union TokenValue { public: char _cval; // private: int priv; }; int main() { TokenValue tp; tp_cval = '\n'; // ok // error: main() cannot access private member // TokenValue::priv tppriv = 1024; }
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A union cannot have a static data member or a member that is a reference A union cannot have a member of a class type that defines either a constructor, destructor, or copy assignment operator For example:
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union illegal_members { Screen s; // error: has constructor Screen *ps; // ok static int is; // error: static member int &rfi; // error: reference member };
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Member functions, including constructors and destructors, can be defined for a union
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union TokenValue { public: TokenValue(int ix) : _ival(ix) { } TokenValue(char ch) : _cval(ch) { } // int ival() { return _ival; } char cval() { return _cval; } private: int _ival; char _cval; // }; int main() { TokenValue tp(10); int ix = tpival(); // }
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Here is an example of how the union TokenValue might be used:
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enum TokenKind { ID, Constant /* and other tokens */ }; class Token { public: TokenKind tok; TokenValue val; };
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An object of type Token might be used as follows:
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int lex() { Token curToken; char *curString; int curIval; // case ID: // identifier curTokentok = ID; curTokenval_sval = curString; break; case Constant: // integer constant curTokentok = Constant; curTokenval_ival = curIval; break; // etc }
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The danger of using a union is the possibility of accidentally retrieving the value currently stored in the union through an inappropriate data member For example, if the last assignment is to _ival, the programmer does not want to retrieve that value through the member _sval Doing so will certainly lead to a program error To help safeguard against this kind of error, a programmer should define an additional object with a purpose of keeping track of the type of the value currently stored in the union This additional object is referred to as the discriminant of the union This is the role of the tok member within the class Token For example:
char *idVal; // verify value of discriminant before referring to sval if ( curTokentok == ID ) idVal = curTokenval_sval;
A good practice when handling a union object that is a class member is to provide a set of access functions for each union data type For example:
#include <cassert> // access function to union member sval string Token::sval() { assert( tok==ID ); return val_sval; }
When defining a union, the name of the union is optional If the name of the union is not used as a type name in the program to declare other objects, there is no reason to provide a name when the union type is defined For example, the following definition of Token is equivalent to its previous definition The only difference is that the union is without a name:
class Token { public: TokenKind tok; // name of union type omitted union { char _cval; int _ival; char *_sval; double _dval; } val; };
There is a special instance of a union referred to as an anonymous union An anonymous union is a union without a name that is not followed by an object definition For example, here is a Token class definition containing an anonymous union:
class Token {
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