C++ Tutorial/Data Types/union

An anonymous union.

<source lang="cpp">#include <iostream>

1. include <cstring>

using namespace std;

int main() {

 // define anonymous union 
 union { 
   long l; 
   double d; 
   char s[4]; 
 } ; 

 // now, reference union elements directly 
 l = 100000; 
 cout << l << " "; 
 d = 123.2342; 
 cout << d << " "; 
 strcpy(s, "hi"); 
 cout << s; 

 return 0; 

}</source>

100000 123.234 hi

An example of a union

<source lang="cpp">#include <iostream> using std::cout; using std::endl; union Number {

  int integer1;       
  double double1;     

}; int main() {

  Number value;
  value.integer1 = 100;
  cout << value.integer1 << "\ndouble: " << value.double1
       << endl;
  value.double1 = 100.0;
  cout << value.integer1 << "\ndouble: " << value.double1
       << endl;
  return 0;

}</source>

100
double: 4.24399e-314
0
double: 100

A union is created with several data types.

<source lang="cpp">#include <iostream.h> union unmany_types {

char c;
int ivalue;
double fvalue;
double dvalue;

} unmy_union;

int main(void) {

unmy_union.c="b";
cout << unmy_union.c << "\n";
unmy_union.ivalue=1990;
cout << unmy_union.ivalue << "\n";
unmy_union.fvalue=19.90;
cout << unmy_union.fvalue << "\n";
unmy_union.dvalue=987654.32E+13;
cout << unmy_union.dvalue << "\n";
// invalid I/O
 
cout << unmy_union.c << "\n";
cout << unmy_union.ivalue << "\n";
cout << unmy_union.fvalue << "\n";
cout << unmy_union.dvalue << "\n";
// union size
cout << "The size of this union is: " << sizeof(unmany_types) << " bytes." << "\n";
return (0);

}</source>

A union with method

<source lang="cpp">#include <iostream> using namespace std;

union u_type {

 u_type(short int a) { i = a; };  
 u_type(char x, char y) { ch[0] = x; ch[1] = y; } 

 void showchars(){ 
   cout << ch[0] << " "; 
   cout << ch[1] << "\n"; 
 } 

 short int i; 
 char ch[2]; 

};

int main() {

 u_type u(1000); 
 u_type u2("X", "Y"); 

 cout << "u as integer: "; 
 cout << u.i << "\n"; 
 cout << "u as chars: "; 
 u.showchars(); 

 cout << "u2 as integer: "; 
 cout << u2.i << "\n"; 
 cout << "u2 as chars: "; 
 u2.showchars(); 

 return 0; 

}</source>

u as integer: 1000
u as chars: F ?
u2 as integer: 22872
u2 as chars: X Y

Put int and float into a union

<source lang="cpp">#include <iostream> using namespace std; int main(void){

  union 
  {
    int my_data;
    float his_data;
  };
  my_data = 3;
  cout << "Value of my_data is " << my_data;
  his_data = 1.2345;
  cout << "\nValue of his_data is " << his_data;

}</source>

Unions and Classes Are Related

<source lang="cpp">#include <iostream> using namespace std;

union swap_byte {

 void swap();
 void set_byte(unsigned short i);
 void show_word();
  
 unsigned short u;
 unsigned char c[2];

};

void swap_byte::swap() {

 unsigned char t;
  
 t = c[0];
 c[0] = c[1];
 c[1] = t;

}

void swap_byte::show_word() {

 cout << u;

}

void swap_byte::set_byte(unsigned short i) {

 u = i;

}

int main() {

 swap_byte b;
  
 b.set_byte(49034);
 b.swap();
 b.show_word();
  
 return 0;

}</source>

Using an anonymous union: members integer1, double1 and charPtr share the same space

<source lang="cpp">#include <iostream> using std::cout; using std::endl; int main() {

  union                                                        
  {                                                            
     int integer1;                                             
     double double1;                                           
     char *charPtr;                                            
  };
  integer1 = 2;
  cout << integer1 << endl;
  double1 = 4.4;
  cout << double1 << endl;
  charPtr = "union";
  cout << charPtr << endl;
  return 0;

}</source>

2
4.4
union