Promotional Hierarchy, structure and union

Promotional Hierarchy of Fundamental data types

long Double                                                      Highest
Double
float
unsigned long int  unsigned long
long int   or long
unsigned int
int
unsigned short int  or unsigned short
short  or short int
unsigned char
char
bool                                                                 Lowest

Conversion from highest type to lowest type will result in loss of data.

//sizeofstructunion.C


int main()
{
   struct {
        char a[20];
        int n;
        union{
                double b;
                struct {
                        char d[15];
                        float e;
                }x;
        }y;
   }z;

   cout << "z.y.x size: " << sizeof(z.y.x) << endl;
   cout << "z.y size: " << sizeof(z.y) << endl;
   cout << "z size: " << sizeof(z) << endl;

    return 0;
}

result:

z.y.x size: 20 // char  array size is 15, float size 4, consider alignment(padding). multiply the size of float to equalize or exceed array size of 15. i.e 4*4=16 (here we got one more than 15). Hence size of struct is 16 of array and 4 of float. 4+16= 20. 

z.y size: 24 // size of struct member (20 bytes)in union is more than double . double b size is 8 bytes. Consider alignment(padding). Multiply the size of double to equalize or exceed the size of struct. Hence we get 8*3= 24.

z size: 48 // sizeof union + char a + int n -> 24+ 20+4 = 48.

//structdeclaration.C

#include<iostream>
using namespace std;

struct test {
        static  int a;
        int b;
        int c;
}obj;
int test::a=10;
int main()
{
 cout << "test" << endl;
cout << "now a:" << obj.a  << endl;
cout << "now b:" << obj.b << endl;
cout << "now c:" << obj.c;
return 0;
}
 

//declare a variable

extern int val;  //declaration of a variable

int iglobal;  // declaration as well as definition, coz global variables intialized to zero

int localVariable; // declaration as well as definition coz local variable are having garbage values.

 in C programming/ C++ programming, global structure members are initialized to zero.

local structure in C/C++ structure members will have garbage value.