Memory Padding

Memory Padding

Lets understand Memory padding by an example.

#include <iostream>

using std::cout;

struct checkSize

{

    int a;

    char b;

    short int c;

    int d;

    char e;

};

int main()

{

  cout <<"Size of structure::"<<sizeof(checkSize);

  return 0;

}

Given Condition:: System is 32 bit. Size of int = 4 bytes, char = 1 byte and short int = 2 bytes.

As we expect the output of this as 12 bytes. But the actual answer is 16 bytes.

Reason:; Memory Padding is used in this.

In a 32 bit system, the bus have limit carry only 32 bits or 4 bytes of data in one time. Or in other word we can say system bus carry data in a group of 4 bytes. Through memory padding technique system tries to group the data in size of 4 bytes in this case also.

int a; 4 byte           -- group 1

char b; 1 byte        -- group 2

short int c; 2 byte  -- group 2

int d; 4 byte           -- group 3

char e;  1 byte       -- group 4

The memory is grouped in the sequence of initialization of the variable.

Group 1 and 3 will contain 4 bytes while group 2 contains 3 bytes and group 4 contains only 1 byte.

If we would have initialized like below, if would have took only 12 bytes of memory.

struct checkSize

{

    char e;

    char b;

    short int c;

    int d;

    int a;

};

Memory padding technique is not used in Class in C++.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.