sizeof operator

sizeof
In the programming languages C and C++, the unary operator 'sizeof' is used to calculate the sizes of datatypes. sizeof can be applied to all datatypes, be they primitive types such as the integer and floating-point types defined in the language, pointers to memory addresses, or the compound datatypes (unions, structs, or C++ classes) defined by the programmer. sizeof is a compile-time operator that returns the size in bytes of the variable or parenthesized type-specifier that it precedes. -- http://en.wikipedia.org/wiki/Sizeof --

Here is an interesting test for the size of several types by myself.

#include "stdio.h"

#define mySizeOf(x) ((char*)(&(x)+1)-(char*)(&x))

struct myType { // This is a structure containing four variables of different types
 char a;
 int b;
 float c;
 double d;
};

struct blankType {  // This is a structure with no variable
};

class CBlankClass {  // This is a class with no member variable
public:
public:
 CBlankClass() {};
 ~CBlankClass() {};

 void function1() {};
 void function2() {};
};

class CMyClass {  // This is a class with a int type member variable
public:
 int memberVar1;

public:
 CMyClass() {};
 ~CMyClass() {};

 void function1() {};
 void function2() {};
};

void main()
{
 char var1;
 printf("Size of char type = %d\n", mySizeOf(var1));

 int var2;
 printf("Size of int type = %d\n", mySizeOf(var2));

 float var3;
 printf("Size of float type = %d\n", mySizeOf(var3));

 double var4;
 printf("Size of double type = %d\n", mySizeOf(var4));

 char var5[] = "AB";
 printf("Size of char array ""AB"" = %d\n", mySizeOf(var5));

 struct myType var6;
 printf("Size of struct myType = %d\n", mySizeOf(var6));

 struct blankType var7;
 printf("Size of struct blankType = %d\n", mySizeOf(var7));

 CBlankClass var8;
 printf("Size of class CBlankClass = %d\n", mySizeOf(var8));

 CMyClass var9;
 printf("Size of class CMyClass = %d\n", mySizeOf(var9));
}

Output:
Size of char type = 1
Size of int type = 4
Size of float type = 4
Size of double type = 8
Size of char array AB = 3
Size of struct myType = 24
Size of struct blankType = 1
Size of class CBlankClass = 1
Size of class CMyClass = 4

From this result, interesting things are these,
Size of struct myType = 24
--> For a struct myType, if it has only "char a", its size is 1. But, if it has "char a" and "int b", its size becomes not 5 but 8. If it has "char a", "int b", and "float c", its size becomes 12.
Size of struct blankType = 1
Size of class CBlankClass = 1
--> Both black structure and class (no member variable) have its size 1.
--> For class, member functions don't affect on the size.
Size of class CMyClass = 4
--> If a class has member variables, its size will be the same to the situation of structre above.

memcpy() vs. memmove() in C

Here is the difference between memcpy() and memmove().

Example:
str = "strings are good";
memmove(str + 8, str + 11, 4);
This returns --> strings are are good
memcpy(str + 8, str + 11, 4);
This returns --> strings are ared

Example:

/* memcpy example */
#include 
#include 

int main ()
{
  char str1[]="Sample string";
  char str2[40];
  char str3[40];
  memcpy (str2,str1,strlen(str1)+1);
  memcpy (str3,"copy successful",16);
  printf ("str1: %s\nstr2: %s\nstr3: %s\n",str1,str2,str3);
  return 0;
}

Output:
str1: Sample string
str2: Sample string
str3: copy successful

Example:

/* memmove example */
#include 
#include 

int main ()
{
  char str[] = "memmove can be very useful......";
  memmove (str+20,str+15,11);
  puts (str);
  return 0;
}

Output:
memmove can be very very useful.