1. Pointers & Data Types

02/02/23

Data Types

bool - true/false. bool and int can be converted implicitly/automatically. non-zero is defined to be true. Can use this inside a while loop;

int x = 6;
while(x)
{
    printf("X is %d\n",x);
    x-=2;
}

Size of types

Size of types can vary based on compilers/operating system. Some guarantees are given:

• Minimum size (bits) - char 8, short 16, long 32.
• Relative sizes - char $\le$ short $\le$ int $\le$ long

void - type is used to mean no return value, no parameters, optional. Cannot create a variable of type void const - means constant/unchangeable. Use const char * instead of char* auto - Allows you to be lazy, don't bother about type. Can only be used for initialised variables.

Pointers

& - Ask for the address of a variable sizeof() - Size in chars - compile time * - Used to denote a pointer. The value of the pointer is an address in memory. The type of a pointer says what type of data the program should expect to find at the address

Can think of pointers whichever what is easier for you

1. As an address in memory and a type/format
2. As a way of pointing to some other data, and a record of what type of data you think the thing pointed as is

Dereferencing operator *

The * operator is used to access the thing that a pointer points at.

char c1 = 'h';
char* pc = &c1; //pc is a pointer to c1
char c3 = *pc; // *pc is thing pointed at

Uninitialised Pointers

Variables are NOT initialised unless you give them an initial value. Value of one is undefined.

Passing pointers as parameters

Each parameter has a single type, so may be one thing. A copy of this is stored in memory for the parameter. To alter something that is external to a function from within a function, need to refer to the thing itself, not a copy of it.

void AlterCopy( int icopy )  
{  
    icopy = 2;  
}  
void AlterValue( int* picopy )  
{  
    *picopy = 3;  
}  
int main( int argc, char* argv[] )  
{  
    int i = 1;  
    printf( "Initial value of i is %d\n", i );  
    AlterCopy( i );  
    printf( "After AlterCopy, value of i is %d\n", i );  
    AlterValue( &i );  
    printf( "After AlterValue, value of i is %d\n", i );  
    return 0;
}