C FAQS
Q:How do I write code that reads data at memory
location specified by segment and offset?
Ans:Use peekb( ) function. This function returns byte(s) read from
specific segment and offset locations in memory. The following program
illustrates use of this function. In this program from VDU memory we have read
characters and its attributes of the first row. The information stored in file
is then further read and displayed using peek( ) function.
#include <stdio.h>
#include <dos.h>
main( )
{
char far *scr = 0xB8000000 ;
FILE *fp ;
int offset ;
char ch ;
if ( ( fp = fopen ( "scr.dat", "wb" ) ) == NULL )
{
printf ( "\nUnable to open file" ) ;
exit( ) ;
}
// reads and writes to file
for ( offset = 0 ; offset < 160 ; offset++ )
fprintf ( fp, "%c", peekb ( scr, offset ) ) ;
fclose ( fp ) ;
if ( ( fp = fopen ( "scr.dat", "rb" ) ) == NULL )
{
printf ( "\nUnable to open file" ) ;
exit( ) ;
}
// reads and writes to file
for ( offset = 0 ; offset < 160 ; offset++ )
{
fscanf ( fp, "%c", &ch ) ;
printf ( "%c", ch ) ;
}
fclose ( fp ) ;
}
Q:What is conversion operator?
Ans:class can have a public method for specific data type conversions.
for example:
class Boo
{
double value;
public:
Boo(int i )
operator double()
{
return value;
}
};
Boo BooObject;
double i = BooObject; // assigning object to variable i of type double. now
conversion operator gets called to assign the value.
Q:What is diff between malloc()/free() and new/delete?
Ans:malloc allocates memory for object in heap but doesn't invoke
object's constructor to initiallize the object.
new allocates memory and also invokes constructor to initialize the object.
malloc() and free() do not support object semantics
Does not construct and destruct objects
string * ptr = (string *)(malloc (sizeof(string)))
Are not safe
Does not calculate the size of the objects that it construct
Returns a pointer to void
int *p = (int *) (malloc(sizeof(int)));
int *p = new int;
Are not extensible
new and delete can be overloaded in a class
"delete" first calls the object's termination routine (i.e. its destructor)
and then releases the space the object occupied on the heap memory. If an
array of objects was created using new, then delete must be told that it is
dealing with an array by preceding the name with an empty []:-
Int_t *my_ints = new Int_t[10];
...
delete []my_ints;
Q:what is the diff between "new" and "operator new" ?
Ans:"operator new" works like malloc.
Q:What is difference between template and macro??
Ans:There is no way for the compiler to verify that the macro parameters
are of compatible types. The macro is expanded without any special type
checking.
If macro parameter has a postincremented variable ( like c++ ), the increment
is performed two times.
Because macros are expanded by the preprocessor, compiler error messages will
refer to the expanded macro, rather than the macro definition itself. Also,
the macro will show up in expanded form during debugging.
for example:
Macro:
#define min(i, j) (i < j ? i : j)
template:
template<class T>
T min (T i, T j)
{
return i < j ? i : j;
}
