Pointers in Java

Drawer PDF417 in Java Pointers

Figure 6-3

Multiple indirection can be carried on to whatever extent desired, but there are few cases where using more than a pointer to a pointer is necessary, or even wise Excessive indirection is difficult to follow and prone to conceptual errors (Do not confuse multiple indirection with linked lists, which are used in databases) A variable that is a pointer to a pointer must be declared as such This is done by placing an additional asterisk in front of its name For example, this declaration tells the compiler that newbalance is a pointer to a pointer of type float

float **newbalance;

It is important to understand that newbalance is not a pointer to a floating-point number but rather a pointer to a float pointer In order to access the target value indirectly pointed to by a pointer to a pointer, the asterisk operator must be applied twice, as is shown in this short example:

#include <stdioh> int main(void) { int x, *p, **q; x = 10; p = &x;

q = &p; printf("%d", **q); /* print the value of x */ return 0; }

Here, p is declared as a pointer to an integer, and q as a pointer to a pointer to an integer The call to printf( ) prints the number 10 on the screen

After a pointer is declared, but before it has been assigned a value, it may contain an unknown value If you try to use the pointer prior to giving it a value, you probably will crash not only your program but also the operating system of your computer a very nasty type of error! By convention, a pointer that is pointing nowhere should be given the value null to signify that it points to nothing However, just because a pointer has a null value does not make it safe If you use a null pointer on the left side of an assignment statement, you still risk crashing your program or operating system Because a null pointer is assumed to be unused, you can use the null pointer to make many of your pointer routines easier to code and more efficient For example, you could use a null pointer to mark the end of a pointer array If this is done, a routine that accesses that array knows that it has reached the end when the null value is encountered This type of approach is illustrated by the search( ) function shown here:

/* Look up a name */ int search(char *p[], char *name) { register int t; for(t=0; p[t]; ++t) if(!strcmp(p[t], name)) return t; return -1; /* not found */ }

The for loop inside search( ) runs until either a match or a null pointer is found Assuming the end of the array is marked with a null, the condition controlling the loop fails when it is reached It is common in professionally written programs to initialize strings You saw an example of this in the serror( ) function shown earlier Another variation on this theme is the following type of string declaration:

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As you can see, the pointer p is not an array The reason this sort of initialization works has to do with the way the compiler operates All C/C++ compilers create what is called a string table, which is used internally by the compiler to store the string constants used by the program Therefore, this declaration statement places the address of "hello world" into the pointer p Throughout the program p can be used like any other string For example, the following program is perfectly valid:

#include <stdioh> #include <stringh> char *p = "hello world\n"; int main(void) { register int t; /* print the string forward and backwards */ printf(p); for(t=strlen(p)-1; t>-1; t--) printf("%c", p[t]); return 0; }