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Thinking in C++
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#### Pointer arithmetic

If all you could do with a pointer that points at an array is treat it as if it were an alias for that array, pointers into arrays wouldn’t be very interesting. However, pointers are more flexible than this, since they can be modified to point somewhere else (but remember, the array identifier cannot be modified to point somewhere else).

Pointer arithmetic refers to the application of some of the arithmetic operators to pointers. The reason pointer arithmetic is a separate subject from ordinary arithmetic is that pointers must conform to special constraints in order to make them behave properly. For example, a common operator to use with pointers is ++, which “adds one to the pointer.” What this actually means is that the pointer is changed to move to “the next value,” whatever that means. Here’s an example:

```//: C03:PointerIncrement.cpp
#include <iostream>
using namespace std;

int main() {
int i[10];
double d[10];
int* ip = i;
double* dp = d;
cout << "ip = " << (long)ip << endl;
ip++;
cout << "ip = " << (long)ip << endl;
cout << "dp = " << (long)dp << endl;
dp++;
cout << "dp = " << (long)dp << endl;
} ///:~```

For one run on my machine, the output is:

```ip = 6684124
ip = 6684128
dp = 6684044
dp = 6684052```

What’s interesting here is that even though the operation ++ appears to be the same operation for both the int* and the double*, you can see that the pointer has been changed only 4 bytes for the int* but 8 bytes for the double*. Not coincidentally, these are the sizes of int and double on my machine. And that’s the trick of pointer arithmetic: the compiler figures out the right amount to change the pointer so that it’s pointing to the next element in the array (pointer arithmetic is only meaningful within arrays). This even works with arrays of structs:

```//: C03:PointerIncrement2.cpp
#include <iostream>
using namespace std;

typedef struct {
char c;
short s;
int i;
long l;
float f;
double d;
long double ld;
} Primitives;

int main() {
Primitives p[10];
Primitives* pp = p;
cout << "sizeof(Primitives) = "
<< sizeof(Primitives) << endl;
cout << "pp = " << (long)pp << endl;
pp++;
cout << "pp = " << (long)pp << endl;
} ///:~```

The output for one run on my machine was:

```sizeof(Primitives) = 40
pp = 6683764
pp = 6683804```

So you can see the compiler also does the right thing for pointers to structs (and classes and unions).

Pointer arithmetic also works with the operators --, +, and -, but the latter two operators are limited: you cannot add two pointers, and if you subtract pointers the result is the number of elements between the two pointers. However, you can add or subtract an integral value and a pointer. Here’s an example demonstrating the use of pointer arithmetic:

```//: C03:PointerArithmetic.cpp
#include <iostream>
using namespace std;

#define P(EX) cout << #EX << ": " << EX << endl;

int main() {
int a[10];
for(int i = 0; i < 10; i++)
a[i] = i; // Give it index values
int* ip = a;
P(*ip);
P(*++ip);
P(*(ip + 5));
int* ip2 = ip + 5;
P(*ip2);
P(*(ip2 - 4));
P(*--ip2);
P(ip2 - ip); // Yields number of elements
} ///:~```

It begins with another macro, but this one uses a preprocessor feature called stringizing (implemented with the ‘#’ sign before an expression) that takes any expression and turns it into a character array. This is quite convenient, since it allows the expression to be printed, followed by a colon, followed by the value of the expression. In main( ) you can see the useful shorthand that is produced.

Although pre- and postfix versions of ++ and -- are valid with pointers, only the prefix versions are used in this example because they are applied before the pointers are dereferenced in the expressions above, so they allow us to see the effects of the operations. Note that only integral values are being added and subtracted; if two pointers were combined this way the compiler would not allow it.

Here is the output of the program above:

```*ip: 0
*++ip: 1
*(ip + 5): 6
*ip2: 6
*(ip2 - 4): 2
*--ip2: 5```

In all cases, the pointer arithmetic results in the pointer being adjusted to point to the “right place,” based on the size of the elements being pointed to.

If pointer arithmetic seems a bit overwhelming at first, don’t worry. Most of the time you’ll only need to create arrays and index into them with [ ], and the most sophisticated pointer arithmetic you’ll usually need is ++ and --. Pointer arithmetic is generally reserved for more clever and complex programs, and many of the containers in the Standard C++ library hide most of these clever details so you don’t have to worry about them.

Thinking in C++
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 Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire