Pointers (C++)
The latest version of this topic can be found at Pointers (C++).
Pointers are declared using the following sequence.
[storage-class-specifiers] [cv-qualifiers] type-specifiers
[ms-modifier] declarator ;
where any valid pointer declarator may be used for declarator. The syntax for a simple pointer declarator is as follows:
* [cv-qualifiers] identifier [= expression]
- The declaration specifiers:
An optional storage class specifier. For more information, see Specifiers.
An optional
constorvolatilekeyword applying to the type of the object to be pointed to.The type specifier: the name of a type representing the type of the object to be pointed to.
- The declarator:
An optional Microsoft specific modifier. For more information, see Microsoft-Specific Modifiers.
The
*operator.An optional
constorvolatilekeyword applying to the pointer itself.The identifier.
An optional initializer.
The declarator for a pointer to function looks like this:
(* [cv-qualifiers] identifier )( argument-list ) [cv-qualifers]
[exception specification] [= expression];
- For an array of pointers, the syntax looks like this:
* identifier [ [ constant-expression ] ]
However, pointer declarators can be more complex. For more information,see Declarators.
Multiple declarators and their initializers may appear together in a single declaration in a comma separated list following the declaration specifier.
A simple example of a pointer declaration is:
char *pch;
The preceding declaration specifies that pch points to an object of type char.
A more complex example is
static unsigned int * const ptr;
The preceding declaration specifies that ptr is a constant pointer to an object of type unsigned int with static storage duration.
The next example shows how multiple pointers are declared and initialized:
static int *p = &i, *q = &j;
In the preceding example, pointers p and q both point to objects of type int and are initialized to the addresses of i and j respectively. The storage class specifier static applies to both pointers.
Example
// pointer.cpp
// compile with: /EHsc
#include <iostream>
int main() {
int i = 1, j = 2; // local variables on the stack
int *p;
// a pointer may be assigned to "point to" the value of
// another variable using the & (address of) operator
p = & j;
// since j was on the stack, this address will be somewhere
// on the stack. Pointers are printed in hex format using
// %p and conventionally marked with 0x.
printf_s("0x%p\n", p);
// The * (indirection operator) can be read as "the value
// pointed to by".
// Since p is pointing to j, this should print "2"
printf_s("0x%p %d\n", p, *p);
// changing j will change the result of the indirection
// operator on p.
j = 7;
printf_s("0x%p %d\n", p, *p );
// The value of j can also be changed through the pointer
// by making an assignment to the dereferenced pointer
*p = 10;
printf_s("j is %d\n", j); // j is now 10
// allocate memory on the heap for an integer,
// initialize to 5
p = new int(5);
// print the pointer and the object pointed to
// the address will be somewhere on the heap
printf_s("0x%p %d\n", p, *p);
// free the memory pointed to by p
delete p;
// At this point, dereferencing p with *p would trigger
// a runtime access violation.
// Pointer arithmetic may be done with an array declared
// on the stack or allocated on the heap with new.
// The increment operator takes into account the size
// of the objects pointed to.
p = new int[5];
for (i = 0; i < 5; i++, p++) {
*p = i * 10;
printf_s("0x%p %d\n", p, *p);
}
// A common expression seen is dereferencing in combination
// with increment or decrement operators, as shown here.
// The indirection operator * takes precedence over the
// increment operator ++.
// These are particularly useful in manipulating char arrays.
char s1[4] = "cat";
char s2[4] = "dog";
char* p1 = s1;
char* p2 = s2;
// the following is a string copy operation
while (*p1++ = *p2++);
// s2 was copied into s1, so now they are both equal to "dog"
printf_s("%s %s", s1, s2);
}
0x0012FEC8
0x0012FEC8 2
0x0012FEC8 7
j is 10
0x00320850 5
0x00320850 0
0x00320854 10
0x00320858 20
0x0032085C 30
0x00320860 40
dog dog
Example
Another example illustrates the use of pointers in data structures; in this case, a linked list.
// pointer_linkedlist.cpp
// compile with: /EHsc
#include <iostream>
using namespace std;
struct NewNode {
NewNode() : node(0){}
int i;
NewNode * node;
};
void WalkList(NewNode * ptr) {
if (ptr != 0) {
int i = 1;
while (ptr->node != 0 ) {
cout << "node " << i++ << " = " << ptr->i << endl;
ptr = ptr->node;
}
cout << "node " << i++ << " = " << ptr->i << endl;
}
}
void AddNode(NewNode ** ptr) {
NewNode * walker = 0;
NewNode * MyNewNode = new NewNode;
cout << "enter a number: " << endl;
cin >> MyNewNode->i;
if (*ptr == 0)
*ptr = MyNewNode;
else {
walker = *ptr;
while (walker->node != 0)
walker = walker->node;
walker->node = MyNewNode;
}
}
int main() {
char ans = ' ';
NewNode * ptr = 0;
do {
cout << "a (add node) d (display list) q (quit)" << endl;
cin >> ans;
switch (ans) {
case 'a':
AddNode(&ptr);
break;
case 'd':
WalkList(ptr);
break;
}
} while (ans != 'q');
}
a
45
d
a
789
d
qa (add node) d (display list) q (quit)
enter a number:
a (add node) d (display list) q (quit)
node 1 = 45
a (add node) d (display list) q (quit)
enter a number:
a (add node) d (display list) q (quit)
node 1 = 45
node 2 = 789
a (add node) d (display list) q (quit)
See Also
C++ Abstract Declarators
Addition of Pointer Types
Indirection Operator: *
Address-of Operator: &