Pointers (C++)

Artigo
01/03/2018

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 const or volatile keyword 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 const or volatile keyword 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)

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