C++ Tutorial - About Pointers

Some Rules with Pointers

These are more common sense than hard and fast rules.

• Always initialize pointer variables.
• If you use C strings, never forget to include the terminating '\0' when calculating the storage size.
• Check that new doesn't generate an exception or return 0.
• For every new there is an equal and opposite delete()!
• For arrays you must use the [] for both new and delete. Don't free with delete() something allocated with new()[] and vice versa.
• After you've deleted a pointer, set it to 0.

For example.

 int * pint=new int ;
 /*
 Do something with pint
 */
 delete pint ;
 pint = 0; 
 

Setting pint to 0 is a safety feature. Just because the memory has been released by the call to delete() does not prevent it accidentally being reused. This could lead to a nasty bug but delete() does nothing with a 0 value pointer.

New is more than just malloc()

You might be wondering why not just malloc(), especially if you know C. Two reasons

1. new is typesafe. Whereas malloc() returns a void *, new returns an object of the specified type.
2. malloc() does not initialise memory whereas new calls a constructor on an object.

Example 3 shows a struct being created.

 // ex3.cpp
 
 #include <iostream>
 using namespace std;
 
 struct s{
   int a;
   double d;
   s() {a =9;d=9.999;}
 };
 
 int main(int argc, char* argv[])
 { 
   s * value = new s;
   cout << "Value of value.a = " << 
    value->a << " value.d = " << value->d << endl;
   return 0;
 }
 
 

The output from this is:

 Value of value.a = 9 value.d = 9.999
 

Download Example 3.

This shows that the constructor s() was called.

On the next page : More examples of using pointers