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Pointers
Overview What are Pointers?
How to use Pointers?
Use of Pointers
Pointer: A variable that can only store the
address of a memory location (usually the address of other variable).
300
x500
a300
pointer pointee
Syntax: type * identifier;
i.e. int *p;
G
p258
pointer
int a=50;
int * p;
p = &a;
p500
pointer
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a300
pointee
p points to a
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* and & operators Dereferencing
*pvalue at where pointer p is
pointing
Address of&aaddress of variable a
Example:
void main(){int *p;int a=50;p = &a;cout<<a<<endl;cout<<&a<<endl;cout<<p<<endl;cout<<*p<<endl;cout<<&p<<endl;
}
Example:
void main(){=> int *p;
int a=50;p = &a;cout<<a<<endl;cout<<&a<<endl;cout<<p<<endl;cout<<*p<<endl;cout<<&p<<endl;
}
p500
pointer
Example:
void main(){int *p;
=> int a=50;p = &a;cout<<a<<endl;cout<<&a<<endl;cout<<p<<endl;cout<<*p<<endl;cout<<&p<<endl;
}
p500
pointer
50
a300
pointee
Example:
void main(){int *p;int a=50;
=> p = &a;cout<<a<<endl;cout<<&a<<endl;cout<<p<<endl;cout<<*p<<endl;cout<<&p<<endl;
}
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p500
pointer
50
a300
pointee
Example:
void main(){int *p;int a=50;p = &a;
=> cout<<a<<endl; // 50cout<<&a<<endl;cout<<p<<endl;cout<<*p<<endl;cout<<&p<<endl;
}
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p500
pointer
50
a300
pointee
Example:
void main(){int *p;int a=50;p = &a;cout<<a<<endl; // 50
=> cout<<&a<<endl; // 300cout<<p<<endl;cout<<*p<<endl;cout<<&p<<endl;
}
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p500
pointer
50
a300
pointee
Example:
void main(){int *p;int a=50;p = &a;cout<<a<<endl; // 50cout<<&a<<endl; // 300
=> cout<<p<<endl; // 300cout<<*p<<endl;cout<<&p<<endl;
}
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p500
pointer
50
a300
pointee
Example:
void main(){int *p;int a=50;p = &a;cout<<a<<endl; // 50cout<<&a<<endl; // 300cout<<p<<endl; // 300
=> cout<<*p<<endl; // 50cout<<&p<<endl;
}
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p500
pointer
50
a300
pointee
Example:
void main(){int *p;int a=50;p = &a;cout<<a<<endl; // 50cout<<&a<<endl; // 300cout<<p<<endl; // 300cout<<*p<<endl; // 50
=> cout<<&p<<endl; // 500}
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p500
pointer
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a300
pointee
Example 2:
void main(){int *p;int a=50;cout<<*p<<endl;
}
What would be the output?Runtime error!!!
p500
pointer
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a300
pointee
Crash!!!
Dangling Pointerint *p;
Note: Dereferencing a dangling pointer is a serious runtime error.
G
p500
pointerDangling pointer
Problem:We can’t differentiate that whether a pointer is dangling or has a valid address.
What’s the solution?
NULL Pointerint *p;p = NULL; // points to
// nothing
Tip: Always initialize a pointer with NULL if you don’t have a valid address. This can save a lot of your time on debugging.
p500
pointer
Example:
void main(){int *p = NULL;int a=50;p = &a;cout<<a<<endl;cout<<&a<<endl;cout<<p<<endl;cout<<*p<<endl;cout<<&p<<endl;
}
Pointer Assignment An assignment operation b/w two
pointers makes them points to the same pointee.
int a=50;
int * p;
p = &a;
int p2 = p;
p500
pointer
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a300
pointee
p points to a
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p2200
pointer Sharing
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Note: Pointer assignment only copies the address and not the memory they are pointing to.
Shallow Copy
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p500
pointer
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a300
pointee
p points to a
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p2200
pointer Sharing
Problem with Shallow Copy
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p500
pointer
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a300
pointee
p points to a
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p2200
pointer Wild Pointer
What if p delete a?
What is the solution to the problem?
Deep Copy
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p500
pointer
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a300
pointee
p points to a
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p2200
pointer
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a2100
pointee
Pointer type and Arithmetic What is type of pointer?
int * p; A pointer has not type!!!
It’s the type of variable the pointer p will point to.
Why we need to specify the type of variable a pointer points to?
1.int a=10;int *p = &a;*p = 4; // how many bytes of copy?
It would be 1 byte in case a was a char.
2. Pointer & Arraysint arr[ ]={5,6,7,8,4,3,5};// size of arr?int *p = arr;// why & isn’t use before arr?cout<<arr<<endl; // 40cout<<p<<endl; // 40p=p+3;
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40
p500
pointer
2. Pointer & Arraysint arr[ ]={5,6,7,8,4,3,5};// size of arr?int *p = arr;// why & isn’t use before arr?cout<<arr<<endl; // 40cout<<p<<endl; // 40p=p+3;cout<<p<<endl; // 46cout<<*p<<endl; // 8
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40 42 44 46 48 50 52
40
p500
pointer
Example 3:
void main( ){=> int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
cout<< arr<<endl; cout<<*arr<<endl; cout<<*(arr+4)<<endl; cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl;}
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40 42 44 46 48 50 52
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5};=> int *p = arr;
cout<< arr<<endl; cout<<*arr<<endl; cout<<*(arr+4)<<endl; cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl;}
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40
p500
pointer
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
=> cout<< arr<<endl; // 0x40 cout<<*arr<<endl; cout<<*(arr+4)<<endl; cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl;}
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p500
pointer
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
cout<< arr<<endl; // 0x40=> cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl;}
5 6 7 8 4 3 5
40 42 44 46 48 50 52
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p500
pointer
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5=> cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl;}
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40 42 44 46 48 50 52
40
p500
pointer
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4=> cout<<p<<endl; //0x40 cout<<&p<<endl; cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl;}
5 6 7 8 4 3 5
40 42 44 46 48 50 52
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p500
pointer
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40=> cout<<&p<<endl; //0x500 cout<<*p<<endl; cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl;}
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40 42 44 46 48 50 52
40
p500
pointer
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40 cout<<&p<<endl; //0x500=> cout<<*p<<endl; // 5 cout<<p[3]<<endl; cout<<p++<<endl; cout<<++*p<<endl;}
5 6 7 8 4 3 5
40 42 44 46 48 50 52
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p500
pointer
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40 cout<<&p<<endl; //0x500 cout<<*p<<endl; // 5=> cout<<p[3]<<endl; // 8 cout<<p++<<endl; cout<<++*p<<endl;}
5 6 7 8 4 3 5
40 42 44 46 48 50 52
40
p500
pointer
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40 cout<<&p<<endl; //0x500 cout<<*p<<endl; // 5 cout<<p[3]<<endl; // 8=> cout<<p++<<endl; // 0x40 cout<<++*p<<endl;}
5 6 7 8 4 3 5
40 42 44 46 48 50 52
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p500
pointer
Example 3:
void main( ){ int arr[ ]={5,6,7,8,4,3,5}; int *p = arr;
cout<< arr<<endl; // 0x40 cout<<*arr<<endl; // 5 cout<<*(arr+4)<<endl; // 4 cout<<p<<endl; //0x40 cout<<&p<<endl; //0x500 cout<<*p<<endl; // 5 cout<<p[3]<<endl; // 8 cout<<p++<<endl; // 0x40=> cout<<++*p<<endl; // 7}
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p500
pointer
Arrays of Pointerdata_type *arr[size];
i.e.int *a[7];
char *c[7];
* * * * * * *
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* * * * * * *
80 82 84 86 88 90 92
Uses: Sharing & Cost saving
Pass-by-ref
Dynamic Memory Allocation
Making Complex Data Structures Linked list, Tree etc.
1. Sharing & cost savingvoid main(){
int a=5,b=8;swap(&a,&b);
}void swap(int *a,int *b){
int t = *a;*a = *b;*b = t;
}
2. DMAvoid main(){
int size=0;int *p=NULL;cout<<“Enter No. of Students”<<endl;cin>>size;p = (int *)malloc(size*sizeof(int));for(int i=0;i<size;i++){
cout<<“Enter St “<<i<<“ Marks”<<endl;cin>>p[i]; //= cin>>*p; p++;
}}
3. Data Structures
DataPointer
DataPointer
DataPointer
DataPointer
References: Pointer And Memory (
http://cslibrary.stanford.edu/102/) Pointer Basics (
http://cslibrary.stanford.edu/106/) http://
www.cprogramming.com/tutorial/c/lesson6.html
http://www.cs.cf.ac.uk/Dave/C/node10.html
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