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Add void pointer to index.md (#22248)

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* Update index.md

I added void pointers, which can be very useful in C programming, especially when dealing with unknown data types but had not been included.

* fixed formatting issues, removed attributions, moved placement to more appropriate location

* adjusted section header size
This commit is contained in:
Farai Mugaviri
2018-11-19 04:23:19 +02:00
committed by Christopher McCormack
parent e759361716
commit e73ffaf2fe
1 changed files with 52 additions and 5 deletions
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guide/english/c/pointers/index.md
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@ -166,7 +166,7 @@ This starts by taking a string (something you'll learn about when you get into a
### Const Qualifer ### Const Qualifer
The qualifier const can be applied to the declaration of any variable to specify that its value will not be changed ( Which depends upon where const variables are stored, we may change value of const variable by using pointer ). The qualifier const can be applied to the declaration of any variable to specify that its value will not be changed ( Which depends upon where const variables are stored, we may change value of const variable by using pointer ).
# Pointer to variable ### Pointer to variable
We can change the value of ptr and we can also change the value of object ptr pointing to. We can change the value of ptr and we can also change the value of object ptr pointing to.
Following code fragment explains pointer to variable Following code fragment explains pointer to variable
```c ```c
@ -189,7 +189,7 @@ int main(void)
return 0; return 0;
} }
``` ```
# Pointer to constant ### Pointer to constant
We can change pointer to point to any other integer variable, but cannot change value of object (entity) pointed using pointer ptr. We can change pointer to point to any other integer variable, but cannot change value of object (entity) pointed using pointer ptr.
```c ```c
#include <stdio.h> #include <stdio.h>
@ -209,7 +209,7 @@ int main(void)
return 0; return 0;
} }
``` ```
# Constant pointer to variable ### Constant pointer to variable
In this we can change the value of the variable the pointer is pointing to. But we can't change the pointer to point to In this we can change the value of the variable the pointer is pointing to. But we can't change the pointer to point to
another variable. another variable.
```c ```c
@ -229,7 +229,7 @@ int main(void)
return 0; return 0;
} }
``` ```
# constant pointer to constant ### Constant pointer to constant
Above declaration is constant pointer to constant variable which means we cannot change value pointed by pointer as well as we cannot point the pointer to other variable. Above declaration is constant pointer to constant variable which means we cannot change value pointed by pointer as well as we cannot point the pointer to other variable.
```c ```c
#include <stdio.h> #include <stdio.h>
@ -249,6 +249,54 @@ int main(void)
} }
``` ```
### Void Pointer
A void pointer is a pointer variable declared using the reserved word in C ‘void’.
Lets illustrate this with a void pointer declaration below:
```C
void *ptr;
```
A pointer variable with the keyword `void`is a general purpose pointer variable.
The pointer can hold an address of any variable of any data type (`int`, `char`...etc).
As illustrated earlier on, the * operator serves its own purpose.
But in the case of a void pointer we need to typecast the pointer variable to dereference it mainly because a void pointer has no specific data type associated with it.
There is no other way the compiler can tell what type of data is pointed to by the void pointer.
So to take the data pointed to by a void pointer we typecast it with the correct type of the data that is held inside the void pointer's location.
Below is an example to illustrate how a void pointer coild be used in a program:
```C
#include<stdio.h>
void main() {
int a = 10;
float b = 35.75;
void *ptr; // Declaration of a void pointer
ptr = &a; // Assigning address of integer to void pointer.
printf("The value of integer variable is = %d",*( (int*) ptr) );// (int*)ptr - is ype typecasting, to point to an int type. Where as *((int*)ptr) dereferences the typecasted void pointer variable.
}
```
The output becomes
```output
The value of integer variable is = 10
```
A void pointer can be useful if the programmer is not sure about the data type of data inputted by the end user.
In such a case the programmer can use a void pointer to point to the location of the unknown data type.
The program can be set in such a way to ask the user to inform the type of data and type casting can be performed according to the information inputted by the user.
Another important point you should keep in mind about void pointers is that pointer arithmetic can not be performed in a void pointer.
Example:
```C
void *ptr;
int a;
ptr=&a;
ptr++; // This statement is invalid and will result in an error because 'ptr' is a void pointer variable.
```
Credits: <http://www.circuitstoday.com/void-pointers-in-c>
# Before you go on... # Before you go on...
## A review ## A review
* Pointers are variables, but instead of storing a value, they store a memory location. * Pointers are variables, but instead of storing a value, they store a memory location.
@ -282,4 +330,3 @@ Most of the time, pointer and array accesses can be treated as acting the same,
p++; /*Legal*/ p++; /*Legal*/
a++; /*illegal*/ a++; /*illegal*/
``` ```