[C] Pointers Cheat Sheet

[MrGrj]

Pointer Cheat Sheet

• A pointer must always be of the same type as the variable it's pointing at.
  
• Declaring a pointer variable does not create the type of variable it points at. It creates a pointer variable.
  
• Though pointers are declared with an asterisk they are not always used with an asterisk.
  
• The asterisk is the unary * operator. It is not the * multiplication operator.
  
• Pointers must be initialized before they can be used.
  
• Initialize a pointer by assigning it to a variable; the variable must be of the same type as the pointer.
  
• To assign a pointer to a variable, use an ampersand with the variable's name.
  
• The address-of unary operator & is not the same as the bitwise & AND operator.
  

m_address = &memory;

To assign a pointer to an array, do not use the ampersand:

 s_address = string;

• The pointer s_address would be used on the string array's elements.
  
• To assign a pointer to an array element, use the ampersand:
  

element = &string[2];

• Without an asterisk, an initialized pointer holds a memory address.
  
• With an asterisk, an initialized pointer references the value stored at its address.
  

Typical Pointer Setup and Use

First, create a pointer of the proper type:

float *f;

Second assign it to a variable's memory location:

f = &boat;

Finally, use the pointer:

printf("%.0f",*f);

• Without an asterisk, the pointer references a memory location.
  
• With an asterisk, the pointer references the value at that memory location.
  
• Always use the same type of pointer as the variables it examines: floats for floats, ints for ints, and so on.
  
• Remember: initialize a pointer before you use it! Set the pointer equal to the address of some variable in memory.
  

Pointers, Parenthesis and Math

[table=width: 500, class: grid, align: center]

[tr]

[td]Pointer Thing[/td]

[td]Memory Address[/td]

[td]Memory Contents[/td]

[/tr]

[tr]

[td]p[/td]

[td]Yep[/td]

[td]Nope[/td]

[/tr]

[tr]

[td]*p[/td]

[td]Nope[/td]

[td]Yep[/td]

[/tr]

[tr]

[td]*p++[/td]

[td]Incremented after value is read[/td]

[td]Unchanged[/td]

[/tr]

[tr]

[td]*(p++)[/td]

[td]Incremented after value is read[/td]

[td]Unchanged[/td]

[/tr]

[tr]

[td](*p)++ [/td]

[td]Unchanged[/td]

[td]Incremented after it's used[/td]

[/tr]

[tr]

[td]*++p[/td]

[td]Incremented before value is read[/td]

[td]Unchanged[/td]

[/tr]

[tr]

[td]*(++p)[/td]

[td]Incremented before value is read[/td]

[td]Unchanged[/td]

[/tr]

[tr]

[td]++*p[/td]

[td]Unchanged[/td]

[td]Incremented before it's used[/td]

[/tr]

[tr]

[td]++(*p)[/td]

[td]Unchanged[/td]

[td]Incremented before it's used[/td]

[/tr]

[tr]

[td]p*++[/td]

[td]Not a pointer[/td]

[td]Not a pointer[/td]

[/tr]

[tr]

[td]p++*[/td]

[td]Not a pointer[/td]

[td]Not a pointer[/td]

[/tr]

[/table]

The ++ operator is used above, though any math operation can be substituted.

A tip: Use parenthesis to isolate part of the pointer problem and the answer will always work out the way you intended.

Pointers and array brackets

[table=width: 500, class: grid, align: left]

[tr]

[td]Array Notation[/td]

[td]Pointer Equivalent[/td]

[/tr]

[tr]

[td]array[0][/td]

[td]*a[/td]

[/tr]

[tr]

[td]array[1][/td]

[td]*(a+1)[/td]

[/tr]

[tr]

[td]array[2][/td]

[td]*(a+2)[/td]

[/tr]

[tr]

[td]array[3][/td]

[td]*(a+3)[/td]

[/tr]

[tr]

[td]array[x][/td]

[td]*(a+x)[/td]

[/tr]

[/table]

Ugly ** notation

[table=width: 500]

[tr]

[td]Doodad[/td]

[td]What It Is[/td]

[td]Seen by The Compiler[/td]

[/tr]

[tr]

[td]array+1[/td]

[td]An address[/td]

[td]A pointer[/td]

[/tr]

[tr]

[td]*(array+1)[/td]

[td]Contents of address[/td]

[td]A string[/td]

[/tr]

[tr]

[td]*(*(array+1)) [/td]

[td]Contents of a character array [/td]

[td]A character[/td]

[/tr]

[tr]

[td]**(array+1)[/td]

[td]Same as above[/td]

[td]Same as above[/td]

[/tr]

[/table]

Edited August 17, 2015 by MrGrj

[gigiRoman]

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