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Macros vs Functions

  • Difficulty Level : Easy
  • Last Updated : 23 Mar, 2021

Macros are pre-processed which means that all the macros would be processed before your program compiles. However, functions are not preprocessed but compiled.

See the following example of Macro:

C




#include<stdio.h>
#define NUMBER 10
int main()
{
     printf("%d", NUMBER);
     return 0;
}

C++




#include<iostream>
#define NUMBER 10
using namespace std;
int main()
{
     cout<<NUMBER;
     return 0;
}
//This code is contributed by Mayank Tyagi

Output: 

10

See the following example of Function:

C




#include<stdio.h>
int number()
{
    return 10;
}
int main()
{
    printf("%d", number());
    return 0;
}

C++




#include<iostream>
using namespace std;
int number()
{
    return 10;
}
int main()
{
    cout<<number();
    return 0;
}
//This code is contributed by Mayank Tyagi

Output: 



10

Now compile them using the command: 

gcc –E file_name.c

This will give you the executable code as shown in the figure: 

b12

This shows that the macros are preprocessed while functions are not.
In macros, no type checking(incompatible operand, etc.) is done and thus use of macros can lead to errors/side-effects in some cases. However, this is not the case with functions. Also, macros do not check for compilation error (if any). Consider the following two codes:

Macros:  

C




#include<stdio.h>
#define CUBE(b) b*b*b
int main()
{
     printf("%d", CUBE(1+2));
     return 0;
}

Output: Unexpected output 

7

Note: This macro is expanded as below

CUBE(1+2)  === 1+2*1+2*1+2 which is equal to 7 [correct but unexpected result]



To fix this we need to replace #define CUBE(b) b*b*b as #define CUBE(b) (b)*(b)*(b). With updated macro, CUBE(1+2) will be expanded as 

CUBE(1+2)  === (1+2)*(1+2)*(1+2) which is equal to 27 [correct and expected result]

Functions:

C




#include<stdio.h>
int cube(int a)
{
     return a*a*a;
}
int main()
{
    printf("%d", cube(1+2));
    return 0;
}

Output: As expected 

27
  • Macros are usually one liner. However, they can consist of more than one line, Click here to see the usage. There are no such constraints in functions.
  • The speed at which macros and functions differs. Macros are typically faster than functions as they don’t involve actual function call overhead.

Conclusion: 
Macros are no longer recommended as they cause following issues. There is a better way in modern compilers that is inline functions and const variable. Below are disadvantages of macros:
a) There is no type checking
b) Difficult to debug as they cause simple replacement.
c) Macro don’t have namespace, so a macro in one section of code can affect other section.
d) Macros can cause side effects as shown in above CUBE() example.

MacroFunction
Macro is PreprocessedFunction is Compiled
No Type Checking is done in MacroType Checking is Done in Function
Using Macro increases the code lengthUsing Function keeps the code length unaffected
Use of macro can lead to side effect at later stagesFunctions do not lead to any side effect in any case
Speed of Execution using Macro is FasterSpeed of Execution using Function is Slower
Before Compilation, macro name is replaced by macro valueDuring function call, transfer of control takes place
Macros are useful when small code is repeated many timesFunctions are useful when large code is to be written
Macro does not check any Compile-Time ErrorsFunction checks Compile-Time Errors

See following for more details on macros: 
Interesting facts about Macros and Preprocessors
 
 This article is contributed by Pranjal Mathur. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 

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