From 9cea2d56a535125af490a13146a1598b0b2419ba Mon Sep 17 00:00:00 2001
From: jagmeethanspal <jagmeet.hanspal@gmail.com>
Date: Sun, 25 Nov 2018 06:03:39 +0530
Subject: [PATCH] Added Special Operators in Macros (#22514)

One small fix and the explanation of Stringizing and Token Pasting Operators that can be used effectively in C macros.
---
 guide/english/c/macros/index.md | 50 ++++++++++++++++++++++++++++-----
 1 file changed, 43 insertions(+), 7 deletions(-)

diff --git a/guide/english/c/macros/index.md b/guide/english/c/macros/index.md
index bab53bac5d..1dc08538e8 100644
--- a/guide/english/c/macros/index.md
+++ b/guide/english/c/macros/index.md
@@ -31,30 +31,66 @@ Function-like uses the same `#define` keyword. The difference is that you use a
 ```C
 #define hello_world() printf("Hello World!")
 ```
+
 So calling:
 ```C
-hello_world()
+hello_world();
 ```
+
 You get:
 ```C
 printf("Hello World!");
 ```
+
 You can set parameters too:
 ```C
 #define hello(X) printf("Hello " X "!")
 ```
+
 Now calling:
 ```C
 hello("World");
 ```
+
 You get the equivalent of:
 ```C
 printf("Hello World!");
 ```
+
+#### Special Operators in Macros
+One can use the special operators # (stringize) and ## (concatenate) in macros for achieving unique functionality.
+
+##### Stringizing Operator (#)
+A macro's parameter preceded by a `#` is converted and treated as a string token.  
+For example, we can define ERROR and WARN macros that print a LOG message.  
+While the LOG message gets prefixed with either an `ERR` or a `WARN`, respectively.
+```C
+#define LOG(level, message) printf(#level ": " #message "\n")
+#define ERROR(msg) LOG(FAIL, msg)
+#define WARN(msg) LOG(WARN, msg)
+```
+
+Now, one can use it as
+```C
+ERROR(Invalid settings);  // Output-> FAIL: Invalid settings
+WARN(Upper threshold);    // Output-> WARN: Upper threshold 
+```
+
+##### Concatenation (or token-pasting) Operator (##)
+Using concatenation the parameters can be joined together to form one single token. Token-pasting is much more powerful in the sense that the resulting token could be an object defined in the C program. 
+```C
+#define NUM(x) number_##x
+
+void foo() {
+  int number_one = 10;
+  int number_two = 15;
+
+  printf("%d + %d = %d\n", NUM(one), NUM(two), NUM(one) + NUM(two));
+  // Output-> 10 + 15 = 25
+}
+```
+
 #### More Information:
-<!-- Please add any articles you think might be helpful to read before writing the article -->
-[GCC Online Documentation: Macros](https://gcc.gnu.org/onlinedocs/cpp/Macros.html)
-
-[GCC Online Documentation: Object-like macros](https://gcc.gnu.org/onlinedocs/cpp/Object-like-Macros.html#Object-like-Macros)
-
-[GCC Online Documentation: Function-like macros](https://gcc.gnu.org/onlinedocs/cpp/Function-like-Macros.html#Function-like-Macros)
+- [GCC Online Documentation: Macros](https://gcc.gnu.org/onlinedocs/cpp/Macros.html)
+- [GCC Online Documentation: Object-like macros](https://gcc.gnu.org/onlinedocs/cpp/Object-like-Macros.html#Object-like-Macros)
+- [GCC Online Documentation: Function-like macros](https://gcc.gnu.org/onlinedocs/cpp/Function-like-Macros.html#Function-like-Macros)