9cea2d56a5
One small fix and the explanation of Stringizing and Token Pasting Operators that can be used effectively in C macros.
97 lines
2.7 KiB
Markdown
97 lines
2.7 KiB
Markdown
---
|
|
title: Macros in C
|
|
---
|
|
## Macros in C
|
|
|
|
A macro is a piece of code with a given name. When the name is used, it is replaced by the content of the macro. The `#define` keyword is used to define new macros. It's followed by a name and a content. By convention, macro names are written in uppercase. There are two type of macros: `Object-like` macros and `Function-like` macros.
|
|
|
|
#### Object-like Macros
|
|
```C
|
|
#define PI 3.14
|
|
```
|
|
|
|
If you use the macro this way:
|
|
```C
|
|
printf("Value of PI: %d", PI);
|
|
```
|
|
Is the same as write this:
|
|
```C
|
|
printf("Value of PI: %d", 3.14);
|
|
```
|
|
#### Undefining Macros
|
|
After defining macros you can also undefine them at any point.
|
|
just Type
|
|
```C
|
|
#undefine PI
|
|
```
|
|
This is used to use macros only for specific lines of code and again undefine it.
|
|
|
|
#### Function-like Macros
|
|
Function-like uses the same `#define` keyword. The difference is that you use a pair of parentheses after the function name.
|
|
```C
|
|
#define hello_world() printf("Hello World!")
|
|
```
|
|
|
|
So calling:
|
|
```C
|
|
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:
|
|
- [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)
|