116 lines
4.2 KiB
Markdown
116 lines
4.2 KiB
Markdown
---
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title: Macros in C
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---
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## Macros in C
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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.
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#### Why Macros?
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The C compiler will go through your code and replace every occurrence of a macro with it's value. This begs the question, what is the point of using macros? The answer: macros are a tool for the programmer, not for the program.
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If you have the number `365` hard-coded into your program, it becomes difficult for you and other people who may look at your code to understand what it means. If you `#define DAYSINYEAR 365` and use that instead of the number, the code makes a lot more sense.
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It's also beneficial if you have many instances of the same thing. If you'd used `2018` as the year in multiple places in your program, and then the year changed to `2019`, you would have to go and find every single line containing `2018` and change it, hoping you didn't miss any. With the macro `#define YEAR 2018`, you can simply change the value of the macro and be confident all of the values have been updated accordingly.
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#### Defining macros
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The `#define` keyword is used to define new macros. It's followed by a name and a content, but no equals sign. By convention, macro names are written in uppercase.
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```C
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#define PI 3.14
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```
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If you use the macro this way:
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```C
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printf("Value of PI: %d", PI);
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```
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Is the same as write this:
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```C
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printf("Value of PI: %d", 3.14);
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```
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#### Undefining Macros
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After defining macros you can also undefine them at any point.
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just Type
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```C
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#undefine PI
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```
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This is used to use macros only for specific lines of code and again undefine it.
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#### Function-like Macros
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Function-like uses the same `#define` keyword. The difference is that you use a pair of parentheses after the function name.
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```C
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#define hello_world() printf("Hello World!")
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```
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So calling:
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```C
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hello_world();
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```
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You get:
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```C
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printf("Hello World!");
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```
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You can set parameters too:
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```C
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#define hello(X) printf("Hello " X "!")
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```
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Now calling:
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```C
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hello("World");
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```
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You get the equivalent of:
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```C
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printf("Hello World!");
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```
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An example to explain how code is replaced by macros
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```c
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#define add(a,b) a+b
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```
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Now calling :
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```c
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int x = 2*add(3,5)*4;
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```
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Here, the value of x will be 26 because the expression will become 2*3+5*4 which evaluates to give 26. Sometimes we think that the macro will give us 3+5 i.e. 8 and the expression will become 2*8*4 but this is not the case. This shows how the code is replaced exactly.
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#### Special Operators in Macros
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One can use the special operators # (stringize) and ## (concatenate) in macros for achieving unique functionality.
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##### Stringizing Operator (#)
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A macro's parameter preceded by a `#` is converted and treated as a string token.
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For example, we can define ERROR and WARN macros that print a LOG message.
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While the LOG message gets prefixed with either an `ERR` or a `WARN`, respectively.
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```C
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#define LOG(level, message) printf(#level ": " #message "\n")
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#define ERROR(msg) LOG(FAIL, msg)
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#define WARN(msg) LOG(WARN, msg)
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```
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Now, one can use it as
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```C
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ERROR(Invalid settings); // Output-> FAIL: Invalid settings
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WARN(Upper threshold); // Output-> WARN: Upper threshold
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```
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##### Concatenation (or token-pasting) Operator (##)
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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.
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```C
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#define NUM(x) number_##x
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void foo() {
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int number_one = 10;
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int number_two = 15;
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printf("%d + %d = %d\n", NUM(one), NUM(two), NUM(one) + NUM(two));
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// Output-> 10 + 15 = 25
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}
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```
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#### More Information:
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- [GCC Online Documentation: Macros](https://gcc.gnu.org/onlinedocs/cpp/Macros.html)
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- [GCC Online Documentation: Object-like macros](https://gcc.gnu.org/onlinedocs/cpp/Object-like-Macros.html#Object-like-Macros)
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- [GCC Online Documentation: Function-like macros](https://gcc.gnu.org/onlinedocs/cpp/Function-like-Macros.html#Function-like-Macros)
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