# typedef & function pointer
###### tags: `C basics`

### Reference
>致謝
Typedef function pointer?
https://stackoverflow.com/questions/4295432/typedef-function-pointer/4295495
C 語言 typedef 的用法
>https://magicjackting.pixnet.net/blog/post/65865174
Function pointer in C
>https://www.geeksforgeeks.org/function-pointer-in-c/
correct way to assign function pointer
https://stackoverflow.com/questions/15280749/correct-way-to-assign-function-pointer

typedef is a language construct that associates a name to a type.
You use it the same way you would use the original type, for instance

 typedef int myinteger;
 typedef char *mystring;
 typedef void (*myfunc)();
 
 myinteger i;   // is equivalent to    int i;
 mystring s;    // is the same as      char *s;
 myfunc f;      // compile equally as  void (*f)();

// Examples of typedef a function or pointer of function
typedef int    IamFunc (int, int);            // (1)
typedef int *  IamFunc (int, int);            // (2)
typedef int  (*IamFunc)(int, int);            // (3)

(1) 定義一個別名IamFunc，傳回值為整數且需要二個整數參數的函數
(2) 只是回傳值由int變為int*，因為IamFunc右邊的運算子(), 比在左邊的取值運算子*，優先權較高
(3) 多加了括號，因此*，會先執行，所以函數變成指向函數的指標

可以簡單分成紅藍黃三個區塊
紅色：function return value
藍色：name to be defined. 有*代表是指到這個函數型態的指標
黃色：function arguments

下面三種寫法都是等價的：

void printA(char *str)
{
    printf("in A: %s\n", str);
    return;
}

int main(int argc, char **argv)
{
    // METHOD 1
    void (*func_ptr1)(char*) = &printA;
    func_ptr1("How about you?");

    // METHOD 2
    myfunc_t* func_ptr2 = &printA;
    func_ptr2("How are you?");

    // METHOD 3
    myfuncptr_t func_ptr3 = &printA;
    func_ptr3("How old are you?");
    
    return EXIT_SUCCESS;
}


method 1有點難看，使用typedef會好看很多 ：>

assign to function pointer也可以寫成

myfunc_t* func_ptr2 = &printA;
myfunc_t* func_ptr2 = printA;

呼叫的方式也可以寫成

func_ptr2("How are you?");
(*func_ptr2)("How are you?");