# c prototype

### 重點
在c語言當中
declaration千萬不要使用f() //not define prototype
compiler並不知道這個函數的參數是什麼
因此參數可以自己在definition的時候才寫,
但這樣很危險
compiler會預設把int, short, char 轉成int, float跟double轉成double
如果用declaration的參數用錯, ex:char用成double接,會是undefined behaviour
建議還是不要這樣用,太危險
建議還是要用f(void) //define parameter is void
- c++ 預設 f() 等價於 f(void),即此函式不帶參數, 故避免了危險的發生
下面是幾個例子

#include <stdio.h>

//no declare prototype is very dangerous
int f();
int f1();
int f2();
//int f(double ,double , double);
//int f1(int, int, int);
//int f2(int,int , double);




int main(){
	int a = 5;
	char k = 'a';

	float t = 10.0f;
	//By default, int,char,float will change to int
	//and float, double will change to double
	f(a,k,t);
	f1(a,k,t);
	f2(a,k,t);
	
}

//correct version
int f2(int a, int b, double c){
		printf("---------------------f2 correct function\n");
		printf("%d\n",a);
		printf("%d\n",b);
		printf("%lf\n",c);
		printf("%lf\n",a+b+c);

		return a+b+c;
}

//int a, int b is correct type, but int c is wrong
//therefore, a and b has been set correct value, but the value of c is undefined behaviour
//i am not sure that the value of a,b will always be correct value....
int f1(int a, int b, int c){
		printf("---------------------f1 function\n");
		printf("%d\n",a);
		printf("%d\n",b);
		printf("%d\n",c);
		printf("%lf\n",a+b+c);

		return a+b+c;
}

//double a, double b is wrong type, but c is wrong
//however, all three value is undefined behaviour
int f(double a, double b, double c){
		printf("%lf\n",a);
		printf("%x\n", a);
		printf("%lf\n",b);
		printf("%x\n", b);
		printf("%lf\n",c);
		printf("%lf\n",a+b+c);

		return a+b+c;
}

output=

#include <stdio.h>

//int f();
//int f1();
//int f2();
int f(double ,double , double);
int f1(int, int, int);
int f2(int,int , double);




int main(){
	int a = 7;
	char k = 'b';

	float t = 11.0f;
	f((double)a,(double)k,(double)t);
	f(a,k,t);
	f1(a,k,t);
	f2(a,k,t);
	
}

int f2(int a, int b, double c){
		printf("---------------------f2 correct function\n");
		printf("%d\n",a);
		printf("%d\n",b);
		printf("%lf\n",c);
		printf("%lf\n",a+b+c);

		return a+b+c;
}

int f1(int a, int b, int c){
		printf("---------------------f1 function\n");
		printf("%d\n",a);
		printf("%d\n",b);
		printf("%d\n",c);
		printf("%lf\n",a+b+c);

		return a+b+c;
}


int f(double a, double b, double c){
		printf("---------------------f function\n");
		printf("%lf\n",a);
		printf("%x\n", a);
		printf("%lf\n",b);
		printf("%x\n", b);
		printf("%lf\n",c);
		printf("%lf\n",a+b+c);

		return a+b+c;
}

output=

###### tags: `C` `prototype`