Implementation of C language default parameter values

 

The C language does not have the concept of parameter default values. You can use macros to simulate parameter default values:

(For a function with multiple parameters, you need to write each parameter in the parameter list during calling, but you can put the default parameter before the non-default parameter)

 

The following code cannot be passed in c:

Define fun ():

Int fun (int a, int B = 10) ... ...

{ .... ... ... ....

Return a + B;

} .. ..

Main function code: .... ....

Int main () ......

{

Printf (% d, fun (10 ));

Return 0;

}

........................................ ......................

An error occurs when fun (10) is called during compilation. Note that this code is correct in C ++.

........................................ ......................

Code after macro improvement:

# Include
# Define funi (a) fun (a, 10)
Int fun (int a, int B)
{
Return a + B;

}
Int main ()
{
Int a = 10;
Printf (% d% D, fun (a, 10), funi ());
Return 0;

}

Result: 20

 

[Cpp]View plaincopy

1. # Include
2.  
3. # Define DEFARG (name, defval) (# name [0])? (Name + 0): defval)
4.  
5. Int _ f1 (int I)
6. {
7. Return 2 * I;
8. }
9. # Define f1 (arg0) _ f1 (DEFARG (arg0, 0 ))
10.  
11. Int _ f2 (int I, int j)
12. {
13. Return I + j;
14. }
15. # Define f2 (arg0, arg1) _ f2 (DEFARG (arg0, 0), DEFARG (arg1, 1 ))
16.  
17. Int main ()
18. {
19. Printf (% d, f1 ());
20. Printf (% d, f1 (1 ));
21.  
22. Printf (% d, f2 (,));
23. Printf (% d, f2 (2 ,));
24. Printf (% d, f2 (, 3 ));
25. Printf (% d, f2 (4, 5 ));
26. Return 0;
27. }
    

    # Include
       
        
    # Define DEFAULT 40/* DEFAULT parameter value */# define FUN (A) fun (# ##-) /* macro used to implement default parameters */int f (int n)/* function used to experiment with default parameters */{return printf (% d, n );} int fun (const char * a)/* determines the function called by the function. The return value type must be the same as that of the f () function to be called */{int n; /* the type of the variable must be the same as that of the f () function parameter */if (a [0] = '-') n = DEFAULT; else sscanf (, % d, & n); return f (n);} int main (void) {FUN (); FUN (67); return 0 ;}
       

    
    Ps: if there is a header file:GetStr. h# Define getStr () _ getStr (TestFun.txt); // set default filename in. c
    Void _ getStr (char filename []);
    GetStr. cVoid _ getStr (char filename []) {
    ...}Main. c# Include getStr. h void main () {getStr ();}
    
    
    Ref:
    Http://blog.csdn.net/broook/article/details/7208408 http://nonoob.is-programmer.com/posts/36769.html http://www.myexception.cn/c/232391.html
    Http://wenku.baidu.com/view/1ffed5d86f1aff00bed51eea.html http://blog.csdn.net/broook/article/details/7208408
    From http://blog.csdn.net/pipisorry/article/details/25437893