typedef用于函数定义的一个例子

typedef用于定义基本类型或者结构体，很简单

再看看怎么定义函数：

不使用typedef的：

#include <iostream>
using namespace std;

double besty(int);
double pam(int);
void estimate(int, double (*pf)( int ));

int main(void)
{
int code;
cout<<"How many lines of code do you need ?"<<endl;
cin >>code;
cout<<"Here is the besty's estimate!"<<endl;
estimate(code,besty);
cout<<"Here is the pam's estimate!"<<endl;
estimate(code,pam);
return 0;
}

double besty(int lns)
{
return 0.5*lns;
}

double pam(int lns)
{
return 0.03*lns+0.0004*lns*lns;
}

void estimate(int lines,double (*pf)(int))
{
cout<<lines<<"line will take!"<<endl;
cout<<(*pf)(lines)<<"hours"<<endl;
}

使用typedef的：

#include "stdafx.h"
#include <iostream>
using namespace std;

typedef double MM(int);

MM besty;
MM pam;
void estimate(int , MM *pf);

int main(void)
{
int code;
cout<<"How many lines of code do you need ?"<<endl;
cin >>code;
cout<<"Here is the besty's estimate!"<<endl;
estimate(code,besty);
cout<<"Here is the pam's estimate!"<<endl;
estimate(code,pam);
return 0;
}

double besty(int lns)
{
return 0.5*lns;
}

double pam(int lns)
{
return 0.03*lns+0.0004*lns*lns;
}

void estimate(int lines,MM *pf)
{
cout<<lines<<"line will take!"<<endl;
cout<<pf(lines)<<"hours"<<endl;
cout<<(*pf)(lines)<<"hours"<<endl;//使用pf()和(*pf)()效果是一样的，在引用函数的时候，这两者表示一个意思。
}

再estimate函数里面要把参数里面的函数定义成double (*pf)(int)也就是指向函数的指针。

因为只有这种函数才可以被其他的函数（同返同参）赋地址。

比如

double (*pf)(int)；

double dd(int)；

double cc(int)；

pf＝dd；是可以的可以被赋地址。

cc＝dd；不可以。

同样在使用的时候上面的程序里面也带了说明，使用pf()和(*pf)()效果是一样的，在引用函数的时候，这两者表示一个意思。

就这个认识的也是有两个流派的，不过，现在两种都认为是对的，兼而容之！

今天修改一下，今天看到一个很不错的程序，linux下面的对目录进行访问的一个程序，里面typedef定义了的部分就很多。看看一下，typedefintMyfunc(const char *, const struct stat *, int);如果再使用Myfunc定义一个变量，那么其实就是定义了一个函数，表现的只是一个地址形式而已，这个函数具有Myfunc函数的所有参数以及返回类型！

看程序吧：

#include "apue.h"
#include <dirent.h>
#include <limits.h>

/* function type that is called for each filename */
typedefintMyfunc(const char *, const struct stat *, int);

static Myfuncmyfunc;
static intmyftw(char *, Myfunc *);
static intdopath(Myfunc *);

static longnreg, ndir, nblk, nchr, nfifo, nslink, nsock, ntot;

int
main(int argc, char *argv[])
{
intret;

if (argc != 2)
err_quit("usage: ftw <starting-pathname>");

ret = myftw(argv[1], myfunc);/* does it all */

ntot = nreg + ndir + nblk + nchr + nfifo + nslink + nsock;
if (ntot == 0)
ntot = 1;/* avoid divide by 0; print 0 for all counts */
printf("regular files = %7ld, %5.2f %%\n", nreg,
nreg*100.0/ntot);
printf("directories = %7ld, %5.2f %%\n", ndir,
ndir*100.0/ntot);
printf("block special = %7ld, %5.2f %%\n", nblk,
nblk*100.0/ntot);
printf("char special = %7ld, %5.2f %%\n", nchr,
nchr*100.0/ntot);
printf("FIFOs = %7ld, %5.2f %%\n", nfifo,
nfifo*100.0/ntot);
printf("symbolic links = %7ld, %5.2f %%\n", nslink,
nslink*100.0/ntot);
printf("sockets = %7ld, %5.2f %%\n", nsock,
nsock*100.0/ntot);

exit(ret);
}

/*
* Descend through the hierarchy, starting at "pathname".
* The caller's func() is called for every file.
*/
#defineFTW_F1/* file other than directory */
#defineFTW_D2/* directory */
#defineFTW_DNR3/* directory that can't be read */
#defineFTW_NS4/* file that we can't stat */

static char*fullpath;/* contains full pathname for every file */

static int/* we return whatever func() returns */
myftw(char *pathname, Myfunc *func)
{
int len;
fullpath = path_alloc(&len);/* malloc's for PATH_MAX+1 bytes */
/* ({Prog pathalloc}) */
strncpy(fullpath, pathname, len);/* protect against */
fullpath[len-1] = 0;/* buffer overrun */

return(dopath(func));
}

/*
* Descend through the hierarchy, starting at "fullpath".
* If "fullpath" is anything other than a directory, we lstat() it,
* call func(), and return. For a directory, we call ourself
* recursively for each name in the directory.
*/
static int/* we return whatever func() returns */
dopath(Myfunc* func)
{
struct statstatbuf;
struct dirent*dirp;
DIR*dp;
intret;
char*ptr;

if (lstat(fullpath, &statbuf) < 0)/* stat error */
return(func(fullpath, &statbuf, FTW_NS));
if (S_ISDIR(statbuf.st_mode) == 0)/* not a directory */
return(func(fullpath, &statbuf, FTW_F));

/*
* It's a directory. First call func() for the directory,
* then process each filename in the directory.
*/
if ((ret = func(fullpath, &statbuf, FTW_D)) != 0)
return(ret);

ptr = fullpath + strlen(fullpath);/* point to end of fullpath */
*ptr++ = '/';
*ptr = 0;

if ((dp = opendir(fullpath)) == NULL)/* can't read directory */
return(func(fullpath, &statbuf, FTW_DNR));

while ((dirp = readdir(dp)) != NULL) {
if (strcmp(dirp->d_name, ".") == 0 ||
strcmp(dirp->d_name, "..") == 0)
continue;/* ignore dot and dot-dot */

strcpy(ptr, dirp->d_name);/* append name after slash */

if ((ret = dopath(func)) != 0)/* recursive */
break;/* time to leave */
}
ptr[-1] = 0;/* erase everything from slash onwards */

if (closedir(dp) < 0)
err_ret("can't close directory %s", fullpath);

return(ret);
}

static int
myfunc(const char *pathname, const struct stat *statptr, int type)
{
switch (type) {
case FTW_F:
switch (statptr->st_mode & S_IFMT) {
case S_IFREG:nreg++;break;
case S_IFBLK:nblk++;break;
case S_IFCHR:nchr++;break;
case S_IFIFO:nfifo++;break;
case S_IFLNK:nslink++;break;
case S_IFSOCK:nsock++;break;
case S_IFDIR:
err_dump("for S_IFDIR for %s", pathname);
/* directories should have type = FTW_D */
}
break;

case FTW_D:
ndir++;
break;

case FTW_DNR:
err_ret("can't read directory %s", pathname);
break;

case FTW_NS:
err_ret("stat error for %s", pathname);
break;

default:
err_dump("unknown type %d for pathname %s", type, pathname);
}

return(0);
}