reading a directory in C++ in portable way

I have a requirement as follows. I have a directory for eg i have /tmp/myFolder

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Under "myFolder" i have files whose names as below

myFile-N001.txt
myFile-N002.txt
myFile-N003.txt

in my application while writing to a file i should write to largest number file in /tmp/myFolder, for eg in above case i should write to myFile-N003.txt.

The above should be achieved in C++ and in portable way preferably in POSIX way.

Can any one provide simple example on how we can achive this in C++.

Thanks

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If you are happy enough to:

• work only on POSIX
• You know the pattern you are looking for

You can use the glob() function in POSIX to get the list.

You can then use std::max with a custom comparison on the result set with your iterator range being globres.gl_pathv[0], globres.gl_pathv[globres.gl_pathc] and a comparison being something like:

bool strless(const char* str1, const char* str2 )
{
   return strcmp( str1, str2 ) < 0;
}

although there is most likely something already that does that

The start of the "man" page for glob:

   #include <glob.h>

   int glob(const char *pattern, int flags,
            int (*errfunc) (const char *epath, int eerrno),
            glob_t *pglob);

   void globfree(glob_t *pglob);

DESCRIPTION

The glob() function searches for all the pathnames matching pattern according to the rules used by the shell (see glob(7)). No tilde expansion or parameter substitution is done; if you want these, use wordexp(3).

The globfree() function frees the dynamically allocated storage from an earlier call to glob().

The results of a glob() call are stored in the structure pointed to by pglob. This structure is of type glob_t (declared in <glob.h>) and includes the following elements defined by POSIX.2 (more may be present as an extension):

 typedef struct 
 {
     size_t   gl_pathc;    /* Count of paths matched so far  */
     char   **gl_pathv;    /* List of matched pathnames.  */
     size_t   gl_offs;     /* Slots to reserve in gl_pathv.  */
 } glob_t;

There was a discussion in October 2004 about bringing glob into boost I don't think it ever got implemented:

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There's code for retrieving the list of files from a directory in a POSIX-compliant way here.

You could build on that by sorting the resulting array and popping the last element off. That would be the file you want to find.

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This is really two problems in one. I'd use Boost.Filesystem to portably iterate over the files and store them in a vector with (untested)

std::string maxname;
unsigned maxnum = 0;

for (fs::path::iterator i(dir.begin()), end(dir.end()); i != end; ++i) {
    std::string name = i->string();
    unsigned num;
    if (std::sscanf(name.c_str(), "myFile-N%u.txt", &num) == 1
     && num >= maxnum) {
        maxnum = num;
        maxname = name;
    }
}

then check whether maxname holds the empty string after the loop. If it doesn't, it holds the filename you want.

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You can use boost::filesystem to achieve this.