c++ read content of a pointer - c++

Question

i'm new in c++ world, i just use it for litle app that help me in my work, now, i need to read the content of a folder, list the folder content, i've made a function that return a pointer with the name of every obj in the folder, but now, i don't know how to read the content of the pointer to just print it in a console, my function look like this
string* listdir (const char *path)
{
string* result = new string[50]; // limit to 50 obj
DIR *pdir = NULL;
pdir = opendir (path);
struct dirent *pent = NULL;
if (pdir == NULL)
{
printf ("\nERROR! pdir could not be initialised correctly");
return NULL;
}
int i = 0;
while (pent = readdir (pdir))
{
if (pent == NULL)
{
printf ("\nERROR! pent could not be initialised correctly");
return NULL;
}
//printf ("%s\n", pent->d_name);
result[i++]= pent->d_name;
}
closedir (pdir);
return result;
}
i've been trying to print the result of teh function
int main()
{
string *dirs;
dirs = listdir("c:\\");
int i = 0;
//while(dirs[i])
//{
//cout<<dirs[i]<<'\n';
//++i;
//}
}
but i really don't know what i'm doing, lol, some help would be perfect
thanks

Answer 1

Examine your while loop condition : dirs[i] is a std::string. You are using a string object in a boolean context : would you expect std::string to convert to bool ?
My recommendation : ditch the fixed sized array and go for std::vector.
void listdir(const char *path, std::vector<std::string> &dirs)
{
/* ... */
while (pent = readdir (pdir))
{
/* ... */
dirs.push_back(pent->d-name);
}
closedir(pdir);
}
int main()
{
std::vector<std::string> dirs;
listdir("c:\\", dirs);
for (std::vector<std::string>::const_iterator it = dirs.begin(), end = dirs.end(); it != end; ++it)
std::cout << *it << std::endl;
}

Answer 2

int main()
{
string *dirs;
dirs = listdir("c:\\");
for (int i = 0; i < 50 && dirs[i].size() > 0; ++i)
{
cout << dirs[i] << '\n';
}
}
dirs is a pointer to an array, so you can index it like an array. You created an array of 50, so you need to limit yourself to 50 here too. Since you might not have populated the whole array, the .size() check allows the printing loop to stop early.

Answer 3

There is some major confusion in your code, especially between arrays of characters, strings and arrays of strings. Also, there is a memory leak.
Here are my questions / concerns:
Issues / Concerns
The opendir function may be called
with a null parameter. You should
check for a null path before calling
opendir.
Returns NULL after declaring some
variables. IMHO, one should check
parameters before declaring
variables.
How does one know how many valid
entries are in the returned array?
If the size of the array (known only
to the listdir function) changes,
the users of the function are
doomed.
Is the type of pent->d_name the
same as string *?
The address of the directory name,
pent->d_name, is copied into the
results array, but not the content
of the directory name. The OS may
reuse this location without telling
you; so copying the address of the
text is not a good idea.
The main function does not delete
the memory allocated for the
results. This is known as a memory
leak.
Suggestions / Fixes
Use std::string within the
function. This takes care of
allocating memory for text.
Use std::vector<string> for the
results. This takes care of knowing
the quantity of directories and no
need to dynamically allocate or
deallocate memory.
Create a std::string from the
pent->d_name and use push_back
to append the string to the results.

Answer 4

In C++, dereferencing a pointer is achieved using the * operator, just like in 'C'.
However, there are a number of problems with your code, which I have addressed here because I was bored...
#include <string>
#include <iostream>
#include <list>
#include <dirent.h>
typedef std::list<std::string> dir_list;
bool listdir(const std::string& path, dir_list& result)
{
bool retval = true;
DIR* pdir = opendir(path.c_str());
if (pdir == NULL)
{
std::cerr << "ERROR! pdir could not be initialised correctly" << std::endl;;
retval = false;
}
else
{
for (dirent* pent = readdir(pdir); pent != NULL; pent = readdir(pdir))
{
if (pent == NULL && result.empty())
{
std::cerr << "ERROR! pent could not be initialised correctly" << std::endl;
retval = false;
}
if (result.size() < 50)
{// *really* limit to 50!
result.push_back(pent->d_name);
}
}
closedir(pdir);
}
return retval;
}
int main()
{
dir_list dirs;
if (listdir("C:/", dirs))
{
for (dir_list::const_iterator iter(dirs.begin()), end(dirs.end()); iter != end; ++iter)
{
std::cout << *iter << std::endl;
}
}
}
Since you're using C++, STL, its string and container classes will save you a World of pointer pain!

Answer 5

your while statement looks strange, it looks like it's expecting dirs[i] to be a pointer, but I believe dirs[i] will be a non-pointer type. Maybe change it to (assuming string is std::string):
while(i < 50 && dirs[i].length() > 0)
{
cout<<dirs[i]<<'\n';
++i;
}