CODEWITHSUNDEEP
c++listclassobjectlistiterator
Jake Young
Jake Young

Reputation: 83

Iterating over a list of objects c++

Basically I am creating a basic shell program for a class project. I am stuck on iterating through the files/folders within a directory.

My error is in the function DisplayDirectoryContents()

class Directory 
{
private:
map<string, pair<list<Folder>, list<File> > > directoryContents;
string directoryPath;

public:
list<Folder> getFolders() {return directoryContents[directoryPath].first;}
list<File> getFiles() {return directoryContents[directoryPath].second;}
string getDirectoryPath() {return directoryPath;}

Directory() 
{
    directoryPath = "root/"; 
    File *file = new File("Test");
    directoryContents[directoryPath].second.push_back(*file);
}
void DisplayDirectoryContents()
{
    // Get files and folders from directory
    list<File> files = this->getFiles();

    for(int i = 0; i < files.size(); i++)
    {
        cout << files->getFileName(); << Error here
    }

}
};

I've tried setting this function up a few different ways but I can't seem to get the right kind of code to make it work.

I would think this would work but it's throwing me errors everytime I try and mess with the DisplayDirectoryContents() function. Does anyone have any tips that I can try to help fix this? Thanks

class File
{
string fileName;
string fileTime;
public:
string getFileTime(){return fileTime;}
string getFileName(){return fileName;}

void setFileTime(){time_t now = time(NULL);
                fileTime = ctime(&now);} 
void setFileName(string newFileName){fileName = newFileName;}
File(){}
File(string fName)
{
    fileName = fName;
    time_t now = time(NULL);
    fileTime = ctime(&now);
}
void MakeFile(string fName);
void RemoveFile(string fName); 
};

Upvotes: 1

Views: 4701

Answers (2)

Aconcagua
Aconcagua

Reputation: 25526

Extension to R Sahu's answer:

The reason you cannot iterate using your original loop is because std::list does not offer an index operator (you would need to use std::vector instead).

Alternative approach to the range based for loop (which I'd prefer, though, unless you have specific reasons not to do so) – or the way to go if you have no C++11 available – is using iterators:

for(std::list<File>::iterator i = files.begin(); i != files.end(); ++i)
{
    std::cout << i->getFileName();
}

With C++11, you can have for(auto i = files.begin(); ....

Use cases for using iterator loop even with C++11 can be comparing items with their successors:

// check for empty list first, as std::prev would fail on!)
for(auto i = files.begin(); i != std::prev(files.end()); ++i)
{
    if(someCondition(*i, *std::next(i))
    {
        // do something else
    }
}

There is already std::remove_if for and you should prefer it (together with a lambda; but don't forget to apply erase afterwards, see erase-remove-idiom!), so just for illustration; starting with the simpler approach:

for(auto i = files.begin(); i != files.end(); ) // no ++i (!)
{
    if(condition)
    {
        i = files.erase(i);
        // fine for std::list; with std::vector, if removing more
        // than one single element, we'd move or even copy subsequent
        // elements multiple times, which is quite inefficient
    }
}

Improved variant (which is what std::remove_if does as well):

auto pos = files.begin();
for(auto i = files.begin(); i != files.end(); ++i)
{
    if(!condition)
    // ^ (!)
    {
        *pos++ = std::move(*i);
    }
}
// here, std::remove_if already stops, but returns pos...
files.erase(pos, files.end());

A minimal problem might remain with move or copy assignment operators not handling self-assignment correctly, which would be covered by if(!condition && i != pos).

Upvotes: 2

R Sahu
R Sahu

Reputation: 206607

If you are able to use C++11 or higher, you can use a range for loop to iterate over the contents of the list.

void DisplayDirectoryContents()
{
    // Get files and folders from directory
    list<File> files = this->getFiles();

    // Iterate over each item in the list.
    // Use a reference to avoid copying of the items.
    for ( File& file : files )
    {
        cout << file.getFileName();
    }
}

Upvotes: 2

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