C++ append one vector to another

Question

I fully understand this question has been asked a lot, but I'm asking for a specific variation and my search-foo has given up, as I've only found algorithms that append one existing vector to another, but not one returned to from a function.

I have this function that lists all files in a directory:

vector<string> scanDir( const string& dir )

which may call itself internally (for subdirectories).

I need a short way of appending the returned value to the caller's vector. I have in my mind something like this (but of course it doesn't exist :( ):

vector<string> fileList;
//...
fileList.append( scanDir(subdirname) );

I fear that storing the return value and inserting it in fileList would bring performance badness. What I mean is this:

vector<string> temp( scanDir(subdirname) );
copy( temp.begin(), temp.end(), back_inserter(fileList) );

Thanks!

PS: I'm not forcing myself to using vector, any other container that performs equally well and can prevent the potential large copy operation is fine by me.

Answer 1

Why not just pass the vector as an argument? Then every invocation can append to the same vector, without copying. Or create an implementation class which accumulates the elements into a member object.

Answer 2

If you're in the position to change scanDir, make it a (template) function accepting an output iterator:

template <class OutIt>
void scanDir(const std::string& dirname, OutIt it) {
  // ...
  // Scan subdir
  scanDir(subdir, it);
  // ...
}

You'll have the additional benefit to be able to fill all sort of data structures like

std::vector<string> vector;
scanDir(dir1, std::back_inserter(vector));
std::set<string> fileset
scanDir(dir1, std::inserter(fileset, fileset.begin()));

etc.

EDIT (see comment ...)

For using this function for class member initialization, you could either call it in the constructor as in

class MyClass {
private:
  std::vector<string> m_fileList;
public:
  MyClass(const std::string& dirname) {
    scanDir(dirname, std::back_inserter(m_fileList);
  }
}

or using a wrapper function

std::vector<string> scanDir(const std::string& dirname) {
  std::vector<string> result;
  scanDir(dirname, std::back_inserter(result);
  return result;
}

class MyClass {
// Same as above..
  MyClass(const std::string& dirname) : m_fileList(scanDir(dirname)) { }
}

I would prefer the first version for performance (and other) reasons ...