C / C++ / MFC

6-Oct-11 21:17

hello Friends

What is Best way to store Array of Dynamic Objects

I am creating a pointer for class A and Initializing 100 objects

And then again I am Initializing 200 objects.

What is Best way to store these arrays.
i used map like map<int,a*> But problem is coming that deleting array pointer is deleting from map also.

My Design Structure is like this

C++

int size =2;
for(int i=0; i<size;i++)
{
   A* objA = new A[100]
   for(int j = 0 ; j<100;j++)
   {
      objA[j].var = TempVar; 
   }
   mapA.insert(make_pair(i,objA));
   
}


But,i am not happy with this way to store array of objects in map .As I am using map as a storage that I can use anywhere in application,and bcoz of not deleting pointer it crashes sometimes.

Suggest me some better way.
Thanks & Regards
Yogesh

Richard MacCutchan6-Oct-11 22:34

Richard MacCutchan

6-Oct-11 22:34

yogeshs wrote:
and bcoz of not deleting pointer it crashes sometimes.

The first thing you need to do is find out why your program crashes and fix that problem. I have explained why you cannot delete the objects in your map and the same will hold true if you put them in a fixed array.

Unrequited desire is character building. OriginalGriff

_AnsHUMAN_ 6-Oct-11 22:47

_AnsHUMAN_

6-Oct-11 22:47

Can you use a vector object?
vector class reference[^]

You talk about Being HUMAN. I have it in my name
AnsHUMAN

002comp6-Oct-11 22:56

6-Oct-11 22:56

Can you use a vector object?
vector class reference[^]

Vector is sequence container.i need some key,data Container.
Linked list is also another option but i dont want to involve in that.

Any Other Ideas?
regards
Yogesh

Stefan_Lang6-Oct-11 23:42

Stefan_Lang

6-Oct-11 23:42

I think what AnsHUMAN was implying is that the class std::vector would take care of the allocation and deallocation - it encapsulates the array object and thereby takes away the responsibility from you. You can achieve the same however by defining your own encapsulation: see my response for details.

Stefan_Lang6-Oct-11 23:06

Stefan_Lang

6-Oct-11 23:06

yogeshs wrote:
bcoz of not deleting pointer it crashes sometimes

Can you clarify that statement, please? Normally, the only abnormal behaviour you can get from not deleting stuff is memory leaks, and, as a result exceptions of the type std::bad_alloc, once you run out of memory. The former will not crash your program though, so may I assume it is the latter? if so, the first thing to do would be to catch that exception.

Apart from that, there's a couple of things you could improve:

1. Storing a map that uses an consecutive integer value as a key indicates that a simple array would do the job just as well, and probably better. The only exception would be if you need to be able to insert values at the start or in the midst of your set later, and you stored key values to specific objects that you want to be able to retrieve later. You haven't mentioned such a requirement, so my advice is to use a C-style array instead. Or, if you need to be able to later add or remove entries, use a std::vector.

2. The objA variable you use to create each entry does contain only one thing, but that thing is being allocated dynamically, on the heap. As you need to make sure this memory gets properly freed again when you no longer need this object, the best way is to make a simple class to hold that variable, and define it's constructor and destructor to automatically allocate and deallocate the memory, as appropriate. That way you no longer need to take care of that anywhere else in your code. See below for an example.

C++

#include <iostream> // for std::cerr
#include <new> // for the definition of std::bad_alloc
class MyData {
private:
   A* myobj;
public:
   MyData(int size = 100) {
      try { // make sure to catch bad_alloc!
         myobj = new A[size];
      }
      catch (std::bad__alloc& ba) {
         myobj = 0; // now at least this MyData object is properly initialized
         throw ba; // rethrow - your application should catch this and deal with it appropriately!
      }
   }
   ~MyData() { //destructor
      if (myobj != 0) { // just added for clarity, this is actually redundant
         delete [] myobj; // be sure to use delete [], not just delete
      }
   }
};

int main() {
   int error = 0;
   MyData *arrayData = 0;
   try { // make sure to catch exceptions!
      arrayData = new MyData[200]; // this will call the constructor MyData::MyData() for each element
   }
   catch (std::bad_alloc& ba) {
      std::cerr << "Error - bad alloc: " << ba.what() << std::endl;
      error = 1;
   }
   if (arrayData != 0) {
      // do something
      // ...
      // now clean up
      delete [] arrayData; // this will also call MyData::~MyData() for all 200 elements!
      arrayData = 0;
   }
   return error;
}

P.S.: As I've seen from a more recent response the first point I've raised may not apply to your problem, i. e. you may need a map after all. My point about creating a class to wrap that 100-A array still holds though.

Moreover, you've used a loop to initialize all your A objects. This is another thing you could improve: use your constructor A::A() to do that! If you didn't define one already, do so now:

C++

class A {
   //...
   A() {
      var = TempVar; // or whatever
   }
   //...
};

002comp7-Oct-11 2:29

7-Oct-11 2:29

Thanks A lot,I will try as u suggest.

Regards
Yogesh

002comp10-Oct-11 18:55

10-Oct-11 18:55

Hello
I want to ask one more Question regarding "Moreover, you've used a loop to initialize all your A objects. This is another thing you could improve: use your constructor A::A() to do that!"

I did this in the loop bcoz I am retreiving the size only in that loop.That object array size is not constant. that can also change for each loop.So,thats why i Initialized in the loop inspite of doing in constructor.

Is there any other idea to do this?
Regards
Yogesh

Stefan_Lang10-Oct-11 23:48

Stefan_Lang

10-Oct-11 23:48

I suppose in that case, using a loop will be the most obvious solution. You should still define a constructor for A, but instead of allocating the array, just initialize the array pointer to 0, so you can be sure that pointer is at least valid.

You could still avoid the loop and do the allocation in your constructor, but that will take a bit more effort:

C++

#include <iostream> // for std::cerr
#include <new> // for the definition of std::bad_alloc

// declare an allocator interface
class MyAllocatorHelper {
public:
   virtual void* allocate() const = 0;
}

// declare a global accessor to the allocator interface
class MyArrayAllocator {
public:
   static MyAllocatorHelper const* p;
   static void* makeArray() {
      return p ? p->allocate() : 0;
   }
}

// data array wrapper class
class MyData {
private:
   A* myobj;
public:
   MyData();
   ~MyData() { //destructor
      if (myobj != 0) { // just added for clarity, this is actually redundant
         delete [] myobj; // be sure to use delete [], not just delete
      }
   }
};

// implement the MyData constructor:
MyData::MyData() {
   try { // make sure to catch bad_alloc!
      myobj = MyArrayAllocator::makeArray();
   }
   catch (std::bad__alloc& ba) {
      myobj = 0; // now at least this MyData object is properly initialized
      throw ba; // rethrow - your application should catch this and deal with it appropriately!
   }
}

// declare an actual implementation for that allocator interface
class MyDataArray : public MyAllocatorHelper {
private:
   MyData* the_array;
   std::size_t current_index;
   void* allocate() const; // MyDataArray implements the allocator interface!
public:
   MyDataArray(std::size_t size);
   MyData* get(std::size_t index);
};

// and now the implementation
MyDataArray::MyDataArray(std::size_t size) : the_array(0), current_index(0) {
   try { // make sure to catch exceptions!
      the_array = new MyData[size];
   }
   catch (std::bad_alloc& ba) {
      if (the_array != 0) {
         // error occurred during element allocation
         // the array elelmnts aren't fully usable
         // - you could check the current_index to see at what point the error occurred
         // anyway, clean up to make sure nothing bad happens:
         delete[] the_array;
         the_array = 0;
      }
      throw(ba); // rethrow to main()
   }
}

void* MyDataArray::allocate() const {
   A* result = 0;
   std::size_t alloc_size = 0;
   // now determine the actual size for the array with index current_index
   // ...
   // increment index counter
   ++current_index;
   // allocate array
   try {
      result = new A[alloc_size];
   }
   catch (std::bad_alloc& ba) {
      throw(ba);
   }
   return result;
}

int main() {
   int error = 0;
   MyDataArray* arrayData = 0;
   try { // make sure to catch exceptions!
      arrayData = new MyDataArray(200);
   }
   catch (std::bad_alloc& ba) {
      std::cerr << "Error - bad alloc: " << ba.what() << std::endl;
      error = 1;
   }
   if (arrayData != 0) {
      // do something
      // ...
      // now clean up
      delete arrayData;
      arrayData = 0;
   }
   return error;
}

I wrote this a bit in a hurry and didn't take the time to test it, there may even be typos. However, I hope even if this doesn't compile it might give you an idea of what I did here:

1. An abstract interface for allocating some unspecified data, which later gets implemented by the wrapper to your data array.
2. A 'static class' that your constructors can access without having to rely on parameters.
3. A local counter variable 'current_index' that keeps track of the number of allocations that have already taken place, implying the index of the data object within your array. You can use this counter to determine the correct size for that particular object in your array.

Function "WriteAllText"

i52camam6-Oct-11 8:52

i52camam

6-Oct-11 8:52

Hi!! I'd like to know if function WriteAllText is avalaible in a MFC project.

If not, ¿which other options are to save a text file? So that you can't use:

escritura << "Example";

Thanks!!

TheGreatAndPowerfulOz6-Oct-11 8:59

TheGreatAndPowerfulOz

6-Oct-11 8:59

I'd like to help, but I'm not sure what you're asking. What is "WriteAllText"?

If your actions inspire others to dream more, learn more, do more and become more, you are a leader." - John Quincy Adams
You must accept one of two basic premises: Either we are alone in the universe, or we are not alone in the universe. And either way, the implications are staggering” - Wernher von Braun

i52camam6-Oct-11 9:06

i52camam

6-Oct-11 9:06

Okay, I'm sorry. WriteAllText is a function of the namespace System.IO:

http://msdn.microsoft.com/es-es/library/ms143375%28v=VS.80%29.aspx[^]

I want to save text of an EditControl to a text file.

Thanks!!

Rajesh R Subramanian6-Oct-11 9:11

Rajesh R Subramanian

6-Oct-11 9:11

i52camam wrote:
Okay, I'm sorry. WriteAllText is a function of the namespace System.IO:

And System.IO is part of the .NET framework, but MFC uses only native code.

If you need to work with files, MFC has the CFile[^] class.

"Real men drive manual transmission" - Rajesh.

TheGreatAndPowerfulOz6-Oct-11 9:14

TheGreatAndPowerfulOz

6-Oct-11 9:14

yes, there is such a thing.

the << operator can be used for such.

or you can use the "standard c" library. There are many ways to accomplish what is wanted.

I'll give you the C++ way.

#include "stdafx.h"
#include <fstream>

using namespace std;

int main() {
    fstream file;
    file.open("filename.txt");
    string foo = "here's some text";
    file << foo;
    file.close();
    return 0;
}

See fstream[^] docs.

If you want to use "MFC" there is CFile[^]

i52camam8-Oct-11 10:07

i52camam

8-Oct-11 10:07

Hi,

I've been using the CFile class unsuccessfully, cause the file doesn't get created. Here is my code:

void Guardar::OnBnClickedGuardar()
{
    SaveToFile();
}

void Guardar::SaveToFile()
{
	CString nombre=_T("\D:\Write_File.dat");
	CFile cfile;
	cfile.Open(nombre , CFile::modeCreate | CFile::modeReadWrite);
	char pbufWrite[100];
	memset(pbufWrite, 'a', sizeof(pbufWrite));
	cfile.Write(pbufWrite, 100);         
	cfile.Flush();
}

Also, I have debugged the program and all the functions are called step by step.... so I don't know where is the problem Dead | X|

Thanks for all.