[RESOLVED] allocating and deallocating

[Aashi]

hi all,

my following program is not working, can anyone please tell me what's wrong with that?

class Example   
{
private:
       char *name;
public:
      Example()
      {
           name = new char[20];
      }
      ~Example()
      {
           delete[] name;
      }
     
      Example(const Example &b)
      {
           name = new char[20];
           strcpy(name, b.name);
      }
	  void setName(char* n)
	  {
				name=n;
	  }
      char* getName()
	  {
			   return name;
	  }
};
int main()
{

Example b;
	char* n=NULL;
	cin>>n;
	b.setName(n);
	cout<<b.getName();
return 0;
}

[D_Drmmr]

How is it not working? Does it not compile? Does it crash? Does it not produce the output you expected?
Also, did you step through your program using the debugger?

[S_M_A]

char* n=NULL;
cin>>n;

What did you expect to happen here? Since you use C++ why not use a std::string instead?

[Aashi]

How is it not working? Does it not compile? Does it crash? Does it not produce the output you expected?
Also, did you step through your program using the debugger?

it complies, takes input and then program crashes.

What did you expect to happen here? Since you use C++ why not use a std::string instead?

I know, but i want to use this for learning.

please tell how can i make my program working? it crashes after taking input.

[S_M_A]

Where is cin supposed to store the input when you make n a NULL-pointer?

[Paul McKenzie]

it complies, takes input and then program crashes.

I know, but i want to use this for learning.

Part of learning how to program is to know how to debug your own code. Just writing code and then dumping it here waiting for us to debug it is not the way you learn how to write a program. You must make the effort to debug your own code. You wrote it, you must be able to debug it.

please tell how can i make my program working? it crashes after taking input.

Can you at the very least identify which line causes the crash? Once you figure out which line it crashes on, look at that line and learn what could have happened. Use the debugger that comes with your compiler to step through the program.

Regards,

Paul McKenzie

[noxiousg]

Modifying these two lines....

	char* n=NULL;
	cin>>n;

like so...

	char n[256] = { '\0' };//Declares n to be a character array of length 256 and initializes all elements to terminating NULL character
	cin.getline(n, 255);//Reads user input, ensuring that room remains for the terminating null character

... should get the program to run for you.

The problem here is that you've declared n as a pointer to an array of characters but you never initialize it. If you do not declare the size of a character array at compile time, then you must either point it to some character array or dynamically allocate a new character array at run time before you can use it in any meaningful way. See this link for an excellent tutorial on pointers (the next chapter will talk about character arrays specifically), and the one after that, about dynamic memory allocation). You should come away from it understanding why what you're trying to do here doesn't work.

[Aashi]

Part of learning how to program is to know how to debug your own code. Just writing code and then dumping it here waiting for us to debug it is not the way you learn how to write a program. You must make the effort to debug your own code. You wrote it, you must be able to debug it.
Can you at the very least identify which line causes the crash? Once you figure out which line it crashes on, look at that line and learn what could have happened. Use the debugger that comes with your compiler to step through the program.

Regards,

Paul McKenzie

alright then, can you please tell me about how can i get input from user in a main using setter function?
i'm not asking for code, here's my code with constructor and destructor where i'm dynamic allocating a char type indentifier. would be appreciated if you help me.

#include <iostream>
using namespace std;

class Example
{
private:
	char* str;
	int len;
	static int num_strings;
public:
	Example(const char* s);
	Example();
	~Example();
	void setName(char* n);
	char* getName();
	
};
int Example::num_strings=0;

Example::Example(const char* s)
{
	len=strlen(s);
	str=new char[len+1];
	strcpy(str,s);
	num_strings++;
}
Example::Example()
{
	len=4;
	str=new char[4];
	strcpy(str,"C++");
	num_strings++;
}
Example::~Example()
{
	num_strings--;
	delete[] str;
}
void Example::setName(char* n)
{
	strcpy(str,n);
}
char* Example::getName()
{
	return str;
}

int main()
{
	Example e("me");
	e.setName("Aashi");
	cout<<e.getName();
	return 0;
}

[Aashi]

here's updated code, it's working but is this the right way?
please respond..

#include <iostream>
using namespace std;

class Example
{
private:
	char* str;
	int len;
	static int num_strings;
public:
	Example(const char* s);
	Example();
	~Example();
	void setName();
	char* getName();
	
};
int Example::num_strings=0;

Example::Example(const char* s)
{
	len=strlen(s);
	str=new char[len+1];
	strcpy(str,s);
	num_strings++;
}
Example::Example()
{
	len=4;
	str=new char[4];
	strcpy(str,"C++");
	num_strings++;
}
Example::~Example()
{
	num_strings--;
	delete[] str;
}
void Example::setName()
{
	
	str=new char[2000];
	cin.getline(str,2000);
}
char* Example::getName()
{
	return str;
}

int main()
{
	Example e("me");
	
	e.setName();
	cout<<e.getName();
	return 0;
}

[Paul McKenzie]

here's updated code, it's working but is this the right way?

No it does not work.

One question -- why are you using char pointers and 'C' functions to do something that can be easily done using std::string?

First, that code will fail miserably if I just did this:

int main()
{
	Example e("me");
        Example e2("you");
        e = e2;
}

The reason why it fails is that in the destructor, the same pointer is deleted twice, causing undefined behaviour. You also now have a memory leak.

Here is another failure:

int main()
{
	Example e("me");
        e.setName();
}

Now you have a memory leak. The reason is that you did not deallocate the original memory used by the str pointer.

Bottom line is that if you quit using char pointers and instead used std::string, all of these issues disappear.

#include <iostream>
#include <string>
using namespace std;

class Example
{
private:
	std::string str;
public:
	Example(const std::string& s);
	Example();
	void setName();
	std::string getName();	
};

Example::Example(const std::string& s) : str(s) 
{}

Example::Example() : str("C++")
{}

void Example::setName()
{
    getline(cin, str);	
}

std::string Example::getName()
{
	return str;
}

int main()
{
	Example e("me");
	e.setName();
	cout<<e.getName();
	return 0;
}

There is no memory allocation being done, no memory leaks, no undefined behaviour. There is also no need to guess how many characters you need, as the string is dynamically resized.

If you insist on using char pointers, then you need to learn how to write copy constructor, assignment operator, and know exactly how to manage dynamically allocated memory correctly. But why do that when the code above does everything, easier and safer.

Regards,

Paul McKenzie

[Joeman]

I know, but i want to use this for learning.

Why not learn from tutorials as well? Also read faqs. They can provide common pitfalls and how to avoid them. Googling your question first is even better if you can generalize it.

void Example::setName() 
{          
    str=new char[2000];     
    cin.getline(str,2000); 
}

What happened to your previous str pointer?
Is len updated to the new string length?
How can I set a name without using the console?

char* Example::getName() 
{     
    return str; 
}

What if I have a const Example me and want to call me.getName()?

[Aashi]

thanks,