substring implementation for String

vbx_wx · 10-18-2010, 04:14 PM

Code:

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

class My_String {
	string s;
public:
	My_String(string ss): s(ss) {}
	string substr(int pos = 0, int n = string::npos)
	{
		string str;
		int j = 0;
		for(int i = pos; i <= n; i++)
		{
			str[j] = s[i];
			j++;
		}
		return str;
	}
};

int main()
{
	My_String test("i wanna be a milionar");
	string obj = test.substr(3,6);
	cout << obj;
}

My code doesnt print nothing,any idee why ? Thank you

Sergei Steshenko · 10-18-2010, 04:25 PM

Quote:

Originally Posted by vbx_wx

Code:

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

class My_String {
	string s;
public:
	My_String(string ss): s(ss) {}
	string substr(int pos = 0, int n = string::npos)
	{
		string str;
		int j = 0;
		for(int i = pos; i <= n; i++)
		{
			str[j] = s[i];
			j++;
		}
		return str;
	}
};

int main()
{
	My_String test("i wanna be a milionar");
	string obj = test.substr(3,6);
	cout << obj;
}

My code doesnt print nothing,any idee why ? Thank you

What did you do to debug your code ? How about inserting diagnostic prints and checking validity of you assumptions step by step - this is what debugging is about.

vbx_wx · 10-18-2010, 04:27 PM

I debug it,dont asume i dont,it works great in function,but it doesnt return,it skips return str :|

Sergei Steshenko · 10-18-2010, 04:31 PM

Quote:

Originally Posted by vbx_wx

I debug it,dont asume i dont,it works great in function,but it doesnt return,it skips return str :|

How do you know "it skips return str" ? And why do you think your 'return' statement is correct in the first place ?

Here is a much simpler code for you:

Code:

int *foo()
  {
  int i = 2;
  int *ptr = &i;
  return ptr;
  }

- is this code correct ?

vbx_wx · 10-18-2010, 04:36 PM

vbx_wx · 10-18-2010, 04:39 PM

If i debug it,i step trough code,so i can see it skips return str;

Sergei Steshenko · 10-18-2010, 04:44 PM

Quote:

Originally Posted by vbx_wx

yes

If you think the code I posted is correct, you are wrong. Think which kind of storage (static/global, stack, heap) 'i' variable is located in.

vbx_wx · 10-18-2010, 04:44 PM

When it reaches end of function -> } str = anna\25 ...thats what teh debugger says

vbx_wx · 10-18-2010, 04:45 PM

its stack....i was refering corect that it is compiling,i dont know what you are trying to do,if you try to return the value,its wrong

Sergei Steshenko · 10-18-2010, 04:47 PM

Quote:

Originally Posted by vbx_wx

its stack....i was refering corect that it is compiling,i dont know what you are trying to do,if you try to return the value,its wrong

So, where is your 'str' located ?

vbx_wx · 10-18-2010, 04:52 PM

It doesnt have to be local ?

Sergei Steshenko · 10-18-2010, 04:54 PM

Quote:

Originally Posted by vbx_wx

It doesnt have to be local ?

I think it's on stack. I.e. I think you are trying to return a pointer to something whose contents are on stack - exactly the same way as in my intentionally wrong example.

vbx_wx · 10-18-2010, 04:59 PM

And a solution ?

Sergei Steshenko · 10-18-2010, 05:01 PM

Quote:

Originally Posted by vbx_wx

And a solution ?

'new'.

vbx_wx · 10-18-2010, 05:03 PM

i tried alocating on the heap,still nothing:

Code:

	string substr(int pos = 0, int n = string::npos)
	{
		string* str = new string;
		int j = 0;
		for(int i = pos; i <= n; i++)
		{
			str[j] = s[i];
			j++;
		}
		return *str;
	}