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Aug 11th, 2005

Pointer question

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Hey guys, I was just wondering when using classes to obtain a name or something like that when a pointer is used how come you don't have to use the new and delete key words.

C Syntax (Toggle Plain Text)
 
class LibBook
 
{
    private:
       char *name
    public:
       LibBook( char* = '\0')
       void showBookName()
 
};
class LibBook

{
    private:
       char *name
    public:
       LibBook( char* = '\0')
       void showBookName()

};

Thanks in advance.

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Jon182

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Aug 11th, 2005

Re: Pointer question

Quote originally posted by Jon182 ...

Hey guys, I was just wondering when using classes to obtain a name or something like that when a pointer is used how come you don't have to use the new and delete key words.

Generally you do. Do you have a more specific example? The one you posted is not really correct. The '\0' should be 0.

Dave Sinkula

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Aug 11th, 2005

Re: Pointer question

I found another example:

C Syntax (Toggle Plain Text)
 
class Date
{
 private:
	 char day[10];
	 char month[10];
	 int year;
 
public:
 
 
	void set(char* day, char* month, int year);
 
        void display(Date datem);
 
        char* get_day();
 
	char* get_month();
 
	int get_year();
 
 };
 
void Date::set(char* d, char* m, int y)
{
 
	strncpy(day, d, 9);
	strncpy(month, m, 9);
	year=y;
 
}
 
 
void Date::display(Date date)
{
	cout << "The day is ";
	cout << date.get_day();
 
	cout << "\nThe month is ";
	cout << date.get_month();
 
	cout << "\nThe year is ";
	cout << date.get_year();
	cout << endl;
}
 
char* Date::get_day()
{
	return day;
}
 
char* Date::get_month()
{
	return month;
}
 
int Date::get_year()
{
	return year;
}
class Date
{
 private:
	 char day[10];
	 char month[10];
	 int year;
  
public:
	

	void set(char* day, char* month, int year);
  
        void display(Date datem);
  
        char* get_day();

	char* get_month();

	int get_year();

 };

void Date::set(char* d, char* m, int y)
{

	strncpy(day, d, 9);
	strncpy(month, m, 9);
	year=y;

}

  
void Date::display(Date date)
{
	cout << "The day is ";
	cout << date.get_day();
	
	cout << "\nThe month is ";
	cout << date.get_month();
	
	cout << "\nThe year is ";
	cout << date.get_year();
	cout << endl;
}
  
char* Date::get_day()
{
	return day;
}

char* Date::get_month()
{
	return month;
}

int Date::get_year()
{
	return year;
}

Jon182

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Aug 11th, 2005

Re: Pointer question

What is your question in regard to pointers about your last posted code? The functions that return a pointer to the beginning of a char array that is private? Or the function parameters that receive a pointer to the beginning of a string to copy?

Dave Sinkula

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Re: Pointer question

I was just wondering with the variables such as day and month as they are pointers how come the new and delete keywords don't have to be used.

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Re: Pointer question

Quote originally posted by Jon182 ...

I was just wondering with the variables such as day and month as they are pointers how come the new and delete keywords don't have to be used.

The day and month variables are declared to be arrays of length 10. You only need to use new and delete[] if the arrays have not been allocated yet.

Quote originally posted by Dave Sinkula ...

The one you posted is not really correct. The '\0' should be 0.

Hi Dave, I was under the impression that '\0' was equal to 0?
I got the output "equal" for the following program.

C Syntax (Toggle Plain Text)
#include <cstdio>
 
int main()
{
  if ( '\0' == 0 )
    printf( "equal" );
  getchar();
}
#include <cstdio>

int main()
{
  if ( '\0' == 0 )
    printf( "equal" );
  getchar();
}

Or is this only restricted to my compiler (mingw)?

Dante Shamest

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Aug 11th, 2005

Re: Pointer question

Quote originally posted by Dante Shamest ...

The day and month variables are declared to be arrays of length 10. You only need to use new and delete[] if the arrays have not been allocated yet.

Thanks for your help Dave & Dante.

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Re: Pointer question

Quote originally posted by Dante Shamest ...

Hi Dave, I was under the impression that '\0' was equal to 0?

It was more of a hint not to get into the habit of writing characters to pointers and confusing NULL and '\0'.

[edit]So yes, I overstated things when I said it wasn't quite correct.

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Re: Pointer question

Ah I see. Yes, one should generally initialize pointers to 0 instead of '\0' to avoid confusion. The Great One himself recommends it as well.

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