help with pointers

**axr0284** · 03-06-2006

Hi i am having some trouble with pointers:

Code:

char *input = NULL; /* The character array to put the input line in */

getTheLine(input); /* discard first line */

void CdistanceFilter::getTheLine(char *output)
{
	std::string inputString = ""; /*A string to put file lines*/
	//char *input;

	std::getline(inputFile,inputString); /* get third line */
	
	output = new char[inputString.length()]; /*get the length of the string*/
	
	memcpy(output, inputString.c_str(), inputString.length()); /*Copy the string removing the \0*/
}

NOTE: the first two lines are in a different function. They are not global

The thing is that when the getTheLine function returns, the pointer input gets 0x000000 but just before the function ends, the pointer output was pointing to the correct location so what gives. Thanks
Amish

**7stud** · 03-06-2006

output is a local variable of the function. It is destroyed when the function ends--leaving the address stored in input unchanged. Anything you do to the address stored in output has no effect on the address stored in input.

You can use output to change the value where output is pointing to, e.g.:

*output = "hello";

Since input points to the same location, the value input points to will also change. However, the address stored in input will not change.

When you call the function with input as the argument, the first thing that happens is the parameter variable output is created, and then a copy of input's address is stored in output.

**ChaosEngine** · 03-06-2006

there are three ways to do what you're tring to accomplish

1. pointer to a pointer (most painful)

Code:

char *input = NULL;
getTheLine(&input);
void CdistanceFilter::getTheLine(char **output)
{
	// snip
	*output = new char[inputString.length()]; /*get the length of the string*/
}

2. reference to a pointer (better, but still ugly)

Code:

char *input = NULL;
getTheLine(input);
void CdistanceFilter::getTheLine(char *&output)
{
	// snip
	output = new char[inputString.length()]; /*get the length of the string*/
}

3. forget the the whole pointer thing and use a reference to a std::string (yay!!)

Code:

std::string input;
getTheLine(input);
void CdistanceFilter::getTheLine(std::string& output)
{
	// snip
	output = "whatever the hell you want";
}

**7stud** · 03-06-2006

...and maybe easier to understand but not as efficient as option 3:

Code:

string input = getTheLine();
....
....

string CdistanceFilter::getTheLine()
{
	string inputString;
	getline(inputFile, inputString);
	
	return inputString;
}

**axr0284** · 03-06-2006

The double pointer worked fine but when I tried to deallocate the memory using delete [] input outside of getTheLine, the program crashed. Any clues
Amish

**ChaosEngine** · 03-06-2006

Originally Posted by axr0284

The double pointer worked fine but when I tried to deallocate the memory using delete [] input outside of getTheLine, the program crashed. Any clues
Amish

post code!

**axr0284** · 03-06-2006

here it is

Code:

int CdistanceFilter::initializeArrays(void)
{
	int errorCode = 0; /* This is the error code that is returned  */
	
	char *input = NULL; /* The character array to put the input line in */
...
getTheLine(&input); /* discard first line */
delete [] input;
...
}

void CdistanceFilter::getTheLine(char **output)
{
	std::string inputString = ""; /*A string to put file lines*/
	//char *input;

	std::getline(inputFile,inputString); /* get third line */
	
	*output = new char[inputString.length()]; /*get the length of the string*/
	
	memcpy(output, inputString.c_str(), inputString.length()); /*Copy the string removing the \0*/
}

I get an exception error when i try to delete it. Again thks for the help. I really appreciate it
Amish

**Salem** · 03-07-2006

> memcpy(output, inputString.c_str(), inputString.length());
Should have been
memcpy(*output, inputString.c_str(), inputString.length());

**axr0284** · 03-07-2006

Thanks everybody for your help, it works now. I must have been staring at the code for too long not to notice that last mistake

Amish

**ChaosEngine** · 03-07-2006

Originally Posted by axr0284

Thanks everybody for your help, it works now. I must have been staring at the code for too long not to notice that last mistake

Amish

this is why I pointers to pointers (2 star programming) is Not A Good Thing(tm) and why I discouraged you from using it. You're lucky in that it was a simple bug and (relatively) easy to spot. With a string reference this problem would never have come up.

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