help with pointers

This is a discussion on help with pointers within the C++ Programming forums, part of the General Programming Boards category; Hi i am having some trouble with pointers: Code: char *input = NULL; /* The character array to put the ...

  1. #1
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    help with pointers

    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

  2. #2
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    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.
    Last edited by 7stud; 03-06-2006 at 05:29 PM.

  3. #3
    semi-colon generator ChaosEngine's Avatar
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    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";
    }
    "I saw a sign that said 'Drink Canada Dry', so I started"
    -- Brendan Behan

    Free Compiler: Visual C++ 2005 Express
    If you program in C++, you need Boost. You should also know how to use the Standard Library (STL). Want to make games? After reading this, I don't like WxWidgets anymore. Want to add some scripting to your App?

  4. #4
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    ...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;
    }

  5. #5
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    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

  6. #6
    semi-colon generator ChaosEngine's Avatar
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    Quote 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!
    "I saw a sign that said 'Drink Canada Dry', so I started"
    -- Brendan Behan

    Free Compiler: Visual C++ 2005 Express
    If you program in C++, you need Boost. You should also know how to use the Standard Library (STL). Want to make games? After reading this, I don't like WxWidgets anymore. Want to add some scripting to your App?

  7. #7
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    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

  8. #8
    and the hat of wrongness Salem's Avatar
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    > memcpy(output, inputString.c_str(), inputString.length());
    Should have been
    memcpy(*output, inputString.c_str(), inputString.length());

  9. #9
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    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

  10. #10
    semi-colon generator ChaosEngine's Avatar
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    Quote 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.
    Last edited by ChaosEngine; 03-07-2006 at 01:29 PM.
    "I saw a sign that said 'Drink Canada Dry', so I started"
    -- Brendan Behan

    Free Compiler: Visual C++ 2005 Express
    If you program in C++, you need Boost. You should also know how to use the Standard Library (STL). Want to make games? After reading this, I don't like WxWidgets anymore. Want to add some scripting to your App?

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