problems with overloaded '+' again

This is a discussion on problems with overloaded '+' again within the C++ Programming forums, part of the General Programming Boards category; Ok look , i have a simple string class with overloaded '+=' , '=' and '+'. The '+=' and '=' ...

  1. #1
    former member Brain Cell's Avatar
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    Angry problems with overloaded '+' again

    Ok look , i have a simple string class with overloaded '+=' , '=' and '+'. The '+=' and '=' work just fine when i use them with object. But the friggin '+' sign is still causing troubles !!!

    I spent like 1 hour on this crap and i still can't figure out how to get around this "Debug Assertion Failed" error (vc++). I tracked execution and found that the problem occurs when one of the objects (don't know which) tries to delete it's sPtr space.

    anyway , the following are my .h , .cpp and main.cpp files. The problem is probably in the operator+ function (hilihgted in blue) , i posted the whole code just incase you need to take a look at something else :

    mystr.h
    Code:
    #ifndef MYSTR_H
    #define MYSTR_H
    
    class mystr
    {
    public :
    	mystr(const char * = "");
    	~mystr();
    	
    	const mystr &operator+=(const mystr &);
    	const mystr &operator=(const mystr &);
    
    	mystr operator+(const mystr &);
    
    	void print();
    
    private :
    	int length;
    	char *sPtr;
    };
    
    #endif
    mystr.cpp
    Code:
    #include <iostream>
    using std::cout;
    using std::endl;
    
    #include <cstring>
    #include "mystr.h"
    
    mystr::mystr(const char *s) // default constructor
    {
    	length = strlen(s);
    	sPtr = new char[ length + 10 ];
    
    	strcpy(sPtr, s);
    }
    
    
    mystr::~mystr()   // destructor
    {
    	delete [] sPtr;
    }
    
    
    const mystr &mystr::operator +=(const mystr &right)
    {
    	size_t newLength = length + right.length;
    
    	char *tempPtr = new char[newLength + 1];
    
    	strcpy(tempPtr, sPtr);
    	strcpy(tempPtr + length, right.sPtr);
    	
    	delete [] sPtr;
    
    	sPtr = tempPtr;
    	length = newLength;
    
    	return *this;
    }
    
    
    mystr mystr::operator+(const mystr &right)
    {
    	mystr temp(sPtr);
    
    	temp += right;
    
    	return temp;
    
    }
    
    
    const mystr &mystr::operator=(const mystr &right)
    {
    	if( &right != this)
    	{
    		delete [] sPtr;
    
    		sPtr = new char[right.length + 1];
    
    		strcpy(sPtr, right.sPtr);
    	}
    
    	return *this;
    }
    
    
    
    void mystr::print()
    {
    	cout << sPtr << endl;
    }
    main.cpp
    Code:
    #include "mystr.h"
    
    int main(void)
    {
    	mystr a("hello"), b("world\n"), c;
    
    	a.print();
    
    	b.print();
    
    	c = a + b;
    
    	c.print();
    
    	return 0;
    }

    it only displays :

    hello
    world

    then shows an error. I hope someone can help before i tear the rest of my hair out.

    p.s : please don't give examples , just show me how to fix my my code.
    Last edited by Brain Cell; 04-13-2005 at 06:12 PM.
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  2. #2
    Software Developer jverkoey's Avatar
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    I've had the same problem before too....The problem's in that when returning the temporary string, the destructor gets called, but AFTER the string is returned, so you end up returning a string with an old, dead pointer value that's just been deleted!

    *pulls hair out*

    The only thing I can think of is a method which involves creating, dynamically, a new string and returning the pointer to it....but this will end up creating a memory leak

  3. #3
    Registered User
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    Your code seems fine, you just need to add a copy constructor.

  4. #4
    Software Developer jverkoey's Avatar
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    Ah hah! So it's just a matter of using the same copy constructor as the overloaded operator is using? As in, in this case, the + operator's using a mystr&, so use a copy constructor that accepts a mystr&

    That's what I just did in the above code and the crash disappeared.

    Code:
    mystr::mystr(const mystr &s)
    {
    	length=s.length;
    	sPtr = new char[ length + 1 ];
    
    	strcpy(sPtr, s.sPtr);
    }
    Code:
    mystr mystr::operator+(const mystr &right)
    {
    	mystr temp(*this);
    
    	temp += right;
    
    	return temp;
    }
    Oh, nevermind, the copy constructor's called when the return statement is hit.
    Last edited by jverkoey; 04-13-2005 at 07:27 PM.

  5. #5
    former member Brain Cell's Avatar
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    yea i just need to add a copy constructor to the original code. Thanks alot Daved and Jeff. If you were girls , i'd fall in love with you both
    My Tutorials :
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  6. #6
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    Quote Originally Posted by Brain Cell
    I spent like 1 hour on this crap and i still can't figure out how to get around this "Debug Assertion Failed" error (vc++). I tracked execution and found that the problem occurs when one of the objects (don't know which) tries to delete it's sPtr space.
    Why? I told you yesterday that you needed a copy constructor.
    If I did your homework for you, then you might pass your class without learning how to write a program like this. Then you might graduate and get your degree without learning how to write a program like this. You might become a professional programmer without knowing how to write a program like this. Someday you might work on a project with me without knowing how to write a program like this. Then I would have to do you serious bodily harm. - Jack Klein

  7. #7
    Senior Member joshdick's Avatar
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    Remember the rule of three: If you implement either the copy constructor, destructor or assignment operator, you should probably implement all three.

  8. #8
    & the hat of GPL slaying Thantos's Avatar
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    Please remember to keep threats of violence out of your thread titles.

  9. #9
    Sweet
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    This is how I implemented my + operator for my little string class
    Code:
    //.h
    friend string operator+(const string &s, const string &s2);
    friend string operator+(const char *s, const string &s2);
    friend string operator+(const char ch, const string &s2);
    friend string operator+(const string &s, const char *s2);
    friend string operator+(const string &s, const char ch);
    Code:
    //.cpp
        string operator+(const string &s, const string &s2)
        {
            string both = s;
            both += s2;
            return both;
        }
        string operator+(const char *s, const string &s2)
        {
            string both = s;
            both += s2;
            return both;
        }
        string operator+(const char ch, const string &s2)
        {
            string both(1,ch);
            both += s2;
            return both;
        }
        string operator+(const string &s, const char *s2)
        {
            string both = s;
            both += s2;
            return both;
        }
        string operator+(const string &s, const char ch)
        {
            string both = s;
            both += ch;
            return both;
        }
    Woop?

  10. #10
    former member Brain Cell's Avatar
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    Quote Originally Posted by pianorain
    Why? I told you yesterday that you needed a copy constructor.
    ooh , sorry. Seems like i skipped that line


    Quote Originally Posted by joshdick
    Remember the rule of three: If you implement either the copy constructor, destructor or assignment operator, you should probably implement all three.
    Useful tip , thanks .

    Quote Originally Posted by Thantos
    Please remember to keep threats of violence out of your thread titles.
    ummm...ok. But you knew i wasn't serious right ? i was only expressing how ***** off i was.
    My Tutorials :
    - Bad programming practices in : C
    - C\C++ Tips
    (constrcutive criticism is very welcome)


    - Brain Cell

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