trouble returning an object by reference

This is a discussion on trouble returning an object by reference within the C++ Programming forums, part of the General Programming Boards category; Hello all, Could anyone tell me why I am losing my object? and possibly how I could correct this problem? ...

  1. #1
    novice fisheromen1031's Avatar
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    trouble returning an object by reference

    Hello all,
    Could anyone tell me why I am losing my object? and possibly how I could correct this problem?

    I am having alot of trouble with this function. I would like to return an object by reference from a function, but somewhere between my "return" statement and assigning the result the reference to my returned object gets lost. I don't know why but the destructor is fired by the time I get back inside my assignment operator.

    I have tried to include the least amount of code needed to show my problem. Please forgive me for its lengthiness.

    Some how after the returning from the operator* and after just entering the assignment (=) operator my returned matrix object is lost. When I go through debugging, the object is still good but is lost before the "if (right.get_length()!=get_length)" statement. I identified this by watching the object.aptr value change from what it was before the return statement. It appears to still be correct when I get to the first line of the assignment operator definition but not after that.

    Thanks for any comments.
    -fisher
    Code:
    #include "Matrix.h"
    
    int main(){
        matrix<int> MAT(3),MAT2(3);
    
        for (int j=1; j<=3; j++){
            for (int i=1; i<=3; i++){
                MAT(i,j)=1;
            }
        }
    
        MAT2=MAT*MAT; //at the moment, this is the troublesome line <---------------------------
       
        return 0;
    }
    Code:
    //Matrix.h
    #ifndef MATRIX_H
    #define MATRIX_H
    #include <iostream>
    #include <cstdarg> //for using variable argument functions
    
    template <class T>
    class matrix
    {
    protected:
      T *aptr;
      int *size; //adjustable array of size of each matrix dimension
      int dim; //number of dimensions of matrix
      int array_length; //total number of elements in matrix
        // array_length=size[0]*size[1]*...*size[dim-2]*size[dim-1]
      char type;
    public:
      // CONSTRUCTOR PROTOTYPES
      matrix();//default constructor
      matrix(int);// two-dimensional square matrix constructor
      matrix(int, int, ...);//an any dimensional matrix  
      ~matrix(){cout<<"matrix distroyed"<<endl;}
    
      //*** --- operators ----***
      T &operator()(int, ...);//reference operator: matrix(i,j,...)=matrix[i-1][j-1]...
      const matrix &operator= (const matrix &);//assignment operator
      //multiplication
      const matrix &operator* (matrix &);
    };
    
    template <class T>//an any dimensional array
    matrix<T>::matrix(int d, int n, ...){
      int i;
      va_list list;
      if(d>0){
        dim=d;
        size= new int[dim];
        size[0]=n;
        array_length=n;
        va_start(list, n);
        for(i=1;i<d;i++){
          size[i]=va_arg(list,int);
          array_length *= size[i];
        }
        aptr= new T[array_length];
        for (i=0;i<array_length;i++)
          aptr[i]=0;
      }
      va_end(list);
    }
    
    template <class T>//reference operator
    T &matrix<T>::operator()(int row, ...){
      int i, prod=0, element=0;
      va_list list;
      va_start(list, row);
      prod=size[0];
      element= (va_arg(list,int)-1)+size[1]*(row-1);
      for(i=2;i<dim;i++){
        prod*= size[i-1];
        element+= prod*(va_arg(list,int)-1);
      }
      va_end(list);
      return aptr[element];
    }
    
    template <class T> //assignment operator
    const matrix<T> &matrix<T>::operator= (const matrix<T> &right){<------object lost between here
      if(&right != this){ //check for self-assignment <-------------------------------------and here
        //reallocate memory if different amount of space needed
        if( array_length!=right.array_length ){
          array_length=right.array_length;
          dim=right.dim;
          delete [] aptr;
          delete [] size;
          size= new int[dim];
          for(int i=0; i<dim ; i++) size[i]=right.size[i];
          aptr= new T[array_length];
        }
        else{//change size of dimensions if necessary
          if(dim!=right.dim){ 
            dim=right.dim;
            delete [] size;
            size= new int[dim];
          }
          for(int i=0; i<dim ; i++){
            if (size[i]!=right.size[i])
              size[i]=right.size[i];
          }
        }
      }
      for(int i=0; i<array_length; i++) 
        aptr[i]=right.aptr[i]; //copy matrix elements
      return *this;
    }
    
    template<class T>//assumes 2D matrixes
    const matrix<T> &matrix<T>::operator* (matrix<T> &right){
      T sum;
      if((*this).size[1]==right.size[0]){
        matrix<T> new_m(2,(*this).size[0],right.size[1]);
        //do dot product of rows and columns
        // in tensor form Cik=Aij*Bjk (sum over j-index)
        for(int i=1;i<=(*this).size[0];i++){
          for(int k=1;k<=right.size[1];k++){
            sum=0;
            for (int j=1; j<=(*this).size[1]; j++){
              sum+=(*this)(i,j)*right(j,k);
            }
            new_m(i,k)=sum;
          }
        }
        return new_m;//this might not be passing back correctly <-----------------------------------
      }
    }
    #endif
    Last edited by fisheromen1031; 02-18-2008 at 10:00 PM.

  2. #2
    Deathray Engineer MacGyver's Avatar
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    Your code produces a number of errors for me.

    Code:
    Matrix.h:101: warning: reference to local variable `new_m' returned
    Bad idea.

  3. #3
    novice fisheromen1031's Avatar
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    I am doing that on purpose. It seemed like the best way for me to accomplish things.

  4. #4
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by fisheromen1031 View Post
    Hello all,
    I am having alot of trouble with this function. I would like to return an object by reference from a function, but somewhere between my "return" statement and assigning the result the reference to my returned object gets lost. I don't know why but the destructor is fired by the time I get back inside my assignment operator.
    You might like to return a reference to new_m, but you can't do that because you are not allowed to return a reference to a local variable. That variable ceases to exist when the function has returned and you have a dangling reference.
    So the solution is to stop trying to do things that are forbidden.
    An even better solution would be to use expresion templates which help eliminate temporaries. For that I'd recommend dropping your implementation and using the CML
    My homepage
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  5. #5
    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by fisheromen1031 View Post
    I am doing that on purpose. It seemed like the best way for me to accomplish things.
    If you had a vehicle that was just above the weight limit for a bridge would you take the wheels off to cross it? That's about as bad as what you've done.

    Damn I'm getting my mileage out of this analogy today...
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  6. #6
    Deathray Engineer MacGyver's Avatar
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    If the "best way" is to do something stupid, then I shudder to think of the rather obtuse ways you probably considered.

    A local variable, in the form of an object, is destroyed upon the end of the function (more or less).

    Let me give you an example where this "technique" provides for unwanted behavior:

    Code:
    #include <iostream>
    
    class abc
    {
    	int x;
    
    public:
    	abc()
    	{
    		std::cout << "abc::abc()" << std::endl;
    	}
    	~abc()
    	{
    		std::cout << "abc::~abc(); integer = " << x << std::endl;
    	}
    	void set(int i)
    	{
    		std::cout << "abc::set(); value = " << i << std::endl;
    		x = i;
    	}
    	void print()
    	{
    		std::cout << "abc::print()" << std::endl;
    		std::cout << "x = " << x << std::endl;
    	}
    };
    
    abc &create(int);
    
    abc &create(int i)
    {
    	abc r;
    	r.set(i);
    	return r;
    }
    
    int main()
    {
    	std::cout << "Before the horrible create()..." << std::endl;
    	
    	abc &z = create(10);
    
    	std::cout << "After the horrible create()...." << std::endl;
    
    	z.set(6);
    
    	std::cout << "Altered value." << std::endl;
    	
    	z.print();
    	
    	std::cout << "Done." << std::endl;
    	
    	return 0;
    }
    Example output on a system of mine:

    Code:
    Before the horrible create()...
    abc::abc()
    abc::set(); value = 10
    abc::~abc(); integer = 10
    After the horrible create()....
    abc::set(); value = 6
    Altered value.
    abc::print()
    x = 4201217
    Done.
    As you can see, returning a local variable totally screws up the life of the object... because the object actually dies, but you're using it anyway.

  7. #7
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    Just more support for the theory of life after death, don't ya know!

  8. #8
    novice fisheromen1031's Avatar
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    possible counter example

    I have made this work fine with the following overloaded operator:

    Code:
    template<class T>
    matrix<T> matrix<T>::operator* (T n){
      matrix<T> new_m;
      new_m=*this;
      for (int i=0; i<array_length; i++){
        new_m.aptr[i]=n*aptr[i];
        //cout << i;
      }
      cout << "done with * "<< endl;
      return new_m;
    }
    Could someone explain to me why this passes back the local object just fine while the previously posted operator* does not? It seems to me that what I have written should be the same as passing back a pointer to my local object.

  9. #9
    C++ Witch laserlight's Avatar
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    Now you are returning a value, not a reference.
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  10. #10
    Kernel hacker
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    Because your new example doesn't pass a REFERENCE to the object, but a copy of the object, which means that the new_m that you have as a local variable doesn't get used after it has been "removed".

    --
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  11. #11
    novice fisheromen1031's Avatar
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    OHHHHHHHHHHHHHHH (smacks forehead)
    Thanks y'all. I'm a bit embarrassed, but I understand now.
    I thought I was being clever and potentially saving myself some overhead for when very large matrixes might need to be passed back and forth.

  12. #12
    Kernel hacker
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    Well, you are saving some overhead, but unfortunately, by using data that is about to be re-used for other purposes - a bit like filling your homebrewed beer into the bottles that are in the recycling bin, leaving them in there to settle for a few days, and then act surprised when you can't find your beer-bottles as the recycling men have collected them... [Keeping your beer in the recycling bin also saves time when you have finished the beer...]

    --
    Mats
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  13. #13
    C++ Witch laserlight's Avatar
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    I thought I was being clever and potentially saving myself some overhead for when very large matrixes might need to be passed back and forth.
    You do have a point there, though. In some cases you might want to save the overhead of copying and operate on the current matrix itself. As such, I suggest that you implement operator*= for scalar multiplication:
    Code:
    template<typename T>
    matrix<T>& matrix<T>::operator*=(const T& n) {
      for (int i = 0; i < array_length; ++i) {
        aptr[i] *= n;
      }
      return *this;
    }
    Then you can implement operator* for matrix multiplication in terms of the assignment operator version:
    Code:
    template<typename T>
    matrix<T> matrix<T>::operator*(const T& n) {
      return matrix<T>(*this) *= n;
    }
    So you get the best of both worlds: if you want to change the current matrix, you can do so, and if you want to generate a new matrix, you can also do so.
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  14. #14
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    Quote Originally Posted by fisheromen1031 View Post
    OHHHHHHHHHHHHHHH (smacks forehead)
    Thanks y'all. I'm a bit embarrassed, but I understand now.
    I thought I was being clever and potentially saving myself some overhead for when very large matrixes might need to be passed back and forth.
    If you have Stroustrup's TC++PL (The C++ Programming Language), he talks about this issue in Section 11.6 (Large Objects).

  15. #15
    novice fisheromen1031's Avatar
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    Much obliged

    Quote Originally Posted by robatino View Post
    If you have Stroustrup's TC++PL (The C++ Programming Language), he talks about this issue in Section 11.6 (Large Objects).
    Thank you very much! I will make a point to look that up. A buddy of mine has a copy.

    Thanks again, all.

    Blessings,
    fisher

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