invalid lvalue in assignment

This is a discussion on invalid lvalue in assignment within the C++ Programming forums, part of the General Programming Boards category; hi. I keep getting the invalid lvalue in assignment for the following code Code: void rbnode::rotate_left(rbnode *x) { printf("rotate left!\n"); ...

  1. #1
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    invalid lvalue in assignment

    hi. I keep getting the invalid lvalue in assignment for the following code

    Code:
    void rbnode::rotate_left(rbnode *x) {
    	printf("rotate left!\n");
    	rbnode *y = new rbnode(0, Red, NULL, NULL, NULL);
    	&y = &x;
    	x->right = y->left;
    	if(y->left != NULL){
    		y->left->parent = x;
    	}
    	y->parent = x->parent;
    
    	if(x == x->parent->left){
    		x->parent->left = y;
    	}
    	else{
    		x->parent->right = y;
    	}
    
    	y->left = x;
    	x->parent = y;
    
      
    } // rbnode::rotate_left
    The error state that &y = &x has error. am I doing something wrong here? I search on google about the reason of getting this error but none of them apply to me here.

    thx for the help

  2. #2
    Captain Crash brewbuck's Avatar
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    You're using '&' to take the address of y. The resulting value is not an l-value, so the assignment is improper.

    I'm not sure what you WANT to accomplish... I assume you're trying to copy the object pointed to by x into the object pointed to by y. In that case, you need to use '*' instead of '&':

    Code:
    *y = *x;
    And yet I have doubts that this is what you want, because if it was, surely you would have just invoked the copy constructor when you call new? Confused.

  3. #3
    Registered User hk_mp5kpdw's Avatar
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    Code:
    rbnode *y = new rbnode(0, Red, NULL, NULL, NULL);
    &y = &x;
    y is a pointer, &y is the address of the pointer variable on the stack. Obviously you can't assign a new address to this variable (it's fixed when you call the function). If you want y to point to x, they are both pointers so you just assign one to another:
    Code:
    y = x;
    However, this constitutes a memory leak as you've just lost what you assigned to y in the previous line of code during the call to new. Are you sure you need to be doing this?

    Echo confused as to what you're trying to do there.
    "Owners of dogs will have noticed that, if you provide them with food and water and shelter and affection, they will think you are god. Whereas owners of cats are compelled to realize that, if you provide them with food and water and shelter and affection, they draw the conclusion that they are gods."
    -Christopher Hitchens

  4. #4
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    thx for the quick reespose.

    All I want to do here is to copy x into y. The reason that I didn't pass in all the parameter when I new the object is I keep getting segmentation fault error. So I was thinking that this way might help me for a bit.

    anyway, I have tried using *y = *x but now the segmentation error came back....What really happen that it gives me segmentation error anyway?Is ther a way to fix it?

  5. #5
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    Code:
    rbnode *y = new rbnode(x->key, x->colour, x->left, x->right, x->parent);
    this was whwat I originally did but segmentation fault occur..

  6. #6
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    Does rbnode have a valid copy constructor and copy assignment operator? What does that class declaration look like?

  7. #7
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    Code:
    // Constructor
    rbnode::rbnode(int s_key, rbcolour s_colour, 
    			   rbnode *s_left, rbnode *s_right, rbnode *s_parent) {
      key = s_key;
      colour = s_colour;
      left = s_left;
      right = s_right;
      parent = s_parent;
    } // rbnode::rbnode
    rbnode only has this constructor.

  8. #8
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    Do you have allocated resources in your class? All those pointers were allocated with new? Where are they deleted?

    Chances are you have some other problems in your code that cause the segmentation fault. I doubt that using a copy constructor would be any different than doing what you tried to do above. I would use this:
    Code:
    rbnode *y = new rbnode(*x);
    to create a new copy of another node, at least until you figure out what's causing your segmentation fault.

  9. #9
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by saipkjai View Post
    Code:
    // Constructor
    rbnode::rbnode(int s_key, rbcolour s_colour, 
    			   rbnode *s_left, rbnode *s_right, rbnode *s_parent) {
      key = s_key;
      colour = s_colour;
      left = s_left;
      right = s_right;
      parent = s_parent;
    } // rbnode::rbnode
    rbnode only has this constructor.
    It also has a default copy constructor. Since all your data seems to be held by value, the default copy constructor should work fine.

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