[SOLVED] Assigning Iterator to Iterator
Oh, your problem is likely that you called cont.begin() when the container was empty. That will return you a nice invalid iterator
Author
Quote:Original post by RDragon1
Oh, your problem is likely that you called cont.begin() when the container was empty. That will return you a nice invalid iterator
I'm sorry, I do not quite understand. When debugging, menuItems was populated with 2 items, and activeItem simply had (error) as a symbol and 0 as its value, and maintained that state ever since its construction in its parent class.
Did it have 2 items when you executed this:
activeItem = menuItems.begin();
?
I suspect it was .empty() at this point
If not, we'll need a full small test case that shows the problem to continue from here
activeItem = menuItems.begin();
?
I suspect it was .empty() at this point
If not, we'll need a full small test case that shows the problem to continue from here
Author
Quote:Original post by RDragon1
Did it have 2 items when you executed this:
activeItem = menuItems.begin();
?
I suspect it was .empty() at this point
If not, we'll need a full small test case that shows the problem to continue from here
Why, yes.
Note the (error) symbol I mentioned.
I then experimented and look at the results:
(again the error)
I meant this line:
activeItem = menuItems.begin();
Not this line:
if( activeItem == menuItems.begin() )
- you showed the latter in the debugger
activeItem = menuItems.begin();
Not this line:
if( activeItem == menuItems.begin() )
- you showed the latter in the debugger
Author
Oh, sorry.
That line is actually here:
That line is actually here:
void Menu::AddItem(MenuPair item){ menuItems.push_back(item); if (menuItems.size() == 1) activeItem = menuItems.begin();}
Are you absolutely sure that's always getting hit before that comparison statement gets executed?
Author
Quote:Original post by RDragon1
Are you absolutely sure that's always getting hit before that comparison statement gets executed?
Yes.
menu.AddItem(MenuPair(PLAY, MenuItem("Play")));menu.AddItem(MenuPair(QUIT, MenuItem("Quit")));is called in the constructor of MainMenuState, and I've stepped through the whole thing many times.
Could it be a bug on my part? I do not quite fully understand the nature of iterators.
Would posting the project help?
void Menu::AddItem(MenuPair item){ menuItems.push_back(item); // if (menuItems.size() == 1) activeItem = menuItems.begin();}Try that. Iterators are invalidated when the internals of the container are moved around in memory. Adding an item to a vector for instance will re-allocate memory if the array is too small accommodate the new item, thus invalidating all previously "purchased" iterators, see below for a code sample demonstrating the problem. Possible fixes include reserve-ing some amount of menu items before hand (may cause problems later), revalidating iterators as the code above shows, using a separate container (such as std::list, which I believe does not suffer this problem), or you can simply refer to the activeItem as an index in the array.
int main() { int* arr = new int[2]; // equivalent to vector::reserve(2) int arrSize = 0; arr[0] = 123; // equivalent to vector::push_back( 123 ) arr[1] = 456; // equivalent to vector::push_back( 456 ) int* selection = &arr[1]; // push_back( 789 ): int* temp = new int[3]; for ( int i = 0; i < 2; ++i ) temp = arr; delete[] arr; arr = temp; temp = 0; arr[2] = 789; *selection = 1; // Boom! delete[] arr;}
Author
Quote:Original post by _fastcall
*** Source Snippet Removed ***
Try that. Iterators are invalidated when the internals of the container are moved around in memory. Adding an item to a vector for instance will re-allocate memory if the array is too small accommodate the new item, thus invalidated all previously "purchased" iterators. Possible fixes include reserve-ing some amount of menu items before hand (may cause problems later), revalidating iterators as the code above shows, using a separate container (such as std::list, which I believe does not suffer this problem), or you can simply refer to the activeItem as an index in the array.
Oh, I see. Revalidating the iterator seems to have fixed the problem. Thanks tons, _fastcall and RDragon1.
++rating; [wink]
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