Memory overwritten with structures

[steflhermitte]

Dear cpp-ians,

I have two structures:

struct metaSegment
{
vector <long double> mean;
bool full;
};

struct segment
{
segmentIter it_meta_segm;
};

and both are linked via the it_meta_segm:

typedef vector <metaSegment>:: iterator segmentIter;
In my program I have:

div_t divresult;
vector <metaSegment> X;
segmentIter itX;
segment *** Y;

for (int NbPixels = 0; NbPixels<10000; NbPixels++)
{
// create temporary segment
metaSegment tempMetaSegment ; // Create temporary metaSegment
divresult = div(counter,100 ); // Retrieve ...
row = divresult.quot; // ... row number and ...
column = divresult.rem; // ... column number.
for (int layer=0; layer<5; layer++)
{
tempMetaSegment .mean.push_back (image[layer][row][column]);
// Assign image[layer][row][column] values...
// ...(long double type) to vector <long double>...
// ... mean of tempMetaSegment
}
tempMetaSegment .full = false; // Assign full value
X.push_back(tem pMetaSegment); // Put tempMetaSegment in...
// ...the vector <metaSegment> X
itX = --X.end(); // Take the last element
Y[row][column]->it_meta_segm = itX; // of X and assign it to Y.
}

When I retrieve now the values of Y[row][column]->it_meta_segm->full,
then I see they do not fullfill my expection of being false (as I
assigned them).
I assume my values are overwritten somewhere in my memory, but I don't
see where I go wrong?

Any advice?

Kind regards and thank you very much in advance,
Stef

[Victor Bazarov]

steflhermitte wrote:

Dear cpp-ians,

I have two structures:

struct metaSegment
{
vector <long double> mean;
bool full;
};

struct segment
{
segmentIter it_meta_segm;
};

and both are linked via the it_meta_segm:

typedef vector <metaSegment>:: iterator segmentIter;
In my program I have:

div_t divresult;
vector <metaSegment> X;
segmentIter itX;
segment *** Y;

for (int NbPixels = 0; NbPixels<10000; NbPixels++)
{
// create temporary segment
metaSegment tempMetaSegment ; // Create temporary metaSegment
divresult = div(counter,100 ); // Retrieve ...
row = divresult.quot; // ... row number and ...
column = divresult.rem; // ... column number.
for (int layer=0; layer<5; layer++)
{
tempMetaSegment .mean.push_back (image[layer][row][column]);
// Assign image[layer][row][column] values...
// ...(long double type) to vector <long double>...
// ... mean of tempMetaSegment
}
tempMetaSegment .full = false; // Assign full value
X.push_back(tem pMetaSegment); // Put tempMetaSegment in...
// ...the vector <metaSegment> X
itX = --X.end(); // Take the last element
Y[row][column]->it_meta_segm = itX; // of X and assign it to Y.
You're holding onto an iterator value here. Since it's an iterator of
a vector, it can go bad (become invalid) after any 'push_back' operation
if that operation involves reallocation. I don't see any attempt to
prevent reallocation by reserving the size of the vector 'X', so your 'Y'
elements are going bad without you ever knowing about it.
}

When I retrieve now the values of Y[row][column]->it_meta_segm->full,
then I see they do not fullfill my expection of being false (as I
assigned them).
I assume my values are overwritten somewhere in my memory, but I don't
see where I go wrong?

Any advice?

If you know the size of 'X' beforehand, use 'X.reserve(thes ize)'. If you
don't know the size, store _indices_ and not iterators. If you can't do
that, change 'X' from 'vector' to 'list', the iterators of 'list' do not
go bad when you push_back.

V

[steflhermitte]

Thank you very much Victor!

X.reserve(thesi ze) works perfectly. I did not now that reallocation can
make an iterator go bad.