向量向量的C ++迭代器

Question

Why I am getting a segmentation fault ??

    std::vector<std::vector<int> > cells;
    std::vector<std::vector<int>::iterator> cellitr;

    for(int i=0;i<10;i++)
    {
        std::vector<int> c;
        std::vector<int>::iterator citr;
        citr= c.begin();
        for(int j=0;j<10;j++)
        {
            c.push_back(j);
        }
        cells.push_back(c);
        cellitr.push_back(citr);
    }
    qDebug()<<cells[5][6];
    int *x = &cells[5][6];
    cells[5].insert(cellitr[5],200);//SEG FAULT HERE
    qDebug()<<cells[5][6];

I have a vector of vectors and vector of iterators and I am trying to insert a value in one of the sub vectors using the index of the iterator.

any idea ??

Answer 1

There are two sources of undefined behavior in your program.

First , when you do this:

c.push_back(j);

You are incrementing the size of the vector. If the size of the vector exceeds its capacity, the vector has to reallocate a bigger storage to hold its current elements and its new element - that's because vectors must always store elements in a contiguous region of storage.

When reallocation occurs, iterators are invalidated. In particular, it is possible that your iterator citr obtained this way prior to insertions:

citr= c.begin();

Will be invalidated. Dereferencing an invalid iterator injects undefined behavior in your program.

Second , when you do this:

cellitr.push_back(citr);

You are storing an iterator to a vector ( c ) that will go out of scope when the loop exits - that will make the stored iterator an invalid iterator. Again, dereferencing an invalid iterator will give you undefined behavior.

---

In order to avoid both sources of undefined behavior, you could change your loop as follows:

cells.reserve(10);
for(int i=0;i<10;i++)
{
    std::vector<int> c;
    for(int j=0;j<10;j++)
    {
        c.push_back(j);
    }

    cells.push_back(c);
    cellitr.push_back(cells.back().begin());
}

Notice, that in C++11 you could avoid the inner loop and make the above more compact:

#include <algorithm> // For std::iota() (C++11 only)

// ...

cells.reserve(10);
for(int i=0;i<10;i++)
{
    std::vector<int> c(10);
    std::iota(c.begin(), c.end(), 0);

    cells.push_back(c);
    cellitr.push_back(cells.back().begin());
}

Answer 2

Here, a copy of c is pushed into cells :

cells.push_back(c);

Here, you store an iterator to a local vector c , whose lifetime is only one single iteration of the for-loop:

cellitr.push_back(citr);

You are storing iterators to local vectors, and trying to access them in a scope where these vectors do not exist.

Besides this, there may be some iterator invalidation issues due to the growth of c . This can be fixed via a call to c.reserve(10) , but you have to fix the other problem first:

std::vector<int> c;
....
cellitr.push_back(cells.back().begin());