memset on vector is making its size 0

Question

I was trying to clear all data in a vector without changing its size using memset. But after memset operation size of vector becomes 0. Here is the exact code:

std::vector<T> buckets[2];
MAX_BUCKET_SIZE=16;

Constructor:

buckets[0].resize(MAX_BUCKET_SIZE);
std::cout << "Actual size0 " << buckets[0].size() << std::endl;
buckets[1].resize(MAX_BUCKET_SIZE);
std::cout << "Actual size1 " << buckets[1].size() << std::endl;

in a function:

std::cout << "Actual size2 0 " << buckets[0].size() << std::endl;
std::cout << "Actual size2 1 " << buckets[1].size() << std::endl;
...
...
while (...){
    delete_data(current_bucket);

    std::cout << "current bucket ** " << current_bucket << std::endl;
    std::cout << "Actual size3 0 " << buckets[0].size() << std::endl;
    std::cout << "Actual size3 1 " << buckets[1].size() << std::endl;
}

delete_data function:

memset(&buckets[bucket_id], 0, sizeof(buckets[bucket_id]));

output:

Actual size0 16
Actual size1 16
Actual size2 0 16
Actual size2 1 16
current bucket ** 1
Actual size3 0 16
Actual size3 1 0
current bucket ** 0
Actual size3 0 0
Actual size3 1 0

Answer 1

I was trying to clear all data in a vector without changing its size using memset.

It's not going to work, because it does not pay attention to the area being filled. The way you apply it also creates a memory leak in situations when std::vector is implemented using pointers, because pointers are being zeroed out.

Use std::fill instead:

std::fill(buckets[0].begin(), buckets[0].end(), 0);

Answer 2

Not to mention memset on non-POD is undefined behavior, it looks like you've just "succeeded" memset-ing the vector's internal data - that could be pointer to implementation or pointers to start/end of the underlying array. You didn't modify the elements and you effectively lost them (and leaked a memory).

You most probably wanted to operate on the underlying array (still, T has to be POD):

memset(&buckets[bucket_id].front(), 0, buckets[bucked_id].size() * sizeof(T));

But nobody does this in C++, because there are STL solutions, like std::fill:

std::fill(buckets[bucket_id].begin, buckets[bucket_id].end(), T{});

which will fill the array with copies of default-constructed T (which can be non-POD).

^{Also note that sizeof(buckets[bucket_id]) gives you completely pointless number for this occasion.}

Answer 3

Don't do that. Trashing the automatic storage of an instance of a class will probably lead to undefined behaviour. For example, I imagine that any destructors of the vector's elements will not be called.

Calling resize to zero will achieve essentially the same thing. Or just reassign buckets to a new instance of std::vector<T>.

Answer 4

You're doing memset on a class, which is undefined behavior at best. How do you know what setting the memory used by the object to zero makes sense for the object? For example, if the vector stores a pointer to the items it contains then you've set that pointer to null, and thus caused a memory leak.

If you're lucky, you'll crash, if you're unlucky you won't.