Why doesn't vector::clear remove elements from a vector?

Question

When I use clear() on a std::vector, it is supposed to destroy all the elements in the vector, but instead it doesn\'t.

Sample

Answer 1

Yes this is normal. clear() doesn't guarantee a reallocation. Try using resize() after clear().

Answer 2

When you access outside of the bounds of a vector, you get Undefined Behavior. That means anything can happen. Anything.

So you could get the old value, garbage, or a seg-fault. You can't depend on anything.

If you want bounds checking, use the at() member function instead of operator []. It will throw an exception instead of invoking Undefined Behavior.

Answer 3

From cppreference:

void clear();

Removes all elements from the container. Invalidates any references, pointers, or iterators referring to contained elements. May invalidate any past-the-end iterators. Many implementations will not release allocated memory after a call to clear(), effectively leaving the capacity of the vector unchanged.

So the reason there is no apparent problem is because the vector still has the memory available in store. Of course this is merely implementation-specific, but not a bug. Besides, as the other answers point out, your program also does have Undefined Behavior for accessing the cleared contents in the first place, so technically anything can happen.

Answer 4

The operator[] is efficient but comes at a price: it does not perform boundary checking.

There are safer yet efficient way to access a vector, like iterators and so on.

If you need a vector for random access (i.e. not always sequential), either be very careful on how you write your programs, or use the less efficient at(), which in the same conditions would have thrown an exception.