STL Vector - clear() works for 2D Vectors?

[madhu]

http://msdn2.microsoft.com/en-us/library/fs5a18ce(VS.80).aspx

vector <int> v1;
v1.push_back( 10 ); //adds 10 to the tail
v1.push_back( 20 ); //adds 20 to the tail
cout << "The size of v1 is " << v1.size( ) << endl;
v1.clear( ); //clears the vector

I have a few questions:

Does clear() deallocates the memory too (like resize())?
Does clear() work for 2D vectors?
Or clear() is to be called for each dimension?

thanks in advance..

 

[ralph]

http://msdn2.microsoft.com/en-us/library/fs5a18ce(VS.80).aspx

vector <int> v1;
v1.push_back( 10 ); //adds 10 to the tail
v1.push_back( 20 ); //adds 20 to the tail
cout << "The size of v1 is " << v1.size( ) << endl;
v1.clear( ); //clears the vector

I have a few questions:

Does clear() deallocates the memory too (like resize())?

This is implementation defined. Same goes for resize().

Does clear() work for 2D vectors?
Or clear() is to be called for each dimension?

What do you mean by 2D vectors?

Ralpe

 

[werasm]

http://msdn2.microsoft.com/en-us/library/fs5a18ce(VS.80).aspx

vector <int> v1;
v1.push_back( 10 ); //adds 10 to the tail
v1.push_back( 20 ); //adds 20 to the tail
cout << "The size of v1 is " << v1.size( ) << endl;
v1.clear( ); //clears the vector

I have a few questions:

Does clear() deallocates the memory too (like resize())?

As far as I'm aware of, both clear() and resize() does not perform any
de-allocation in terms of the memory allocated for the items. It only
calls the destructors of the items that were erased. A vector's
capacity (which is relative to the amount of contigious memory that it
represents) grows with amortized constant time as new items are added.
It never shrinks, unless you do this:

std::vector<T> newv; //empty
oldv.swap( newv );

As far as 2D vectors are concerned, clear will erase all the items in
the first (or outer) dimension vector. This will cause destructors of
all items to be called, which effectively deletes all the unerlying
vectors - which of course erases the items that they contained, so YES.

R(r)esize will compare the current size, and erase items if excessive
items exist. If to little items exist, it may perform re-allocation,
causing all existing iterators to become invalid. This (invalidated
iterators) will obviously be the case for clear too.

Regards,

Werner

 

[Mark P]

http://msdn2.microsoft.com/en-us/library/fs5a18ce(VS.80).aspx

vector <int> v1;
v1.push_back( 10 ); //adds 10 to the tail
v1.push_back( 20 ); //adds 20 to the tail
cout << "The size of v1 is " << v1.size( ) << endl;
v1.clear( ); //clears the vector

I have a few questions:

Does clear() deallocates the memory too (like resize())?

clear() doesn't necessary deallocate memory-- you don't need to worry
about this as it's the vector destructor's job to make sure that any
allocated memory is eventually freed. It does, however, invoke the
destructor of any object that gets cleared out of the vector.

Does clear() work for 2D vectors?

There's no such thing as a 2D vector (except in Physics class). What I
assume you're asking about is a vector of vectors and in this case, yes,
calling clear() does what you would expect it to: it invokes the
destructor of each of its contained vectors and, in the course of its
destruction, each of these vectors does the same for all of its
contained objects.

 

[divya_rathore_]

What I
assume you're asking about is a vector of vectors and in this case, yes,
calling clear() does what you would expect it to: it invokes the
destructor of each of its contained vectors and, in the course of its
destruction, each of these vectors does the same for all of its
contained objects.

That's interesting..
But does this mean that a single call would remove all the objects in
all the dimenstions?
Or it is to be done iteratively for each dimention?

I well could have a vector of a vector of a vector (a.k.a. 3D).. or
maybe even higher.

- Divya Rathore
(remove underscores for email ID)

 

[Mark P]

That's interesting..
But does this mean that a single call would remove all the objects in
all the dimenstions?
Or it is to be done iteratively for each dimention?

I well could have a vector of a vector of a vector (a.k.a. 3D).. or
maybe even higher.

Only one call is needed to start the recursive process.

Clearing or destructing a vector will destruct all of its contents. If
those contents are vectors then destructing any of those vectors will
destruct all of its contents. If those contents are vectors then
destructing any of those vectors will destruct all of its contents. If
those contents are vectors... get the idea?