size vs. capacity in a container, such as vector

[puzzlecracker]

Say I have a vector<intintVe c, and I reserve some sapce

intVec.reserve( 100);

Is it fair to assume the vector contains enough space for 100
elements. In which case, the capacity -- ability to store 100 elements
without a relocation -- is 100.

Then what is the difference for intVec.resize(1 00)?

Just trying to understand the difference between these two beasts,
size and capacity.


Thank you.

[Ian Collins]

Re: size vs. capacity in a container, such as vector

puzzlecracker wrote:

Say I have a vector<intintVe c, and I reserve some sapce
>
intVec.reserve( 100);
>
Is it fair to assume the vector contains enough space for 100
elements. In which case, the capacity -- ability to store 100 elements
without a relocation -- is 100.
>
Then what is the difference for intVec.resize(1 00)?
>
Just trying to understand the difference between these two beasts,
size and capacity.
>

reserve() reserves space (capacity), but size() will not change.

try:

#include <vector>
#include <iostream>

int main()
{
std::vector<int v1;
std::vector<int v2;

v1.reserve(100) ;
v2.resize(100);

std::cout << "v1 " << v1.size() << ' ' << v1.capacity() << std::endl;
std::cout << "v2 " << v2.size() << ' ' << v2.capacity() << std::endl;
}

--
Ian Collins

[Victor Bazarov]

Re: size vs. capacity in a container, such as vector

puzzlecracker wrote:

Say I have a vector<intintVe c, and I reserve some sapce
>
intVec.reserve( 100);
>
Is it fair to assume the vector contains enough space for 100
elements. In which case, the capacity -- ability to store 100 elements
without a relocation -- is 100.
>
Then what is the difference for intVec.resize(1 00)?
>
Just trying to understand the difference between these two beasts,
size and capacity.

Try to be curious enough to experiment...

std::vector<int intVec;
std::cout << intVec.size() << std::endl;
intVec.reserve( 100);
std::cout << intVec.size() << std::endl;
intVec.resize(1 00);
std::cout << intVec.size() << std::endl;

V
--
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I do not respond to top-posted replies, please don't ask

[Juha Nieminen]

Re: size vs. capacity in a container, such as vector

puzzlecracker wrote:

Say I have a vector<intintVe c, and I reserve some sapce
>
intVec.reserve( 100);
>
Is it fair to assume the vector contains enough space for 100
elements. In which case, the capacity -- ability to store 100 elements
without a relocation -- is 100.
>
Then what is the difference for intVec.resize(1 00)?

The difference is that, among other things, the new elements will
*not* be initialized with reserve(), and the push_back() and at()
functions will behave differently. Accessing elements beyond size() with
operator[] (even if space has been reserved for those elements) is
probably undefined behavior, especially if the element type is a class
which requires initialization.

[James Kanze]

Re: size vs. capacity in a container, such as vector

On Oct 12, 10:08 am, Juha Nieminen <nos...@thanks. invalidwrote:

puzzlecracker wrote:

Say  I have a vector<intintVe c, and I reserve some sapce

intVec.reserve( 100);

Is it fair to assume the vector contains enough space for
100 elements. In which case, the capacity -- ability to
store 100 elements without a relocation -- is 100.

Then what is the difference for  intVec.resize(1 00)?

The difference is that, among other things, the new elements
will *not* be initialized with reserve(), and the push_back()
and at() functions will behave differently. Accessing elements
beyond size() with operator[] (even if space has been reserved
for those elements) is probably undefined behavior, especially
if the element type is a class which requires initialization.

There's no probably...espe cially about it. It's undefined
behavior (except for at(), which is guaranteed to throw). In
any good implementation, something like

int
main()
{
std::vector< int v ;
v.reserve( 100 ) ;
v[ 50 ] ;
return 0 ;
}

will result in a runtime error (core dump under Unix).

More generally, there are two main uses of reserve(): to ensure
the validity of iterators and pointers to elements, and as an
optimization measure. It doesn't affect the logical state of
the container.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
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[Hendrik Schober]

Re: size vs. capacity in a container, such as vector

puzzlecracker wrote:

[...]
Just trying to understand the difference between these two beasts,
size and capacity.

One is the number of objects the vector actually has, the
other is the number of objects the vector could have without
needing to allocate more memory.

Thank you.

HTH,

Schobi