Asp Forum - size vs. capacity in a container, such as vector

puzzlecracker

10/12/2008 12:38:00 AM

Say I have a vector<int> intVec, 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(100)?

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

Thank you.

5 Answers

Ian Collins

10/12/2008 1:09:00 AM

puzzlecracker wrote:
> Say I have a vector<int> intVec, 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(100)?
>
> 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

10/12/2008 1:14:00 AM

puzzlecracker wrote:
> Say I have a vector<int> intVec, 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(100)?
>
> 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(100);
std::cout << intVec.size() << std::endl;

V
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Juha Nieminen

10/12/2008 8:09:00 AM

puzzlecracker wrote:
> Say I have a vector<int> intVec, 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(100)?

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

10/12/2008 10:12:00 AM

On Oct 12, 10:08 am, Juha Nieminen <nos...@thanks.invalid> wrote:
> puzzlecracker wrote:
> > Say I have a vector<int> intVec, 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(100)?

> 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...especially 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.kanze@gmail.com
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Hendrik Schober

10/14/2008 7:29:00 AM

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

comp.lang.c++