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std::vector

hi all,

i don't understand what's wrong:
1)
std::vector<dou ble[3]> p(10);

doesn't compile:

/usr/include/c++/3.2.2/bits/stl_vector.h: In constructor
`std::vector<_T p, _Alloc>::vector (unsigned int) [with _Tp = double[3],
_Alloc = std::allocator< double[3]>]':
main.cpp:9: instantiated from here
/usr/include/c++/3.2.2/bits/stl_vector.h:34 2: ISO C++ forbids casting
to an array type `double[3]'

2)
std::vector<dou ble[3] p;
p.resize(10);

doesn't compile:

/usr/include/c++/3.2.2/bits/stl_vector.h: In member function `void
std::vector<_Tp , _Alloc>::resize (unsigned int) [with _Tp = double[3],
_Alloc = std::allocator< double[3]>]':
main.cpp:10: instantiated from here
/usr/include/c++/3.2.2/bits/stl_vector.h:70 3: ISO C++ forbids casting
to an array type `double[3]'

3)
std::vector<dou ble[3]> p;
p.reserve(10);

it's ok.

i'm using g++ 3.2.2
Jul 19 '05 #1
4 12242


enzo wrote:

hi all,

i don't understand what's wrong:

1)
std::vector<dou ble[3]> p(10);


You can't put arrays into vectors. Arrays don't fullfil the properties
needed for std::vector: They are not copyable and not assignable.

Why not:

struct Point
{
double x;
double y;
double z;
};

std::vector< Point > p;

--
Karl Heinz Buchegger
kb******@gascad .at
Jul 19 '05 #2

"enzo" <pr************ *@yahoo.com> wrote in message news:6c******** *************** ***@posting.goo gle.com...
hi all,

i don't understand what's wrong:
1)
std::vector<dou ble[3]> p(10);

The type used in a vector (or other standard container) must be copy constructable
and assignable. Plain arrays do not meet this requirement (you can't assign them).
Your choice is to wrap a class around the vector:"

struct tripple { double d[3]; }
vector<triple> p(10);
Jul 19 '05 #3
"Ron Natalie" <ro*@sensor.com > wrote in message news:<3f******* *************** *@news.newshost ing.com>...
"enzo" <pr************ *@yahoo.com> wrote in message news:6c******** *************** ***@posting.goo gle.com...
hi all,

i don't understand what's wrong:
1)
std::vector<dou ble[3]> p(10);
The type used in a vector (or other standard container) must be copy constructable
and assignable.


ok
so why this works? but mostly how can work?

3)
std::vector<dou ble[3]> p;
p.reserve(10);
p[1][2] = 3.0;
std::cout << p[1][2] << std::endl;
Plain arrays do not meet this requirement (you can't assign them).
Your choice is to wrap a class around the vector:"

struct tripple { double d[3]; }
vector<triple> p(10);

Jul 19 '05 #4
> > > i don't understand what's wrong:
1)
std::vector<dou ble[3]> p(10);

The type used in a vector (or other standard container) must be copy constructable and assignable.


ok
so why this works? but mostly how can work?

3)
std::vector<dou ble[3]> p;
p.reserve(10);
p[1][2] = 3.0;
std::cout << p[1][2] << std::endl;


It may work with the implementation of the standard library you are
using, but it is not guaranteed to work with other standard library
implementations . The reason why it works in your case is that reserve
immediately allocates the requested space and you have a POD type which
does not have to be initialized before use.

The reason that std::vector::re size() and the vector constructor
versions in your original post don't compile is that those try to
initialize the elements. std::vector::re serve() doesn't initialize
elements, it is just a hint so the vector can manage memory more
effectively. In other words std::vector::re serve() is not an alternative
for std::vector::re size().

--
Peter van Merkerk
peter.van.merke rk(at)dse.nl
Jul 19 '05 #5

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