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const vector<A> vs vector<const A> vs const vector<const A>

 
 
Javier
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      09-04-2007
Hello,
thanks for the replies to my questions.
I have one more:
class A
{
public:
m1() const;
m2();
};

is there any difference between

std::vector<const A> v;
const std::vector<A> v;
const std::vector<const A> v;

and, what about:

A a1;
v.push_back(a1);
v[0].m1()
v[0].m2()

in the three cases?

 
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Neelesh Bodas
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      09-04-2007
On Sep 4, 10:53 pm, Javier <(E-Mail Removed)> wrote:
> Hello,
> thanks for the replies to my questions.
> I have one more:
> class A
> {
> public:
> m1() const;

missing return value
> m2();

missing return value;
>
> };
>


> is there any difference between
>
> std::vector<const A> v;


> const std::vector<A> v;

v is a const vector whose elements are of type A. Effectively,
v.push_back() (or any other operation that changes the vector) is not
allowed on v
> const std::vector<const A> v;

doesnot compile
>
> and, what about:
>
> A a1;
> v.push_back(a1);
> v[0].m1()
> v[0].m2()

Talking about the 2nd case (const std::vector<A> v), since v is a
const vector, non-const member functions cannot be called, const
member functions can be.

-N

 
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James Kanze
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Posts: n/a
 
      09-04-2007
On Sep 4, 7:53 pm, Javier <(E-Mail Removed)> wrote:

> thanks for the replies to my questions.
> I have one more:
> class A
> {
> public:
> m1() const;
> m2();
> };


> is there any difference between


> std::vector<const A> v;
> const std::vector<A> v;
> const std::vector<const A> v;


Yes. The first and third aren't legal. An element of a
container must support assignment, and A const doesn't.

In general, in the standard library, declaring a container to be
const (i.e. your second declaration) means that 1) the topology
(number and order of elements) cannot change, and 2) the value
of the individual elements cannot change.

> and, what about:


> A a1;
> v.push_back(a1);
> v[0].m1()
> v[0].m2()


> in the three cases?


Illegal. As I said above, the declaration of v is illegal in
the first and third cases above. And if v is declared as in the
second case, push_back cannot be used on it. Given something
like:

std::vector< A > const v( 1, A() ) ;

however, v[0].m1() is legal, v[0].m2() no.

--
James Kanze (GABI Software) email:(E-Mail Removed)
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

 
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