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Dimitar
Guest
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 Posted: Tue Aug 29, 2006 3:04 am Post subject: Templates and const function name resolution |
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Hello,
I have the following class C and the function F:
class C
{
public:
const vector<int>& GetV() const { return m_v; }
private:
vector<int>& GetV() { return m_v; }
vector<int> m_v;
};
template<class T1, class T2>
void F(const typename vector<T1>::iterator& first,
const typename vector<T1>::iterator& last)
{
for (vector<T1>::iterator it = first; it != last; ++it)
{
const vector<T2>& v = it->GetV();
}
}
I am trying to call F as:
....
vector<C> v;
F<C, int>(v.begin(), v.end());
....
But this does not compile (with MS VC++ 7) - the compiler refuses to
find the public GetV() function, instead it finds the private one.
However, if I replace
const vector<T2>& v = it->GetV();
with
const vector<T2>& v = vector<T1>::const_iterator(it)->GetV();
the code compiles.
I am wondering why the code as presented above does not compile and how
can I make it compile without introducing const iterator?
Thanks,
Dimitar
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Bo Persson
Guest
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| Posted: Tue Aug 29, 2006 4:05 am Post subject: Re: Templates and const function name resolution |
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"Dimitar" <dimitar.gospodinov (AT) gmail (DOT) com> skrev i meddelandet
news:1156791128.327334.276570 (AT) p79g2000cwp (DOT) googlegroups.com...
| Quote: | Hello,
I have the following class C and the function F:
class C
{
public:
const vector<int>& GetV() const { return m_v; }
private:
vector<int>& GetV() { return m_v; }
vector<int> m_v;
};
template<class T1, class T2
void F(const typename vector<T1>::iterator& first,
const typename vector<T1>::iterator& last)
{
for (vector<T1>::iterator it = first; it != last; ++it)
{
const vector<T2>& v = it->GetV();
}
}
I am trying to call F as:
...
vector<C> v;
F<C, int>(v.begin(), v.end());
...
But this does not compile (with MS VC++ 7) - the compiler refuses to
find the public GetV() function, instead it finds the private one.
|
Yes, the compiler has to first select candidates for overload
resolution, and if one is selected, check if it is also accessible.
| Quote: |
However, if I replace
const vector<T2>& v = it->GetV();
with
const vector<T2>& v = vector<T1>::const_iterator(it)->GetV();
the code compiles.
I am wondering why the code as presented above does not compile and
how
can I make it compile without introducing const iterator?
|
If you have both a const and a non-const function, the call is based
on the const-ness of the object you call it for ('*it' in this case).
You have several options, like
- make 'it' a const iterator in the for loop
- remove the private overload
- rename the const one GetConstV
Bo Persson
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Jens Theisen
Guest
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| Posted: Tue Aug 29, 2006 4:06 am Post subject: Re: Templates and const function name resolution |
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Dimitar wrote:
| Quote: | I am wondering why the code as presented above does not compile and how
can I make it compile without introducing const iterator?
|
Access control comes after overload resolution: If the non-const version
is a better match, then it will be taken; and if that's not possible
because it being private, it's an error.
If it's called on a non-const object, it is a better match, and
otherwise the const version is the only match.
If you use const_iterators, the value_type (what you get when
dereferencing the iterator) is C const&, so the const version will be
taken and with iterators it's the non-const version.
This is desirable: You should use const_iterators when you don't want to
modify your object.
Jens
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kanze
Guest
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| Posted: Tue Aug 29, 2006 11:51 pm Post subject: Re: Templates and const function name resolution |
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Dimitar wrote:
| Quote: | I have the following class C and the function F:
class C
{
public:
const vector<int>& GetV() const { return m_v; }
private:
vector<int>& GetV() { return m_v; }
vector<int> m_v;
};
template<class T1, class T2
void F(const typename vector<T1>::iterator& first,
const typename vector<T1>::iterator& last)
{
for (vector<T1>::iterator it = first; it != last; ++it)
{
const vector<T2>& v = it->GetV();
}
}
I am trying to call F as:
...
vector<C> v;
F<C, int>(v.begin(), v.end());
...
But this does not compile (with MS VC++ 7) - the compiler
refuses to find the public GetV() function, instead it finds
the private one.
|
It finds both. It then uses overload resolution to choose the
best match. In this case, the non-const version. (A non-const
function always beets a const function, if it can be called with
the given arguments).
Finally, having chosen the function, the compiler checks whether
it is accessible. It isn't, so you have an error.
| Quote: | However, if I replace
const vector<T2>& v = it->GetV();
with
const vector<T2>& v = vector<T1>::const_iterator(it)->GetV();
the code compiles.
|
Yes. Because the non-const function cannot be called, and is
not part of the overload set.
| Quote: | I am wondering why the code as presented above does not
compile and how can I make it compile without introducing
const iterator?
|
Change the name of one of the functions. It is almost always a
bad idea for functions with the same name to have different
access rights.
--
James Kanze GABI Software
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
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Greg Herlihy
Guest
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| Posted: Wed Aug 30, 2006 5:13 pm Post subject: Re: Templates and const function name resolution |
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|
kanze wrote:
| Quote: | Dimitar wrote:
I have the following class C and the function F:
class C
{
public:
const vector<int>& GetV() const { return m_v; }
private:
vector<int>& GetV() { return m_v; }
vector<int> m_v;
};
template<class T1, class T2
void F(const typename vector<T1>::iterator& first,
const typename vector<T1>::iterator& last)
{
for (vector<T1>::iterator it = first; it != last; ++it)
{
const vector<T2>& v = it->GetV();
}
}
....
I am wondering why the code as presented above does not
compile and how can I make it compile without introducing
const iterator?
Change the name of one of the functions. It is almost always a
bad idea for functions with the same name to have different
access rights.
|
I would go further and question whether declaring a private, accessor
method makes much sense in the first place. After all, any context in
which the private GetV() method could be called is also a context that
that enjoys direct access to the private m_v member variable. And if
C's implemention cannot handle its own m_v data member according to its
own specifications, then there's little chance that any other code in
the program will do any better.
Greg
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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kanze
Guest
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| Posted: Thu Aug 31, 2006 8:00 pm Post subject: Re: Templates and const function name resolution |
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|
Greg Herlihy wrote:
| Quote: | kanze wrote:
Dimitar wrote:
I have the following class C and the function F:
class C
{
public:
const vector<int>& GetV() const { return m_v; }
private:
vector<int>& GetV() { return m_v; }
vector<int> m_v;
};
template<class T1, class T2
void F(const typename vector<T1>::iterator& first,
const typename vector<T1>::iterator& last)
{
for (vector<T1>::iterator it = first; it != last; ++it)
{
const vector<T2>& v = it->GetV();
}
}
...
I am wondering why the code as presented above does not
compile and how can I make it compile without introducing
const iterator?
Change the name of one of the functions. It is almost
always a bad idea for functions with the same name to have
different access rights.
I would go further and question whether declaring a private,
accessor method makes much sense in the first place. After
all, any context in which the private GetV() method could be
called is also a context that that enjoys direct access to the
private m_v member variable. And if C's implemention cannot
handle its own m_v data member according to its own
specifications, then there's little chance that any other code
in the program will do any better.
|
In a simple case like the above, I would agree. On the other
hand, I've used private accessors for objects that were lazily
constructed, and I could see there use any time there was a
chance that the function do more than just return the immediate
object.
--
James Kanze GABI Software
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
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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