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CoolPint
Guest
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 Posted: Tue Dec 23, 2003 10:28 am Post subject: Help Please: Passing Functor Object with Template type param |
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I have implemented a generic priority queue below and tested it works
fine, but I have one small problem I cannot understand. I have type
parameter F which determines the priority so that users can
instantiate in the following ways
PQueue<int> pq1;
PQueue<int, Functor> pq2; // where Functor is a name of user-defined
class
I also added another constructor to accept a function pointer so that
users can use normal functions as callback too. For example,
PQueue<int, bool (*) (int, int)> pq3(function);
// where function is a user-defined function which accepts ints and a
bool
So far so good. I then got a bit curious and issued something like
this:
PQueue<int, Functor> pq4(Functor()); <------- This causes the error
thinking this would assign Functor class to F and then assign an
object of Functor class to cmp, but it got me a compiler error which I
cannot understand.
I am interested to know what the error message means and what is
causing the problem. Can somebody kindly explain what is happening?
The compiler error I got when I tried to compile above with g++ is
something like below:
pqdriver.cpp:23: request for member `enQueue' in `numq(isGreater
(*)())', which
is of non-aggregate type `PQueue
(*)())'
I would very much appreciate a kind explanation as to what the problem
is what is causing it. And maybe a solution,, but I am more interested
in knowing what's happening so that I can learn from it. Thanks a lot
in advance!
class DefaultHeapOrder {
public:
template <typename T>
bool operator()(const T & a, const T & b)
{ return ( a < b ? true : false); }
};
template
class PQueue {
public:
PQueue();
explicit PQueue(F);
template <typename Iterator>
PQueue(Iterator, Iterator);
bool isEmpty() const;
bool isFull() const;
bool enQueue(const T &);
bool deQueue(T &);
bool getTop(T &);
void clear();
private:
int counter;
DArray<T> heap;
F cmp;
void buildHeap();
void shiftdown(int);
};
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tom_usenet
Guest
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| Posted: Tue Dec 23, 2003 11:48 am Post subject: Re: Help Please: Passing Functor Object with Template type p |
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On 23 Dec 2003 02:28:51 -0800, [email]coolpint (AT) yahoo (DOT) co.uk[/email] (CoolPint) wrote:
| Quote: | I have implemented a generic priority queue below and tested it works
fine, but I have one small problem I cannot understand. I have type
parameter F which determines the priority so that users can
instantiate in the following ways
PQueue<int> pq1;
PQueue<int, Functor> pq2; // where Functor is a name of user-defined
class
I also added another constructor to accept a function pointer so that
users can use normal functions as callback too. For example,
PQueue<int, bool (*) (int, int)> pq3(function);
// where function is a user-defined function which accepts ints and a
bool
So far so good. I then got a bit curious and issued something like
this:
PQueue<int, Functor> pq4(Functor()); <------- This causes the error
|
That's a function declaration! (a function called pq4 taking a Functor
and returning a PQueue
//extra parentheses prevent parsing as function decl
PQueue<int, Functor> pq4((Functor()));
or
// named object also works.
Functor f;
PQueue<int, Functor> pq4(f);
Tom
C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
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CoolPint
Guest
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| Posted: Wed Dec 24, 2003 5:06 am Post subject: Re: Help Please: Passing Functor Object with Template type p |
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| Quote: | PQueue<int, Functor> pq4(Functor()); <------- This causes the error
That's a function declaration! (a function called pq4 taking a Functor
and returning a PQueue
|
That's why! Thanks! How silly can I get? Now you mention it, it indeed
looks
like a function prototype...
| Quote: | //extra parentheses prevent parsing as function decl
PQueue<int, Functor> pq4((Functor()));
|
I am afraid this still gives me an error. g++ says
pqdriver.cpp:21: parse error before `)' token
I again feel stupid not to be able to see what makes the compiler
complain. Would you kindly help me out one more time?
| Quote: | or
// named object also works.
Functor f;
PQueue<int, Functor> pq4(f);
|
Yes, this works and of course it should....
Thank you, Tom... I think you helped me last time too. This group is
turning out to be a very valuable place for learning experience.
Thanks.
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tom_usenet
Guest
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| Posted: Mon Dec 29, 2003 3:25 pm Post subject: Re: Help Please: Passing Functor Object with Template type p |
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On 23 Dec 2003 21:06:53 -0800, [email]coolpint (AT) yahoo (DOT) co.uk[/email] (CoolPint) wrote:
| Quote: | PQueue<int, Functor> pq4(Functor()); <------- This causes the error
That's a function declaration! (a function called pq4 taking a Functor
and returning a PQueue
That's why! Thanks! How silly can I get? Now you mention it, it indeed
looks
like a function prototype...
//extra parentheses prevent parsing as function decl
PQueue<int, Functor> pq4((Functor()));
I am afraid this still gives me an error. g++ says
pqdriver.cpp:21: parse error before `)' token
I again feel stupid not to be able to see what makes the compiler
complain. Would you kindly help me out one more time?
|
(Delayed by xmas!)
That's a gcc bug. I reported it a while back, and I think it is marked
as closed for gcc 3.4 (due soon?), which includes a brand new
hand-coded parser.
Tom
C++ FAQ: http://www.parashift.com/c++-faq-lite/
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
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