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Geoff Macartney · Guest

Trying to get my head round partial template specialisation and
I'd be grateful for advice on this error; hope this isn't too
verbose.

-- Background --

I've a template for a hash table based on key-value pairs, where
the key must be an unsigned int. (The hash is done on the key and
the value gets put in the table.) Simplifying the name it looks like:

template < unsigned M, typename T >
class THashTable
....
int insert( unsigned key, const T & value );

where M is the size of the table and T is the type of object you want
to store in the table.

To avoid an ambiguity between overloads of the "search" method, type T
cannot be "unsigned". I want to do a partial specialisation of the
hash table to allow storage of unsigneds:

template < unsigned M >
class THashTable<M,unsigned>
....
int insert( unsigned value );

My intention was to implement this specialisation using a hash table to
store the unsigned, by having a dummy int value 0 as the T parameter and
using the value you want to store as the key:
private:
THashTable<M,int> m_Table;
....
insert(key, 0);

-- The Problem --

When I try compiling I get an error that looks like the compiler
doesn't spot that the 2 param insert above belongs to a THashTable<M,int>
but thinks it belongs to the specialisation, which only has
the one parameter insert, so it fails.

The listing below is an example with all the meat cut out of it
but the same structure as my code. It fails to compile, with the
following errors:

line 33: Error: Could not find a match for THashTable<M>::insert(unsigned, int).
line 40: Where: While instantiating
THashTable<73,unsigned>::insert(unsigned)
line 40: Where: Instantiated from non-template code.

Surely it ought to be ok to have a specialisation implementation that
uses a member variable of the type of the general template, and the
compiler ought to be able to spot that since m_Table is of type
THashTable<M,int> it should use that version of the function?

Geoff

1 #include <iostream.h>
2
3 template <unsigned M, typename T>
4 class THashTable
5 {
6 public:
7
8 int insert( unsigned, const T & );
9 };
10
11 template <unsigned M, typename T>
12 int
13 THashTable<M,T>::insert( unsigned , const T & )
14 {
15 return 0;
16 }
17
18 //specialisation
19 template < unsigned M >
20 class THashTable<M,unsigned>
21 {
22 public:
23 int insert ( unsigned );
24
25 private:
26 THashTable<M,int> m_Table;
27 };
28
29 template < unsigned M >
30 int
31 THashTable<M,unsigned>::insert( unsigned key )
32 {
33 return m_Table.insert(key, 0 ); // 0 is dummy
34 }
35
36 int main()
37 {
38 THashTable<73,unsigned> table;
39
40 cout << table.insert(10) << endl;
41 }

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Thomas Mang · Guest

Geoff Macartney schrieb:

[problem snip]

Geoff Macartney · Guest

Thomas Mang <a9804814 (AT) unet (DOT) univie.ac.at> wrote

[....]

Geoff Macartney · Guest

Wait a moment, I have gcc somewhere on this box, let's try that...
well, blow me down, it works.

So is it maybe the compiler that's at issue, and not me? (That's a
first!)

I know that there are some issues with template support with Forte
6 (and its successors, I think) - e.g. it doesn't support
template template parameters. But I'd have thought this much should
be ok.

Geoff

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