Re: C++ help, please? (fwd)

[Joe Buck <jbuck at Synopsys dot COM>]

> I got this. Is this an egcs bug or is the code bogus?

In the future please ask people to use gnu.g++.help for these.

The code is extremely bogus, however egcs-1.1.1 gets messed up by it.
The current snapshot compiler gives much better diagnostics, so it
doesn't look like we have remaining bugs.

Bogosities in the code:

1. attempt to write to a const object: note

   typedef const unsigned char ByteKeyID[8];

   the user then writes
	ByteKeyID kid;
   and proceeds to assign to its elements.

2. wrong interface to 'insert' function.  There is no insert(key,value)
   interface.  The user could write
	hm[key] = value;
   which is easy to write and to understand.  insert takes a pair.

3. hash<T> specializations are only provided for certain types,
   and arrays of 8 unsigned chars are not among them.  The user must
   supply a hash function.

4. The user appears to be counting on being able to pass arrays by value.
   One way to do this safely is to wrap the array in a struct:

struct ByteKeyID {
	unsigned char data[8];
};

   but it simply won't work otherwise.

> ...
> Here's the code:
> 
> #include <hash_map>
> 
> typedef const unsigned char ByteKeyID[8];
> 
> struct kcmp {
>   bool operator() (ByteKeyID a, ByteKeyID b) const {
>     return (memcmp(a, b, 8) == 0);
>   }
> };
> 
> typedef hash_map<ByteKeyID, 
>   int, 
>   hash<ByteKeyID>, 
>   kcmp> 
> hmap;
> 
> int main(int argc, char **argv)
> {
>   int i=0;
>   ByteKeyID kid;
>   hmap hm;
> 
>   for (i = 0; i < 8; i++) kid[i] = 8-i;
>   hm.insert(kid, 2);
> 
>   for (i = 0; i < 8; i++) kid[i] = i;
>   hm.insert(kid, 10);
> 
>   for (i = 0; i < 8; i++) kid[i] = 8-i;
> 
>   cout << "Extracted first value: ";
>   cout << hm[kid] << endl;
> }
> 
> Here's what the compiler says:
> 
> uri@alisan:/usr2/src/crypto-3.0 > g++ -I. -g -o v2-t v2-t.cc
> /usr/include/g++-2/stl_hash_map.h: In instantiation of `hash_map<const unsigned char[8],int,hash<const unsigned char[8]>,kcmp,__default_alloc_template<false,0> >::hash_map<unsigned char[8], int, hash<const unsigned char[8]>, kcmp, alloc>()':
> v2-t.cc:21:   instantiated from here
> /usr/include/g++-2/stl_hash_map.h:78: new declaration `hash_map<unsigned char[8],int,hash<const unsigned char[8]>,kcmp,__default_alloc_template<false,0> >::hash_map<>()'
> /usr/include/g++-2/stl_hash_map.h:78: ambiguates old declaration `hash_map<const unsigned char[8],int,hash<const unsigned char[8]>,kcmp,__default_alloc_template<false,0> >::hash_map<unsigned char[8], int, hash<const unsigned char[8]>, kcmp, alloc>()'
> /usr/include/g++-2/stl_hash_map.h: In method `hash_map<const unsigned char[8],int,hash<const unsigned char[8]>,kcmp,__default_alloc_template<false,0> >::hash_map<unsigned char[8], int, hash<const unsigned char[8]>, kcmp, alloc>()':
> /usr/include/g++-2/stl_hash_map.h:78: confused by earlier errors, bailing out
> uri@alisan:/usr2/src/crypto-3.0 > g++ -v
> Reading specs from /usr/lib/gcc-lib/i586-pc-linux-gnulibc1/egcs-2.91.60.1/specs
> gcc version egcs-2.91.60.1 19990115/Linux (egcs-1.1.1 release)
> uri@alisan:/usr2/src/crypto-3.0 > 
>