Re: Template trouble
Re: Template trouble
- Subject: Re: Template trouble
- From: "Edward K. Chew" <email@hidden>
- Date: Tue, 29 Aug 2006 00:09:24 -0400
First of all, thanks for the various replies!
So, after another day of pounding on it, I was able to condense the
specialization problem down to a simple example:
//__________________________________________________
#include <cstring>
#include <iostream>
namespace AccessFunctions
{
template<typename T> int dataLength(T value)
{
return sizeof value;
}
template<typename T> int dataLength(T* value)
{
return sizeof(T);
}
template<> int dataLength(const char* str)
{
return std::strlen(str);
}
}
template<typename T>
class TAccessClass
{
public:
static int dataLength(T value)
{
return AccessFunctions::dataLength<T>(value);
}
};
int main (int argc, char * const argv[]) {
using namespace std;
const char str[] = "Hello, world!";
cout << "Case 1: ";
cout << AccessFunctions::dataLength(str);
cout << endl;
cout << "Case 2: ";
cout << TAccessClass<const char*>::dataLength(str);
cout << endl;
return 0;
}
//__________________________________________________
So when I pass my string to the dataLength function, the compiler
could conceivable choose one of three implementations. The first
function should return 4: the size of a pointer (assuming you are
compiling in 32-bit mode). The second should return 1 (the size of
the char being pointed to). The third should return 13 (the length
of the string). What gcc gives me is:
Case 1: 13
Case 2: 4
Having an intermediary class, then, was the source of my grief. On
further examination, I determined that if I remove the "<T>" from the
line "return AccessFunctions::dataLength<T>(value);", case 2 would
also return 13. Is there something illegal/deprecated about the way
I had it? If so, it would have been nice to get a compiler warning.
I vaguely recall having to put that in there to assuage some ancient
compiler (FWIW, CW 10 doesn't seem to care one way or the other). I
am lazy, and wouldn't have added it without reason. :-)
Anyway, so that's one down, at least...
-Ted
On Aug 28, 2006, at 8:11 PM, Steve Checkoway wrote:
On Aug 28, 2006, at 1:00 PM, Edward K. Chew wrote:
Oh, I forgot to mention, I have run into trouble with template
specialization. For example, I have a function along the lines:
template <typename T> UInt32 myHashFunction(T value, int
hashBitLength);
I specialized this for C strings:
template <> UInt32 myHashFunction(const char *str, int
hashBitLength);
In most cases, I use a string and get the specialized version, but
in a few situations, I get the general implementation instead. I
experimented with partial specialization (i.e. ...myHashFunction
(const T *pointer... ), but it still goes with the general case.
The debugger says that the class from which I call myHashFunction
is of type myClass<char const *>, but I believe "const char *" and
"char const *" are equivalent in C++. Anyway, I tried every
permutation I could think of to no avail.
I have a guess why this might be happening.
[PBG4:~/temp] steve$ cat template.cc
#include <cstdio>
template <typename T> void f( T t ) { puts( "general" ); }
template<> void f<const char *>( const char *s ) { puts( s ); }
int main()
{
char foo[] = "non const";
const char *bar = "const";
f( foo );
f( bar );
return 0;
}
[PBG4:~/temp] steve$ g++ -Wall -std=c++98 template.cc
[PBG4:~/temp] steve$ ./a.out
general
const
As you can see, the general version will be called for char*.
Personally, I can never remember the matching rules so I always
have to test this stuff out.
--
Steve Checkoway
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