C++ function pointers inside templates - c++

Question

I have this problem:
template<typename T> class Bubu
{
...
int (*comparer)(const T t1, const T t2);
...
public:
Bubu(int (*_comparer)(const T t1, const T t2))
{
comparer = _comparer;
}
};
And in another file:
Bubu<char*> asd(strcmp);
Error:
error C2664: 'Bubu<T>::Bubu(int (__cdecl *)(const T,const T))' :
cannot convert parameter 1 from 'int (__cdecl *)(const char *,
const char *)' to 'int (__cdecl *)(const T,const T)'
I don't understand why. Shouldn't the compiler see a "char*" instead of "T" there?
EDIT: the Ideone.com-ready code:
int asdf(const char* a, const char* b)
{ return 0; }
template class Bubu
{
int (*comparer)(const T t1, const T t2);
public:
Bubu(int (*_comparer)(const T t1, const T t2))
{
comparer = _comparer;
}
};
int main(int argc, char* argv[])
{
Bubu asd(asdf);
}

Answer 1

When T is char*, const T is char* const which isn't the same thing as const char *. You need:
Bubu<const char*> asd(strcmp);
Top level const is ignored for function signatures so
int (*)( const char* const, const char* const );
is the same type as
int (*)( const char*, const char* );
so you're OK on the extra top level const although it doesn't gain you anything over the simpler int (*comparer)(T t1, T t2);.

Answer 2

If T is specialized as char*, const T means char* const (i.e. immutable pointer to a mutable char), rather than const char* == char const* (i.e. mutable pointer to an immutable char).
Bubu<const char*> asd(strcmp)
will compile.

Answer 3

Not sure if this is your problem, but your * in your function pointer declaration is in the wrong place (at least compared to what I've ever seen). Instead of:
int (comparer*)(const T t1, const T t2);
It should be:
int (*comparer)(const T t1, const T t2);

Answer 4

I think this does what you want:
#include <cstring>
#include <iostream>
template<class T>
class Bubu {
public:
typedef int (*Comparator)(T, T);
Bubu(Comparator inComparator) : mComparator(inComparator) { }
int compare(T a, T b)
{
return mComparator(a, b);
}
private:
Comparator mComparator;
};
int main()
{
Bubu<const char*> obj(strcmp);
std::cout << obj.compare("one", "two") << std::endl;
return 0;
}