C++ function pointers inside templates

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);
}

Tidy this up, please! I got about five unrelated errors when I tried to compile it. Then I gave up. Give us a single block of code that we can put through ideone.com! — TonyK
– TonyK, Commented Mar 18, 2011 at 7:50

Lara Bailey · Accepted Answer · 2011-03-18 07:48:17Z

9

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);.

answered Mar 18, 2011 at 7:48

Lara Bailey

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3 Comments

cantrem Over a year ago

You're right! And I am most happy because of that. But what would happen if I'd want to change some T's inside my template? since Bubu's type has the const keyword in it?

Lara Bailey Over a year ago

@cantrem: You have number of options. You could write a wrapper function that calls strcmp but takes pointers to non-const char; you could template your class in the character type and not the pointer type and used int (*)( const T*, const T* ); you could use some traits-style meta functions to recover char from T = char* and manufacture const char* from there. It really depends on what your template is supposed to provide flexibility for and what it must assume about its parameters.

cantrem Over a year ago

I went for the quick and very very dirty solution of wrapping strcmp... but since it's a school project, it deserves to be that way! I guess the best solution would be to use std::string, like normal people do.

Maxim Razin · Accepted Answer · 2011-03-18 07:53:02Z

3

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.

answered Mar 18, 2011 at 7:53

Maxim Razin

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2 Comments

cantrem Over a year ago

Yes, but the thing is I need that chars mutable. Any idea how to keep my Bubu<char*> declaration as it is?

cantrem Over a year ago

To wrap strcmp into a function with non-const char* parameters is so ugly!

Josh Haberman · Accepted Answer · 2011-03-18 07:49:43Z

0

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);

answered Mar 18, 2011 at 7:49

Josh Haberman

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1 Comment

cantrem Over a year ago

it is (at least now, sorry if it was a typo before the post edit)

StackedCrooked · Accepted Answer · 2011-03-18 08:59:01Z

0

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;
}

answered Mar 18, 2011 at 8:59

StackedCrooked

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