Re: mem_fun error

From:

"mlimber" <mlimber@gmail.com>

Newsgroups:

comp.lang.c++

Date:

7 Mar 2007 14:13:34 -0800

Message-ID:

<1173305614.594773.134940@t69g2000cwt.googlegroups.com>

On Mar 7, 4:56 pm, "joseph cook" <joec...@gmail.com> wrote:

I am getting a compile error on any compiler I try, so I know I have
an error here. Can anyone see it?

//includes

class Foo
{
public:
Foo(int a){m_hi = a;}
int hi(){return m_hi;}

int m_hi;

};

int main()
{
std::vector<Foo *> data;
data.push_back(new Foo(3));

max_element(data.begin(),
data.end(),
std::mem_fun(&Foo::hi));

}

gives a compile error:
in gcc: stl_algo.h:4565: error: no match for call to
'(std::mem_fun_t<int, Foo>) (Foo*&,Foo*&)'
stl_function.h:600: note: candidates are: _Ret
std::mem_fun_t<_Ret,_Tp>::operator()(_Tp*) const [with _Ret = int,
_Tp = Foo]

help!

}

You're specifying a member function adapter where you need a
comparator. A member function is invoked like:

some_mem_fun( *i )

where some_mem_fun is the functor resulting from your mem_fun call and
i is an iterator in the range [begin, end). Note that the functor only
takes one parameter. A comparator, on the other hand, has a signature
like:

bool less_than( a, b )

Probably what you intended is something like this:

for_each( data.begin(),
           data.end(),
           mem_fun(&Foo::hi));

or

int mx = numeric_limits<int>::min();
for( vector<Foo*>::const_iterator i=data.begin();
      i != data.end(); ++i )
{
   mx = max( (*i)->hi(), mx );
}

(Of course, this would require Foo::hi() to be a const function.)

Cheers! --M