std::mem_fun_ref problem: Works as functor broken when called as a member function

0
votes

The problem is compiler errors with the code snippet below.

Here's a very simple program to fill a list with random integers and increment each element. I use a std::for_each call to a functor to increment each member of my collection and all compiled. Focus on the for_each in main(). Next I simply alter the for_each to call List::increment rather than the functor Increment (notice the commented out code in main). For this task I use mem_fun_ref since I am not dealing with a collection of pointers but of classes. Also I don't think I need bind1st or bind2nd.

Here's my compiler errors:

/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_algo.h: In function _Function std::for_each(_InputIterator, _InputIterator, _Function) [with _InputIterator = std::_List_iterator, _Function = std::mem_fun1_ref_t]': blahblah.cpp:53: instantiated from here

/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_algo.h:158: error: no match for call to (std::mem_fun1_ref_t) (unsigned int&)'

/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/bits/stl_function.h:826: note: candidates are: void std::mem_fun1_ref_t::operator()(_Tp&, _Arg) const [with _Tp = List, _Arg = unsigned int&]

#include <list>
#include <string>
#include <algorithm>
#include <functional>
#include <iostream>

static const unsigned int LIST_SIZE = 25;

typedef std::list<unsigned int> List_t;

class List
{
    public:
        List() {};
        ~List() {};
        void push_back(unsigned int in) {m_list.push_back(in);};
        List_t::iterator begin( ) {m_list.begin();}
        List_t::iterator end( ) {m_list.end();};
        void print_list ( ) const
        {
            std::cout << std::endl;
            for (List_t::const_iterator iter = m_list.begin(); iter != m_list.end(); ++iter)
            {
                std::cout << *iter << ' ';
            }
            std::cout << std::endl << std::endl;

        }
        void increment( unsigned int & input)
        {
            ++input;
        }
    private:
        List_t m_list;
};
struct Increment{
    void operator ()(unsigned int &input)
    {
       ++input;
    }
};

int main()
{
    List l;
    for (unsigned int i= 0; i <= LIST_SIZE; i++)
    {
        unsigned int x = rand() % 100;
        l.push_back(x);
    }
    l.print_list(); // pre incremented
    //for_each(l.begin(),l.end(),Increment());
    for_each(l.begin(),l.end(),std::mem_fun_ref(&List::increment));
    l.print_list(); // incremented
    return 1;
}
2
c++stlmem-fun
How do I change the above for_each call so as to compile without the errors listed above. – John of Palestine

2 Answers

3
votes

mem_fun_ref in conjunction with for_each wants to have a member function of each individual item, not of the container class itself.

Why do you need increment to be a member of List? It doesn't operate on its state. I'd say it's perfectly fine as a free-functor.

0
votes
static void increment(unsigned int &input)
{
    ++input;
}

int main()
{
    List l;
    for (unsigned int i= 0; i <= LIST_SIZE; i++)
    {
        unsigned int x = rand() % 100;
        l.push_back(x);
    }
    l.print_list(); // pre incremented
    for_each(l.begin(),l.end(),std::ptr_fun(&increment));
    l.print_list(); // incremented
    return 1;
}