How to use std::for_each on a function pointer list

Question

On the following code, how can i rewrite the for loop by using a std::for_each instruction. I tried to use boost::lambda::_1, boost::bind, but I could not make it working.

#include <vector>
#include <iostream>
#include <cstring>
#include <cstdlib>

int main() 
{ 
  std::vector<int(*)(const char*)> processors; 
  processors.push_back(std::atoi); 
  processors.push_back(reinterpret_cast<int(*)(const char*)>(std::strlen)); 

  const char data[] = "1.23"; 

  for(std::vector<int(*)(const char*)>::iterator it = processors.begin();
      it != processors.end(); ++it) 
    std::cout << (*it)(data) << std::endl;
}

Any hint to help me solve this problem are welcome.

EDIT: Solution

#include <vector>
#include <iostream>
#include <cstring>
#include <cstdlib>
#include <boost/function.hpp>
#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>

int main() 
{ 
  std::vector<boost::function<int(const char*)> > processors; 
  processors.push_back(std::atoi); 
  processors.push_back(std::strlen); 

  const char data[] = "1.23"; 

  namespace bl = boost::lambda;
  std::for_each(processors.begin(), processors.end(),
      std::cout << bl::bind(bl::_1, data) << "\n");
}

Note that casting between function-pointer types and then dereferencing them is undefined. — Oliver Charlesworth
– Oliver Charlesworth, Commented Apr 30, 2011 at 0:50
What went wrong when you used std::for_each? What did you pass to for_each as the third parameter... i.e. the thing to do for each element of the range? — Brian Rothstein
– Brian Rothstein, Commented Apr 30, 2011 at 0:53
Since you're already using Boost's functional implementation, you should be able to use boost::function<int(const char*)> instead of int(*)(const char*). It will handle the return-type conversion for std::strlen (from size_t to int). — James McNellis
– James McNellis, Commented Apr 30, 2011 at 0:59
@Brian: I actually did tried something like that boost::bind(std::cout << boost::bind(_1, data) << "\n" and std::cout << (int(*)(const char*))boost::lambda::_1(data) << "\n" — Phong
– Phong, Commented Apr 30, 2011 at 1:01

Ise Wisteria · Accepted Answer · 2011-04-30 04:49:59Z

2

If boost::lambda and '\n' instead of endl are allowed, does the following code meet the purpose?

namespace bl = boost::lambda;
std::for_each( processors.begin(), processors.end()
             , std::cout << bl::bind( bl::_1, data ) << '\n' );

answered Apr 30, 2011 at 4:49

Ise Wisteria

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3 Comments

Phong Over a year ago

Yes i did tried it before but there is a compile error: it looks like it is not capable to recognize the type of bl::_1.

Ise Wisteria Over a year ago

Strange... For your information, the above code worked when I tested on VC2005 and ideone(gcc4.3.4).

Phong Over a year ago

You are right!, I misunderstood: boost::bind and boost::lambda::bind. It is working perfectly. Now i understand why all my previous attempt did fail.

Oliver Charlesworth · Accepted Answer · 2011-04-30 01:10:00Z

1

void blah(int (*func)(const char *), const char *data)
{
    std::cout << func(data) << std::endl;
};

...

std::for_each(processors.begin(), processors.end(), boost::bind(blah, _1, data));

edited Apr 30, 2011 at 1:10

answered Apr 30, 2011 at 0:53

Oliver Charlesworth

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3 Comments

Phong Over a year ago

Thx for the answer, But i would like to avoid creating another class for this purpose.

Phong Over a year ago

I see..., Is it poosible to reproduce blah function using a boost::bind function ? I know it wont be really pretty. but it is for improving my understanding of boost::bind.

Phong Over a year ago

I found my answer: stackoverflow.com/questions/5838679/…

Emile Cormier · Accepted Answer · 2011-04-30 04:11:59Z

1

You might find it easier to use BOOST_FOREACH:

#include <vector>
#include <iostream>
#include <cstring>
#include <cstdlib>
#include <boost/foreach.hpp>
#include <boost/function.hpp>

int main()
{
    typedef boost::function<int(const char*)> ProcessorFunc;
    std::vector<ProcessorFunc> processors;
    processors.push_back(std::atoi);
    processors.push_back(std::strlen);

    const char data[] = "1.23";

    BOOST_FOREACH(ProcessorFunc& proc, processors)
    {
        std::cout << proc(data) << std::endl;
    }

}

Or you could use a ranged-based for loop from C++0x.

answered Apr 30, 2011 at 4:11

Emile Cormier

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