How to pass member functions into a function object...

[TeenWolf]

C++ has many algorithms in the form:

algo_if ( iterator, iterator, unary_function_object );

Take, for example:

algo_if (myContainer.begin(), myContainer.end(), bind1st (equal_to<size_t> (), 7))

This is all well and good if myContainer contains ints, but what if it contained strings?

equal_to, or perhaps algo_if, implicitly passes in myContainer.begin() as the first element for comparison, then myContainer.begin()+1, etc.

What if I didn't want to pass in the element of a container...what if I wanted to pass in the string length of each element?!

I have at my disposal strlen, which takes a string argument...
and length, which takes no argument other than the implicit this.

[laserlight]

You can write your own function object to compare the string lengths, though you still have to pass in elements of the container.

[TeenWolf]

But how do I "grab at" this implicit iterator that has no name? I can't just say myIter->length() or strlen(*myIter) because there is no myIter.

I researched the mem_fun family but I'm still confused.

Broadly speaking, how can I get access to the member functions of each element being implicitly passed into the above function algo_if?

[JaWiB]

> What if I didn't want to pass in the element of a container...what if I wanted to pass in the string length of each element?!

I don't think this should be a problem. For example, if you were using find_if on a container of strings, and you wanted to find an element with length equal to 7, you can fairly easily write your own functor, e.g.:

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

template <class T>
struct length_is_equal: public std::binary_function<T, typename T::size_type, bool>
{
  bool operator()(T obj, typename T::size_type len) const
  {
    return obj.length()==len;
  }
};


int main()
{  
  std::vector<std::string> strings;
  strings.push_back("Hello");
  strings.push_back("World");
  strings.push_back("abcefg");
  std::vector<std::string>::iterator it = std::find_if(strings.begin(),strings.end(), std::bind2nd(length_is_equal<std::string>(),5));
  if (it!=strings.end())
    std::cout<<*it;
}

Granted, the syntax can be a bit verbose/ugly, but it's not too difficult when you get used to it. (And if you use boost, it might be able to simplify things somewhat)