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

This is a discussion on How to pass member functions into a function object... within the C++ Programming forums, part of the General Programming Boards category; C++ has many algorithms in the form: Code: algo_if ( iterator, iterator, unary_function_object ); Take, for example: Code: algo_if (myContainer.begin(), ...

  1. #1
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    How to pass member functions into a function object...

    C++ has many algorithms in the form:
    Code:
    algo_if ( iterator, iterator, unary_function_object );
    Take, for example:
    Code:
    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.

  2. #2
    C++ Witch laserlight's Avatar
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    You can write your own function object to compare the string lengths, though you still have to pass in elements of the container.
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    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?

  4. #4
    carry on JaWiB's Avatar
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    > 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.:
    Code:
    #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)
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    I know them not: not therefore am I short
    Of knowing what I ought."
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