109. Missing binders for non-const sequence elements

Section: 99 [depr.lib.binders] Status: CD1 Submitter: Bjarne Stroustrup Opened: 1998-10-07 Last modified: 2016-02-10

Priority: Not Prioritized

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Discussion:

There are no versions of binders that apply to non-const elements of a sequence. This makes examples like for_each() using bind2nd() on page 521 of "The C++ Programming Language (3rd)" non-conforming. Suitable versions of the binders need to be added.

Further discussion from Nico:

What is probably meant here is shown in the following example:

class Elem { 
  public: 
    void print (int i) const { } 
    void modify (int i) { } 
}; 
int main() 
{ 
    vector<Elem> coll(2); 
    for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::print),42));    // OK 
    for_each (coll.begin(), coll.end(), bind2nd(mem_fun_ref(&Elem::modify),42));   // ERROR 
}

The error results from the fact that bind2nd() passes its first argument (the argument of the sequence) as constant reference. See the following typical implementation:

template <class Operation> 
class binder2nd 
  : public unary_function<typename Operation::first_argument_type, 
                          typename Operation::result_type> { 
protected: 
  Operation op; 
  typename Operation::second_argument_type value; 
public: 
  binder2nd(const Operation& o, 
            const typename Operation::second_argument_type& v) 
      : op(o), value(v) {} 
 typename Operation::result_type 
  operator()(const typename Operation::first_argument_type& x) const { 
    return op(x, value); 
  } 
};

The solution is to overload operator () of bind2nd for non-constant arguments:

template <class Operation> 
class binder2nd 
  : public unary_function<typename Operation::first_argument_type, 
                          typename Operation::result_type> { 
protected: 
  Operation op; 
  typename Operation::second_argument_type value; 
public: 
  binder2nd(const Operation& o, 
            const typename Operation::second_argument_type& v) 
      : op(o), value(v) {} 
 typename Operation::result_type 
  operator()(const typename Operation::first_argument_type& x) const { 
    return op(x, value); 
  } 
  typename Operation::result_type 
  operator()(typename Operation::first_argument_type& x) const { 
    return op(x, value); 
  } 
};

Proposed resolution:

Howard believes there is a flaw in this resolution. See c++std-lib-9127. We may need to reopen this issue.

In 99 [depr.lib.binders] in the declaration of binder1st after:

typename Operation::result_type
 operator()(const typename Operation::second_argument_type& x) const;

insert:

typename Operation::result_type
 operator()(typename Operation::second_argument_type& x) const;

In 99 [depr.lib.binders] in the declaration of binder2nd after:

typename Operation::result_type
 operator()(const typename Operation::first_argument_type& x) const;

insert:

typename Operation::result_type
 operator()(typename Operation::first_argument_type& x) const;

[Kona: The LWG discussed this at some length.It was agreed that this is a mistake in the design, but there was no consensus on whether it was a defect in the Standard. Straw vote: NAD - 5. Accept proposed resolution - 3. Leave open - 6.]

[Copenhagen: It was generally agreed that this was a defect. Strap poll: NAD - 0. Accept proposed resolution - 10. Leave open - 1.]