Re: hasMember

James Kanze <>
Thu, 9 Dec 2010 03:59:21 -0800 (PST)
On Dec 9, 7:01 am, Paavo Helde <> wrote:

Andrea Crotti <> wrote

Jeff Flinn <> writes:

To avoid the memory allocation and ownership issues use

  #include <boost/optional.hpp>

  typedef boost::optional<int> optional_int;

  optional_int Cont::getValue(int idx) const
     std::map<int, int>::const_iterator itr = content.find(idx);
     return (itr != cont.end())? itr->second : optional_int();


  if(optional_int i = obj.getValue(idx)) // safe bool idiom
     int x = *i + 123; // deref for value

Using exceptions in this case could be good?

Seems out of place for the use you've described, given what little
info you've provided.

Interesting the optional_int, but I can't use boost...

I'm sure boost::optional is quite easy (and instructive) to implement by

Except that he really needs optional<int&>. Which in turn means
partial specialization for reference types, and the partial
specialization would be nothing more than a wrapper around
a pointer. There's really no point in not using a pointer to
begin with.


Have you considered the simple way of having another output parameter in
addition to the return value?

bool Cont::getValue(int idx, int& value_out) const
   std::map<int, int>::const_iterator itr = content.find(idx);
   if (itr != content.end()) {
        value_out = itr->second;
        return true;
   } else {
      return false;

// ...

   int x;
   if (obj.getValue(idx, x)) {
        // x found and valid, do something with x

This has the drawback that the lexical scope of x is too broad, but if
this is a very low-level class used only by a couple of slightly higher-
level classes this might be OK.

If his functions are reasonably small, the increased lexical
scope is not a problem. Defining an uninitialized int is more

But what's wrong with the pointer solution? That's what I'd
like to know. It's certainly the most idiomatic solution for
this problem.

James Kanze

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