Re: getting values in a map - map wrapper class

 James Kanze <>
Tue, 02 Oct 2007 08:25:28 -0000
On Oct 1, 5:35 pm, wrote:

"Chris ( Val )" <> wrote:

std::string getKeyValue( int index ) const
  // Code...

  return TestMap[ index ];

Can std::map<> operator [](size_t) be used in a const member function?


Of course not.

The first problem is that he is deriving from std::map, when he
probably should be using containment; derivation is a very poor
way to "wrap" something. But having an std::map as a member
doesn't change the problem: you still can't use operator[] in a
const function.

The real question, of course, is what should happen if the value
isn't present in the map. Depending on what the map is used
for, several possibilities can be considered:

 -- It's an error. The user should check beforehand (e.g. with
    a contains() function which you provide). If the value
    isn't present, it's an assertion failure, or possibly only
    an exception.

 -- It's an expected condition. The simplest solution here is
    to return a pointer, returning NULL if the element isn't

 -- The element is by definition present, with a default value.
    Basically, you implement something like the precedent, but
    return "ptr == NULL ? defaultValue : *ptr".

 -- The element is automatically inserted, with a default value.
    This is the behavior of std::map, but it means that you
    cannot read a const map.

The function behind whatever you do (except for the last) is
std::map<>::find. For the second solution (which is the most
general, and easiest to map into the other solutions), you do
something like:

    ValueType const*
    Map::get( KeyType const& key ) const
                            elem = myMap.find( key ) ;
        return elem == myMap.end()
            ? NULL
            : &elem->second ;

James Kanze (GABI Software)
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