Re: hasMember

James Kanze <>
Wed, 8 Dec 2010 03:35:46 -0800 (PST)
On Dec 8, 10:03 am, Andrea Crotti <> wrote:

Suppose I have a class like this

class Cont
        std::map<int, int> content;

        bool hasMember(int idx);
        int getValue(int idx);

Now what I'm doing now in cases like this is to make sure that every
time I call getValue I'm sure that the object is there, so I have to
call first hasMember.

In the caller I always do

if (obj.hasMember(idx))
   int idx = obj.getValue(idx);

in this way I don't need to handle special cases when the thing is not
found, BUT in many cases this brings to useless computations, since
sometimes I have to scan the container twice.

Is there a better pattern in these situations which is not "return -1
if the object is not found"?

The simplest solution is to return a pointer to the map's
contents, or NULL if the value isn't there, e.g.:

    int const* Cont::getValue(int idx) const
        std::map<int, int>::const_iterator result
                = content.find(idx);
        return result == content.end()
            ? NULL
            : &result->second;


    int const* i = obj.getValue(idx);
    if ( i != NULL ) {

James Kanze

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