James Kuyper wrote:
>
> Sean Shubin wrote:
> >
> > Since the map operator [] can be used for both reading and writing
> > values to a map, what should I use to access a given value of a map
> > that is const?
> >
> > #include <iostream>
> > #include <string>
> > #include <map>
> >
> > using std::map;
> > using std::string;
> >
> > void
> > test(const map<string,string>& x)
> > {
> >   using std::cout;
> >   cout << x["hello"] << endl;
>
>         cout << x.find("hello") << endl;

I think

      cout << x.find("hello")->second << endl;

will me more like what he wants.

[...]


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Christopher Eltschka wrote:
>
> James Kuyper wrote:
> >
> > Sean Shubin wrote:
> > >
> > > Since the map operator [] can be used for both reading and writing
> > > values to a map, what should I use to access a given value of a map
> > > that is const?
> > >
> > > #include <iostream>
> > > #include <string>
> > > #include <map>
> > >
> > > using std::map;
> > > using std::string;
> > >
> > > void
> > > test(const map<string,string>& x)
> > > {
> > >   using std::cout;
> > >   cout << x["hello"] << endl;
> >
> >         cout << x.find("hello") << endl;
>
> I think
>
>       cout << x.find("hello")->second << endl;
>
> will me more like what he wants.

It certainly will! Sorry.


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Joe Buck wrote:
>
> sean.shubin@www.anything.com (Sean Shubin) writes:
> >Since the map operator [] can be used for both reading and writing
> >values to a map, what should I use to access a given value of a map
> >that is const?
>
> The reason you can't use [] with a const map is because there isn't
> anything that can be done if the key is not present in the map
> (well, OK, you could throw an exception).

If the const operator[] returned by value, it could also return
a default constructed value. This is what the non-const operator[]
does as well, except that it puts that into the map as well.
If you use only the operator[] interface, you even won't see
the difference at all. If you are using the iterator interface,
you'll see the difference, of course - but since it's logical
that a const operator[] won't change the map, I don't see a
problem with this.

[...]
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Since the map operator [] can be used for both reading and writing
values to a map, what should I use to access a given value of a map
that is const?

#include <iostream>
#include <string>
#include <map>

using std::map;
using std::string;

void
test(const map<string,string>& x)
{
  using std::cout;
  cout << x["hello"] << endl;
//Does not compile because
//map operator [] is not const
}

int
main(int argc,char** argv)
{
  map<string,string> x;
  x["hello"] = "world";
  test(x);
  return 0;
}



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Sean Shubin wrote:
>
> Since the map operator [] can be used for both reading and writing
> values to a map, what should I use to access a given value of a map
> that is const?
>
> #include <iostream>
> #include <string>
> #include <map>
>
> using std::map;
> using std::string;
>
> void
> test(const map<string,string>& x)
> {
>   using std::cout;
>   cout << x["hello"] << endl;

 cout << x.find("hello") << endl;

Of course, this is appropriate only if you're certain that there is a
"hello" key in the map. If not, you'll have to add logic checking
whether or not x.find("hello")==x.end(). You haven't indicated how you
want that case handled, so I didn't insert the appropriate logic.

> //Does not compile because
> //map operator [] is not const
> }
>
> int
> main(int argc,char** argv)
> {
>   map<string,string> x;
>   x["hello"] = "world";
>   test(x);
>   return 0;
> }


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sean.shubin@www.anything.com (Sean Shubin) writes:
>Since the map operator [] can be used for both reading and writing
>values to a map, what should I use to access a given value of a map
>that is const?

The reason you can't use [] with a const map is because there isn't
anything that can be done if the key is not present in the map
(well, OK, you could throw an exception).

>void
>test(const map<string,string>& x)
>{
>  using std::cout;
>  cout << x["hello"] << endl;
>//Does not compile because
>//map operator [] is not const
>}

The answer is to use the find method, which returns an iterator.

// assume appropriate using, etc.
void test(const map<string,string>& x)
{
   map<string,string>::const_iterator p = x.find("hello");
   // p either points to a pair<const Key,Value> or to "one past the end"
   if (p == x.end())
 cout << "No entry for x["hello"]\n";
   else
 cout << (*p).second << endl;
}

(If you have an up-to-date compiler you can write p->second).


--
The reasonable man adapts himself to the world; the unreasonable man
persists in trying to adapt the world to himself. Therefore, all progress
depends on the unreasonable man.         -----George Bernard Shaw
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