Re: STL / iterator / map - I dont get it

From:

Mark P <usenet@fall2005REMOVE.fastmailCAPS.fm>

Newsgroups:

comp.lang.c++

Date:

Thu, 22 Feb 2007 18:38:29 GMT

Message-ID:

<FqlDh.361$LF6.332@newssvr11.news.prodigy.net>

cppquest wrote:

Hi all,

I am doing something terrible wrong, but I dont understand why!
Maybe I am sitting too long in front of this box!
This is a breakdown of some code:

#include <map>

class MyMap : public std::map< double* , int > { };

MyMap test;

void findMyDouble(double* d)
{
    MyMap::iterator iter;
    for( iter = test.begin(); iter != test.end(); ++iter)
    {
        if(*iter->first == 2.0)
        {
            //assign t2 to d
            d = iter->first;
            // d == t2
            // *d == 2.0
            // okay until here
            break;
        }
    }
}

int main(int argc, char** argv)
{
    double* t1 = new double(1.0);
    double* t2 = new double(2.0);
    double* t3 = new double(3.0);

    test.insert(MyMap::value_type(t1,10));
    test.insert(MyMap::value_type(t2,11));
    test.insert(MyMap::value_type(t3,12));

    double* search;

    findMyDouble(search);

If you want to modify the value of search by calling findMyDouble, then
findMyDouble needs to take a reference (or pointer) to double*. Declare
it as:

void findMyDouble( double*& d);

     //search should be:
    //search == t2!
    //*search == 2.0
    //but search is again not defined

    MyMap::reverse_iterator iter;
    for( iter = test.rbegin(); iter != test.rend(); ++iter)
        delete iter->first;

    return 0;
}

I need to extract some map-key, which is a pointer.
The key is the pointer-itself, but i have to find a specific pointer-
value.
in the local scope of findMyDouble everything is okay, but in main
nothing happens at all.

Thanks for help for a tired cpp-victim.