Re: hash_map elements deletion

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 07 Mar 2007 06:50:52 -0500

Message-ID:

<esm8us$oda$1@murdoch.acc.Virginia.EDU>

lokki wrote:

Hello,

can anybody tell me what's wrong with following example code?

    char *k, *v;

    k = new char[3];
    strcpy(k, "a2");

    v = new char[10];
    strcpy(v, "987654321");

    mp.insert(std::pair<const char*, const char*>(k, v));

What is mp?

    std::cout << "map size " << mp.size() << std::endl;

    // find
    TStringMapConstIterator i = mp.find("a1");
    if (i != mp.end())
    {
        std::cout << " found: " << i->second << std::endl;
    }
    else
    {
        std::cout << "not found" << std::endl;
    }

    // remove
    for (TStringMapIterator j = mp.begin(); j != mp.end(); j++)
    {
        delete[] (*j).first;
        delete[] (*j).second;
    }
    mp.clear();

Very likely, you corrupted the integrity of the map by deallocating all the
pointers. Does the comparision object / hash function for this map /
unordered_map access the pointees or just the stored pointers? If the
former is the case, the previous run through the list makes it very likely
that you enter undefined behavior.

Suggested fix: don't use char*, use std::string instead.

    std::cout << "deleted" << std::endl;

I'm using gnu c++ compiler and there are few typedefs before this code
to ensure proper comparison and hash computing of const char* in
hash_map

namespace __gnu_cxx
{
      template<> struct hash<std::string>
      {
                 size_t operator()(const std::string & __s) const
                 { return __stl_hash_string(__s.c_str()); }
      };

      struct tmt_eq_str__
      {
             bool operator() (const char *a, const char *b) const
             {
                  return !strcmp(a, b);
             }
      };

}

typedef __gnu_cxx::hash_map<const char*, const char*,
__gnu_cxx::hash<const char *>, __gnu_cxx::tmt_eq_str__> TStringMap;
typedef __gnu_cxx::hash_map<const char*, const char*,
__gnu_cxx::hash<const char *>,
__gnu_cxx::tmt_eq_str__>::const_iterator TStringMapConstIterator;
typedef __gnu_cxx::hash_map<const char*, const char*,
__gnu_cxx::hash<const char *>, __gnu_cxx::tmt_eq_str__>::iterator
TStringMapIterator;

The problem is removal of the elements. Both key and value pairs were
created by new operator. That's why I assumed, that they should be
deleted before callig mp.clear() function.

Problem is that program hangs in infinite loop when deleting elements.
There wasn't such problem when compiling similar code under VC++ 8.0

Thanks in adavance