Thread: STL map problem

What's wrong with this code? It seems to work fine most of the time, but I'm getting EXC_BAD_ACCESS on some occasions.

Here is the debug stack:

Code:

#4	0x00018016 in std::map<std::string, Item*, std::less<std::string>, std::allocator<std::pair<std::string const, Item*> > >::find at stl_map.h:542

#3	0x00017f51 in std::_Rb_tree<std::string, std::pair<std::string const, Item*>, std::_Select1st<std::pair<std::string const, Item*> >, std::less<std::string>, std::allocator<std::pair<std::string const, Item*> > >::find at stl_tree.h:1378

#2	0x00017efe in std::less<std::string>::operator() at stl_function.h:227

#1	0x00002fff in std::operator< <char, std::char_traits<char>, std::allocator<char> > at basic_string.h:2228

#0	0x97dab336 in std::string::compare

Why would this crash on string compare? I've tried several variations on the format of the string being passed to find(), but can't get any of them to work.

Thanks so much for your help!

Code:

// Group.h
#include "Item.h"

typedef map<string, Item*> GroupMap;

class Group {
public:
	Group();
	
	void addItem(string name, Item* item);
	Item* getItem(string name);

protected:
	GroupMap items;
};

// Group.cpp
#include "Group.h"

Group::Group() {}

void Group::addItem(string name, Item* item) {
	items.insert(pair<string,Item*>(name.data(),item));
}

Item* Group::getItem(string name){
	GroupMap::iterator it;
		
	it=items.find(name.data());
	if(it != items.end()) {
		return (*it).second;
	}
	return NULL;
}

Hey,
Thanks.
Unfortunately, that isn't the problem. I also tried find(name) and got the same results.

Did you fix it in items.insert() as well? I'm a bit confused as to why your backtrace doesn't go down into your actual code.

Hi,
Thanks for your help. Here is the modified code. Still produces the same problem.

Code:

// Group.h
#include "Item.h"

typedef map<string, Item*> GroupMap;

class Group {
public:
	Group();
	
	void addItem(string name, Item* item);
	Item* getItem(string name);

protected:
	GroupMap items;
};

// Group.cpp
#include "Group.h"

Group::Group() {}

void Group::addItem(string name, Item* item) {
	items.insert(pair<string,Item*>(name,item));
}

Item* Group::getItem(string name){
	GroupMap::iterator it;
		
	it=items.find(name);
	if(it != items.end()) {
		return it->second;
	}
	return NULL;
}

Are you using gdb? Can you go up (as many times as necessary) until you get to your code? That would give you information like (1) what line we're on in your code (2) what the values of the variables like name and item and things are.

Hey,
Sorry, here is a more complete backtrace:

Code:

#6	0x00012085 in GroupController::getInfo at GroupController.cpp:128
NOTE: here is what the calling method looks like...
void GroupController::getInfo() {
	Item* myItem = theGroup->getItem("Info");
	// ... nothing else in this function relates to the problem...
}

#5	0x000176f3 in Group::getItem at Group.cpp:22
NOTE: Group.cpp:22 is "it=items.find(name);"

#4	0x00018016 in std::map<std::string, Item*, std::less<std::string>, std::allocator<std::pair<std::string const, Item*> > >::find at stl_map.h:542

#3	0x00017f51 in std::_Rb_tree<std::string, std::pair<std::string const, Item*>, std::_Select1st<std::pair<std::string const, Item*> >, std::less<std::string>, std::allocator<std::pair<std::string const, Item*> > >::find at stl_tree.h:1378

#2	0x00017efe in std::less<std::string>::operator() at stl_function.h:227

#1	0x00002fff in std::operator< <char, std::char_traits<char>, std::allocator<char> > at basic_string.h:2228

#0	0x97dab336 in std::string::compare

thanks again. weird problem... can't make sense of it.

Thanks again for your help.
I believe the problem has to do with the string I'm passing to find() in the method getItem().
Within my program, getItem() is often evoked during a callback event. It's only in these cases that the find function crashes. I have the suspicion that the string can't be accessed properly during the event callback because the string is already being accessed from somewhere else? I'm not doing any threading, but is it possible that my callback events could cause similar problems to thread accessing of strings? Is there any way I can test this? Would making some sort of copy of the string help at all? Sorry for my confusion... can't make head or tail of this problem.

You're passing the string by copy anyway. Again, if you type "up" five times (assuming you're running this in GDB, or are attaching GDB to it), you can see exactly what name and all your other variables are (by doing "print name" or "print Item" etc.). So do that, and you'll know whether you're getting passed garbage or what.

Oh, thanks.

Here is what print name gives back:

Code:

// WITHOUT CALLBACK EVENT
$1 = {
  static npos = 4294967295, 
  _M_dataplus = {
    <std::allocator<char>> = {
      <__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, 
    members of std::basic_string<char,std::char_traits<char>,std::allocator<char> >::_Alloc_hider: 
    _M_p = 0x1b0d05c "a"
  }
}

// WITH CALLBACK EVENT
$1 = {
  static npos = 4294967295, 
  _M_dataplus = {
    <std::allocator<char>> = {
      <__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, 
    members of std::basic_string<char,std::char_traits<char>,std::allocator<char> >::_Alloc_hider: 
    _M_p = 0x342174a9 <Address 0x342174a9 out of bounds>
  }
}

How can I fix this?

If this came from a callback, then up one more level on your call stack had better be, therefore, the function where the callback is happening. Go there, and look around, and see why it's passing basically a bad string to you.