Jump to content

  • Log In with Google      Sign In   
  • Create Account

Interested in a FREE copy of HTML5 game maker Construct 2?

We'll be giving away three Personal Edition licences in next Tuesday's GDNet Direct email newsletter!

Sign up from the right-hand sidebar on our homepage and read Tuesday's newsletter for details!


Struggling with maps in C++


Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.

  • You cannot reply to this topic
8 replies to this topic

#1 Nauraushaun   Members   -  Reputation: 100

Like
0Likes
Like

Posted 16 January 2011 - 06:14 PM

I've got a map that contains char pointers, and when I try to access it in certain ways it doesn't let me, it just creates a new object that's identical to the one that's already there. Very frustrating. Here's a code sample:
[/code]
int nBuffer = m_mKeyValues[uKeyNameBuffer]; // nBuffer == 0, the item is not found
char* uKeyNameBuffer2 = "Q";
int nBuffer = m_mKeyValues[uKeyNameBuffer2]; // nBuffer == 16, the item is found
[/code]

In the first instance, using breakpoints and debugging I've deciphered that uKeyNameBuffer contains "Q". But it won't find the "Q" item that exists in the map, it instead creates one with the exact same key (which should be an impossibility as far as I can tell). The second variable, uKeyNameBuffer2, when checked with breakpoints contains the exact same data in every way as uKeyNameBuffer...so why is the map treating it differently?

So far as I can tell it's a glitch in the map class. The entire point of each item having a key is that you access it with that key, and you can't create another item with the same key. But that's exactly what it's forcing me to do. Of course, it's probably just my stupidity getting in the way, and it's extremely unlikely that the map class would have such a large flaw...but what on earth is going on?

Sponsor:

#2 nobodynews   Crossbones+   -  Reputation: 2021

Like
1Likes
Like

Posted 16 January 2011 - 06:21 PM

I've got a map that contains char pointers

I'm not going to bother reading the rest of your post and just tell you to use std::string instead of char *. There's a good chance your problem is due to std::map not comparing the strings but the addresses of the char *s. Use std::string and if the problem persists then get back to us.

C++: A Dialog | C++0x Features: Part1 (lambdas, auto, static_assert) , Part 2 (rvalue references) , Part 3 (decltype) | Write Games | Fix Your Timestep!


#3 PrestoChung   Members   -  Reputation: 184

Like
0Likes
Like

Posted 16 January 2011 - 06:25 PM

A char* is an address, no? It's probably not looking further than that.

#4 Nauraushaun   Members   -  Reputation: 100

Like
0Likes
Like

Posted 16 January 2011 - 06:31 PM

That'd be it, it's got a different address...But then why does it have the same address as the version in the map if I use the "Q" method? Using debugging I've found that it does indeed have the same address as the item in the map, but how can this happen? I choose to store a Q in a variable, and it automatically makes it the same address as another Q? Is it some kind of optimization?

#5 nobodynews   Crossbones+   -  Reputation: 2021

Like
0Likes
Like

Posted 16 January 2011 - 06:32 PM

I hesitate to bring this up because I think the best solution is to use std::string, but in the interest of full-disclosure there is a way to use std::map and c-strings. Example here:

http://www.sgi.com/tech/stl/Map.html


C++: A Dialog | C++0x Features: Part1 (lambdas, auto, static_assert) , Part 2 (rvalue references) , Part 3 (decltype) | Write Games | Fix Your Timestep!


#6 Juanxo   Members   -  Reputation: 189

Like
0Likes
Like

Posted 16 January 2011 - 07:39 PM

Is it some kind of optimization?


yes, some compilers store const char* in the same address as an optimization

#7 iMalc   Crossbones+   -  Reputation: 2313

Like
0Likes
Like

Posted 17 January 2011 - 12:18 AM

Is it some kind of optimization?


yes, some compilers store const char* in the same address as an optimization

Absolutely, and some compilers go even further than that. An app with strings "Hello world" and "world" might result in pointers to the string "world" pointing about six characters into the middle of "Hello world".
"In order to understand recursion, you must first understand recursion."
My website dedicated to sorting algorithms

#8 Nauraushaun   Members   -  Reputation: 100

Like
0Likes
Like

Posted 20 January 2011 - 04:58 PM

Sorry I didn't post in a while, but I got through it by comparing using strcmp, rather than the equality operator.
Thanks everyone for all your help.

#9 Zao   Members   -  Reputation: 882

Like
1Likes
Like

Posted 21 January 2011 - 02:28 AM

As some may have hinted to above, the default comparator for a std::map is std::less<Key>, which defaults to operator <, which for pointers just compares addresses.
As mentioned, string literals may be folded together in static storage, ending up comparing equal by coincidence.

The proper solution here if you really must use char const* as your key is to provide a comparator to your std::map, which would do a string comparison with strcmp or a similar function.
Of course, by using raw pointers you run into ownership issues, as you have no way of properly handling the memory the pointers point to. Changing the key type to std::string would most probably be a Good Thing.

Note that operator [] on a std::map default-constructs a value if the key could not be found and inserts it into the container. If you do not desire that behavior, use the find(key) member function which returns an iterator to a key-value pair if the key was found, or the end() iterator if it was not found.
To make it is hell. To fail is divine.




Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.



PARTNERS