Chapter 39: In which the STL comes out of the closet

[mrlachatte]

Riddle me this.  I've got defitinitions like this:

--------------

class State
{
    private:
       GUI *gui;
       std::map<char *, int> iVars;

    public:
       State()
       {
          set_ivar("test", 1);
          gui = new GUI(this);
       }

       void set_ivar(char *name, int value)
       {
          iVars[name] = value;
       }

       int get_ivar(char *name)
       {
          if(iVars[name])
             return iVars[name];
          else return 0;
       }
};

class GUI
{
    public:
       GUI(State *s)
       {
          printf("value is %d", s->get_ivar("test"));
       }
};

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Now, ignoring the nice circular dependency going on there, explain why I would get a 0 in the printf() in the GUI constructor.  The real problematic code is more complex, but I've tested, and the values in the map<> are valid before the call to create the GUI.  However, inside the GUI constructor, suddenly they don't exist anymore.  Help :(

Comments

[kvance.livejournal.com]

Here's a theory off the top of my head: std::map doesn't do anything special for the char* data type, which you want to use as a string. You're really just mapping a pointer to an int.

The string "test" in State::State() is in a different memory location than the string "test" in GUI::GUI(). Since that pointer isn't in the map, you get the zero return value. If you had used a std::string as the key type, the strings' VALUES would have been compared and your int would have been found.

I could, of course, be wrong.

[inmatarian.livejournal.com]

Wow, shows what I know. I looked at it for a few minutes and couldn't figure it out. F'ing Lua messing with my understanding of associative containers.

An alternate fix would be to have a const char * str_test = "test"; in a common scope between them and use the str_test identifier.

[mrlachatte.livejournal.com]

Oh man, I'm feeling pretty dumb right now. Thanks all of you.

[highlyeccentric.livejournal.com]

o josh, i'm begining to fear for your sanity...