Question

In my SDL program, I am using a map construct to simulate an "infinite" array of objects within a certain class. The code manages to compile fine, but when I run the program, as soon as one of the functions using the maps is trigger, the program crashes, returning a value of 3.

So, here s exactly what I m doing:

class MyClass
{
     public:
           int MyFunction();
     protected:
           std::map< int, MyObject > MyMap;
}

int MyClass::MyFunction()
{
     ...
     int i;

     if( MyMap.empty() )
     {
          i = 1;
     }
     else
     {
         i = MyMap.size() + 1;
     }

     MyMap[ i ] = PreviouslyDefinedObject;

     return i;

}

When MyFunction() is called from a MyClass object, the crash occurs. It seems to happen whenever anything of use is done with MyMap: it crashes if you comment out the penultimate line and just try to return i, and it crashes if you just set i = 1 and then assign an object to MyMap[i]

This is the first time I ve ever used a map, so I m not certain I m using them right. Is this a basic mistake somewhere? Can anyone point me in the right direction? Cheers.

Answer 1

Maybe you are calling the function from an uninitialized pointer, like this:

MyClass *obj;
obj->MyFunction();

Answer 2

Maps are used to associate a key with a value. If you re looking for an array, you should use a vector. It will do a better job of simulating an "infinite array" than a map, because a map isn t an array.

Note you can allocate a lot of elements with a vector, usually. If you re really trying to simulate a large array, I d recommend wrapping up a vector of vectors. With some math, you could create an operator[] for it that indexes into the correct array, to the correct element.

As for your code, there really isn t enough information to determine why it should be crashing, you d have to try to create a minimal program for us to compile or look at.

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