Question

I am trying to define a class in the global scope which contains some dynamically-allocated arrays. When the class’ constructor is called, the program does not have access to user-defined parameters read through a parameter file (i.e. the number of years in a simulation) thus it cannot allocate memory to the proper size. My idea was to allocate memory within a private function in the class, and then deallocate it using the destructor. Some example code:

class Simulation{
private:
    int initial_call; //a flag used to initialize memory
    double *TransferTracker;
public:
    Simulation();
    ~Simulation();
    void calc();
};

Simulation simulator; //global instance of Simulation

Simulation::Simulation()
{
   initial_call = 1;
}
Simulation::~Simulation()
{
    //when calling the destructor, though, the address is
    //0xcccccccc and the following attempt to delete produces
    //the compiler error.
    delete [] TransferTracker; //see error
}
void Simulation::calc()
{
    for (int i = 0; i < num_its; i++)
    {
         if (initial_call)
         {
             TransferTracker = new double [5];
             //The address assigned is, for example, 0x004ce3e0
             initial_call = 0;
         }
    }
    //even if this calc function is called multiple times, I see
    //that the address is still 0x004ce3e0.
}

The error I receive from the above code fragment is:

 Unhandled exception at 0x5d4e57aa (msvcr100d.dll) in LRGV_SAMPLER.exe: 0xC0000005: Access    
 violation reading location 0xccccccc0.

This error makes sense because I checked the memory address of TransferTracker when entering the destructor. My question is, why do we lose the address when entering the destructor? It probably has something to do with the fact that simulator is global; this paradigm seems to work fine if the class was not global. I am new to object-oriented programming so any help is appreciated!

EDIT: This was basically a blunder on my part and was helped by the answers. Two problems occurred: (1) the pointers were never set to NULL, thus creating confusion on trying to delete unallocated pointers. (2) There were actually two instances of the class in my scope, which was a mistake on my part. In the final code, there will only ever be one instance. Thanks everyone!

Answer 1

You have to initialize the value of the instance variable TransferTracker to 0 in the constructor. The problem you’re having is the destruction of the Simulation class without actually having assigned dynamic memory to TransferTracker.

Calling delete[] in the destructor with a null pointer is safe. The problem is that if you don’t give a value to TransferTracker, it may have any undefined value, that will cause trouble trying to deallocate with delete[].

EDIT:

As per your edit, how do you assure that there is only one instance of the Simulation class? This has to do with if you include several .o files in your build, etc.