Question

I try to realise an external merge sort (wiki) and I want to open 2048 ifstreams and read data to personal buffers.

ifstream *file;
file = (ifstream *)malloc(2048 * sizeof(ifstream));

for (short i = 0; i < 2048; i++) {
    itoa(i, fileName + 5, 10);
    file[i].open(fileName, ios::in | ios::binary); // Access violation Error
    if (!file[i]) {
        cout << i << ".Bad open file" << endl;          
    }
    if (!file[i].read((char*)perfile[i], 128*4)) {
        cout << i << ". Bad read source file" << endl;
    }       
}

But, it crashes with

Unhandled exception at 0x58f3a5fd (msvcp100d.dll) in sorting.exe: 0xC0000005: Access violation reading location 0xcdcdcdfd.

Is it possible to use so much opened ifstreams?
Or maybe it is very bad idea to have 2048 opened ifstreams and there is a better way to realize this algorithm?

Answer 1

Arrays of non-POD objects are allocated with new, not with malloc, otherwise the constructors aren’t run.

Your code is getting uninitialized memory and “interpreting” it as ifstreams, which obviously results in a crash (because the constructor of the class hasn’t been run not even the virtual table pointers are in place).

You can either allocate all your objects on the stack:

ifstream file[2048];

or allocate them on the heap if stack occupation is a concern;

ifstream *file=new ifstream[2048];
// ...
delete[] file; // frees the array

(although you should use a smart pointer here to avoid memory leaks in case of exceptions)

or, better, use a vector of ifstream (requires header <vector>):

vector<ifstream> file(2048);

which do not require explicit deallocation of its elements.

(in theory, you could use malloc and then use placement new, but I wouldn’t recommend it at all)

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… besides, opening 2048 files at the same time doesn’t feel like a great idea…