Is there a way to expand a dynamic memory array? like this:
int *a = new int[5];
*a = new int[2];
Is this legal?
You cannot expand this type of a dynamic memory array. You can use malloc and realloc though if you need this facility but I would advice against that and suggest including <vector> and using std::vector instead. It has a resize method.
Also, what you described won't compile. The following will:
1: int *a = new int[5];
2: a = new int[2];
The above will allocate two memory blocks, neither of which will be destroyed. Second line will simply assign a new array to the same int *a pointer. When an allocated memory stops being referenced by any pointer, this is called a memory leak. The above code loses any reference to new int[5] and there is no way to free this memory to the operating system.
Although this is not a very practical example, there are multiple ways to resize an array/vector. As it is usually practical to increase the array size, I will do just this:
{ // C++ vector on the stack (although internally vector uses memory from the heap)
std::vector<int> a(1024);
// do smth
a.resize(4096); // note: this does not always reallocate
// do smth else
}
{ // C++ everything on the heap
std::vector<int> *a = new std::vector<int>(1024);
// do smth
a->resize(4096); // note: this does not always reallocate
// do smth else
delete a;
}
{ // C style
int *a = (int*)malloc(1024*sizeof(int));
// do smth
a = realloc(a, 4096*sizeof(int));
// do smth else
free(a);
}
It is worth to note that realloc does not do anything smart. All it does is:
mallocmemcpyfree