C ++：在没有cstdlib的情况下重新分配内存

Question

Couldn't find exact answer to the question:

Is freeing memory and allocating again the only way to reallocate memory without using cstdlib? If not so, then what are the possible solutions?

Thanks in advance.

Answer 1

Perhaps you mean like what is done with a std::vector (or other container) when you load it with memory, remove all the elements, then call clear to free the memory, then allocating new items within it, thus allocating more memory? In this case, as the memory in the container grows, the container may realloc its memory as needed.

Since you mention you are creating a Vector:

In projects where we needed to do this because we did not have a vector implementation (embedded), it is common to allocate a set chunk of memory larger than what is intiially required to prevent constant memory reallocations, which incur large copy costs and cause memory fragmentation. A common scheme was to allocate a "reasonable" size for the app, then double that size if the limit is reached. If the user ever requested the buffer be reduced in size, or set to a size at initialization, then we ignore this heuristic and use the requested size.

Answer 2

If you are implementing your own vector class, then you do need to properly copy the contents of the vector, not use realloc , since you don't know what the object itself is doing when it is being copied (or in C++11 for relevant cases, moved). Imagine for example that you have an object that does something like this:

class B;

class A
{
  private:
    B* bp;
  public:
    A(B *p) : bp(p)
    {
    }
};


class B
{
  public:
   A a;
   B() : A(this)
   {
      ... 
   }
 };

MyVector<B> v; 

If you copy the object to a different address, without calling the constructor, the bp pointer in A will point to some "random" place. That would be a pretty nasty bug to try to find.

[And yes, there is a bunch of stuff missing in the above classes - it is not meant as a complete class declaration, etc, etc]