How do you `realloc` in C++?

Question

Any ideas how to realloc in C++?

It seems to be the missing language language - there is new and delete but not resize!

I need it because as my program reads more data, I need to reallocate the buffer to hold it.

I don't think deleteing the old pointer and newing a new, bigger one, is the right option. Please enlighten me.

Thanks, Boda Cydo.

Answer 1

The right option is probably to use a container that does the work for you, like std::vector.

new and delete cannot resize, because they allocate just enough memory to hold an object of the given type. The size of a given type will never change. There are new[] and delete[] but there's hardly ever a reason to use them.

What realloc does in C is likely to be just a malloc, memcpy and free, anyway, although memory managers are allowed to do something clever if there is enough contiguous free memory available.

Answer 2

Use ::std::vector!

Type* t = (Type*)malloc(sizeof(T)*n) 
memset(t, 0, sizeof(T)*m)

becomes

::std::vector<T> t(n, 0);

Then

t = (Type*)realloc(t, sizeof(T) * n2);

becomes

t.resize(n2);

If you want to pass pointer into function, instead of

Foo(t)

use

Foo(&t[0])

It it absolutely correct c++ code, because vector is a smart c-array.

Answer 3

Resizing in C++ is awkward because of the potential need to call constructors and destructors.

I don't think there's a fundamental reason why in C++ you couldn't have a resize[] operator to go with new[] and delete[], that did something similar to this:

newbuf = new Type[newsize];
std::copy_n(oldbuf, newbuf, std::min(oldsize, newsize));
delete[] oldbuf;
return newbuf;

Obviously oldsize would be retrieved from a secret location, same is it is in delete[], and Type would come from the type of the operand. resize[] would fail where the Type is not copyable - which is correct, since such objects simply cannot be relocated. Finally, the above code default-constructs the objects before assigning them, which you would not want as the actual behaviour.

There's a possible optimisation where newsize <= oldsize, to call destructors for the objects "past the end" of the newly-ensmallened array and do nothing else. The standard would have to define whether this optimisation is required (as when you resize() a vector), permitted but unspecified, permitted but implementation-dependent, or forbidden.

The question you should then ask yourself is, "is it actually useful to provide this, given that vector also does it, and is designed specifically to provide a resize-able container (of contiguous memory--that requirement omitted in C++98 but fixed in C++03) that's a better fit than arrays with the C++ ways of doing things?"

I think the answer is widely thought to be "no". If you want to do resizeable buffers the C way, use malloc / free / realloc, which are available in C++. If you want to do resizeable buffers the C++ way, use a vector (or deque, if you don't actually need contiguous storage). Don't try to mix the two by using new[] for raw buffers, unless you're implementing a vector-like container.