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c++arraysresize
Jarx
Jarx

Reputation: 51

C++: What is the proper way of resizing a dynamically allocated array?

In C, I would have done it using realloc.

In C++, one would normally consider using the STL vector class.

But how do I properly resize an array in C++ without using any of the above solutions?

Upvotes: 5

Views: 13319

Answers (4)

Jerry Coffin
Jerry Coffin

Reputation: 490088

If you insist on doing this yourself in C++, you probably want to do roughly like std::vector does, and allocate raw memory using the global new operator, then use placement new to create objects "in place".

Otherwise, you end up constructing objects in all the memory you allocate, and then assigning over them when you use them. Unless you know that the initial (default) object construction is lightweight, you'd rather avoid it if possible.

Upvotes: 0

templatetypedef
templatetypedef

Reputation: 372714

There is no good equivalent of realloc in C++. You'll need to manually duplicate the array and copy the old elements over. Fortunately, thanks to the std::copy function in <algorithm>, this isn't too bad:

size_t k =  /* ... */
T* buffer = /* .. get old buffer of size k. .. */

T* newBuffer = new T[newSize];  // Assume newSize >= k
std::copy(buffer, buffer + k, newBuffer);

delete [] buffer;
buffer = newBuffer;

Hope this helps!

EDIT: Reordered the last two lines! Whoops!

Upvotes: 8

Armen Tsirunyan
Armen Tsirunyan

Reputation: 132984

int* arr = new int[20];
...
//wanna resize to 25?
int* temp = new int[25];
std::copy(arr, arr+20, temp);
delete [] arr;
arr = temp;
... //now arr has 25 elements

But, of course, you shouldn't do this :)

Upvotes: 0

Nemanja Trifunovic
Nemanja Trifunovic

Reputation: 24551

By doing what vector and realloc do internaly: make a new, bigger array, copy the content of the old one, and destroy the old one.

Upvotes: 3

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